The Pest That Is Not Feeding, Breeding or Biting: Overwintering Invaders and a Window That Closes Before Anyone Calls

1. Introduction: a category that breaks the rules

Structural pest management is organised around pests that do something. They feed, they breed, they damage material, they transmit disease, or they bite. Almost every control logic in this journal depends on at least one of those being true.

Overwintering invaders do none of them, and the category is handled badly as a result.

1.1 Why this produces bad outcomes

An occupant who finds flies in January reasonably concludes something is breeding. A contractor who agrees treats a void. Both are responding to a model of the situation that is not merely incomplete but inverted, and §14 sets out the four independent reasons the response cannot work.

2. Three species, one behaviour

The category is defined by a shared behaviour rather than shared biology, which is why identification matters more here than usual.

2.1 Cluster flies

Pollenia is a genus of flies commonly known as cluster flies or attic flies. Unlike typical carrion-feeding blow flies, these insects are parasitoids or predators of earthworms and other invertebrates. The genus contains approximately six species in North America, with Pollenia rudis and P. pediculata most common.⁴

They measure about 6 to 10 millimetres, larger than the common house fly, with a dull grey body marked by black lines and dense golden-yellow hairs on the thorax.⁵ Those golden hairs wear off with age, and the flies lack the bright white face of house flies. Species-level identification requires microscopic examination of setae number, position and length.⁴

2.2 The lady beetle

The multicoloured Asian lady beetle is up to three eighths of an inch, more oval than round, with colours varying from yellow to red and spotting that is variable in number and colour. The diagnostic character is a W or M pattern on the pronotum.⁷

2.3 The boxelder bug

Understanding the overwintering behaviour of boxelder bugs, Boisea trivittata, is described as critical for preventing initial aggregation and secondary movement inside structures.⁹

2.4 Why the distinction matters operationally

The three respond to the same intervention, exclusion, but they differ in what they do once inside, in whether they stain or bite, and in where they originate outdoors. Section 3 shows the cluster fly's outdoor biology rules out a conclusion the other two do not.

3. The cluster fly and the earthworm

The single most useful fact about this species is its host.

Cluster fly larvae are parasites of earthworms. Adult females lay their eggs in the soil near the entrance to earthworm burrows, and the larvae hatch and travel in search of an earthworm host.¹

Upon hatching, the young larvae move into the burrows and penetrate the earthworms' bodies. After feeding on the worms, the fly larvae pupate in the soil.³

3.1 The host species

Many cluster flies develop on Eisenia rosae, an introduced European earthworm now widely found in irrigated lawns and moist soil along waterways.² Other recorded hosts include Aporrectodea caliginosa, A. chlorotica, Eisenia lucens, Lumbricus rubellus and Lumbricus terrestris.⁵

3.2 The introduction history

Pollenia rudis is native to the Palaearctic and likely originated in Europe, and was introduced with European earthworms. P. vagabunda and P. labialis were introduced in the 1950s and 1960s, and the genus is now found across the United States and southern Canada.⁴

That is a satisfying piece of invasion ecology. The parasite arrived with its host, which is why the distribution of the fly follows the distribution of introduced European earthworms rather than any property of the buildings it occupies.

3.3 Generations and activity

There are multiple generations per year, up to four, with overwintering adults emerging in spring to mate and lay eggs, and peak adult activity from March to June.⁴

They do not reduce earthworm populations to the extent that they disrupt soil ecology.¹

4. What that rules out

The logical consequence is worth stating as bluntly as the literature does.

They are not breeding in your house. They physically cannot, because there are no earthworms in a wall.¹⁰

4.1 Where the flies are coming from

What looks like reproduction is a standing population emerging gradually. A large cohort entered in autumn, is dormant in voids, and individuals become active and appear in living space on warm days, as §6 sets out.

Just when an occupant thinks they have them under control, more appear the next day.³ Each is a survivor from September, not an offspring.

4.2 The diet point

Cluster flies are unique in that they do not feed on garbage, human food, or decaying matter.¹¹

This distinguishes them from house flies in the way that matters for public health, and it also means sanitation measures that would reduce a house fly problem do nothing here.

5. The seasonal move

The entry behaviour is consistent across all three species and is the point at which intervention is possible.

During late summer and early fall, cluster flies will increasingly be seen resting on sun-warm walls of buildings and, if entry points are available, will later move behind walls.²

The last generation flies to sun-warmed walls in late August and September and moves into wall voids, attics and upper storeys to overwinter as dormant adults.¹⁰

5.1 Where they concentrate

When abundant, cluster flies may aggregate in loose clusters in a wall void or other overwintering site, which is the origin of the name. Concentrations are largest in upper floors of buildings, where they can become very abundant on occasion.²

5.2 How they reach living space

Flies that have entered a home through the exterior wall may gain entrance into interior living spaces through gaps around ceiling lights and exhaust fans, cracks behind baseboards, and poorly-fitting doors to attic spaces.¹

That list is worth memorising, because it is the interior half of the problem and the half a homeowner can actually address in January when the exterior window has closed.

5.3 The rural gradient

Rural areas with earthworm populations support higher numbers,⁴ and problems tend to be most severe in higher elevation communities.²

They can still be a problem in urban homes if given the opportunity to enter and settle for the winter.¹¹

6. Dormancy and the warm winter day

The mechanism that produces the recurring complaint.

For the cool season, cluster flies remain in a dormant state, ceasing reproduction and moving little. However, during warm periods some may become active and incidentally enter living areas.²

They are particularly noticeable on warm winter days when they become active and find their way into living quarters.³ Adults are often seen spinning on their sides on windowsills or floors while buzzing loudly.¹

6.1 The energy budget

The fly will overwinter until spring, living off of its own fat.⁵

This explains the sluggishness and the tendency to spin rather than fly properly. An insect that has been metabolising stored reserves for four months in a cold void is not in condition, and the behaviour occupants find so unsettling is a starving animal in a space it cannot leave.

6.2 The word incidentally

The extension description says warmed flies incidentally enter living areas.² They are not seeking the occupants or the food. They are responding to warmth and light, and the living space is on the other side of the gaps listed in §5.2.

7. The lady beetle and the cliff face

The Asian lady beetle occupies buildings for a reason that is entirely about substitution.

In many places in their native range, multicoloured Asian lady beetles overwinter in cracks and crevices in rockfaces or cliffs. Where these are lacking, the next best thing is your house.⁸

In mountainous areas, aggregations are often formed under south-facing rocks mid-way up a mountain. In other areas, aggregations may be formed on buildings, often on walls with a southern or western exposure.⁶

7.1 The substitution is precise

A building's sunlit wall with crevices is a cliff face with crevices, at the same aspect, with the same thermal behaviour. The beetle is not adapting to human structures so much as failing to distinguish them from the habitat it evolved to use.

This is a cleaner version of the thermal refuge argument this journal has made for cockroaches, pharaoh ants and termites. Those species exploit buildings for heat they could not otherwise obtain. This one is selecting buildings because they resemble rock.

7.2 Where aggregations form

Upon arrival at an aggregation site, they form mass aggregations in dark, concealed locations.⁶ Favourable sites include attics, basements and living areas.⁷

8. Visual orientation

How the beetles find the building is documented and it is not chemical.

They are attracted to prominent isolated objects with high colour contrasts, which in many areas are homes or other buildings.⁸

Published work specifically addresses the orientation of Asian lady beetles to buildings and the role of visual contrast in alighting behaviour at overwintering sites.¹²

8.1 The timing trigger

They begin to look for places to overwinter during warm days, around 18 degrees Celsius, after a significant drop in temperatures, usually to near-freezing.⁸

That is a specific and useful signature. The flight is not triggered by cold alone but by warmth following cold, which is exactly the weather pattern of a prairie autumn and explains why the arrival appears so sudden.

8.2 What visual orientation implies

A pale isolated building against a dark background is a target. That is not a modifiable feature of most properties, which means orientation cannot be managed and only entry can.

Boxelder bugs and lady beetles are described as attracted to the light that reflects off the sides and windows of buildings.¹³

9. Diapause physiology

The internal state matters because it explains why the insects are unreachable.

Most of the winter is passed in a state of diapause that appears to be regulated by the corpus allatum.⁶

Diapause is entered with an empty digestive tract and an enlarged fat body.⁶

9.1 The corpus allatum connection

The corpus allatum is the gland that synthesises juvenile hormone, as set out in the insect growth regulator article published in this journal. Diapause regulation by that gland places overwintering under the same endocrine system that governs metamorphosis and reproduction.

We note as a research direction, flagged as our own thought, that a juvenile hormone analogue would therefore be acting on the system controlling diapause. We have found no work testing that against overwintering invaders and cannot say what it would do.

9.2 Why an empty gut matters

An insect entering diapause with an empty digestive tract and stored fat is not going to feed during winter, which eliminates every bait-based approach at a stroke.

Combined with §14, this leaves exclusion and physical removal as the only options with a mechanism behind them.

10. Cold tolerance

The physiological preparation is measurable and impressive.

In Japan, Harmonia axyridis acclimates to winter by decreasing its supercooling point and lower lethal temperature to approximately minus 19 and minus 16 degrees Celsius respectively. The decrease in supercooling point was significantly correlated with an increase in myo-inositol content.⁶

10.1 What supercooling point means

The supercooling point is the temperature at which body fluids spontaneously freeze. Depressing it allows an insect to remain unfrozen below the normal freezing point of water, and myo-inositol is functioning as a cryoprotectant.

10.2 The Manitoba implication

Minus 19 degrees is a Winnipeg January afternoon, not an extreme. A beetle with that supercooling point is not surviving a Manitoba winter outdoors in the open.

Which sharpens the point of §7: for this species in this climate, a building is not a preference. The insulated void is doing work that a rock crevice in its native range would do, and the margin is narrow.

We should be careful here, since the figures are from Japanese populations⁶ and local populations may acclimate differently. The direction of the argument holds regardless.

11. The aggregation pheromone disagreement

A genuine contradiction in the literature that is worth reporting rather than resolving.

University extension material states that adults apparently secrete an aggregation pheromone, which results in large numbers of beetles gathering at favourable sites.⁷

The peer-reviewed review states the opposite: it seems unlikely that volatile chemical cues are used for formation of overwintering aggregations.⁶

11.1 How to hold the disagreement

We give more weight to the review, which cites a specific study on the question, than to the extension summary, which uses the hedge apparently.

The two can also be partly reconciled. If beetles orient visually to a prominent contrasting object,⁸ many arriving independently at the same conspicuous building would produce an aggregation with no chemical signal at all. Aggregation is observed; a pheromone is inferred; the inference may not be necessary.

11.2 Why it matters practically

If aggregation is pheromone-mediated, then removing beetles and their residues should reduce the following year's arrival. If it is visually mediated, it should not.

This is a testable question with a direct operational consequence, and the honest position is that it appears unsettled.

12. The boxelder bug and the unfinished work

The third species illustrates how early this science still is.

Like many Hemiptera, boxelder bugs exhibit behaviours driven by allomones including host cues, and field observations suggest that alarm pheromones and chemical cues mediate their end-of-season aggregation.⁹

12.1 What was actually done

Researchers mapped the external morphology, ultrastructure and distribution of the antennal sensilla of overwintering boxelder bugs using scanning electron microscopy. They identified 20 subtypes distributed across seven categories of sensilla, namely sensilla trichodea, basiconica, chaetica, campaniformia, coeloconica, ampullacea and bell-mouthed, with distribution varying considerably by antennal segment.⁹

The work was undertaken in preparation for further pheromone work.⁹

12.2 Reading that honestly

The paper's own subtitle describes this as the first step in understanding overwintering aggregation behaviour.⁹ The chemical cues are suggested by field observation and not yet identified.

For comparison, the bed bug aggregation pheromone took until 2015 to characterise fully, as the bed bug behaviour article in this journal described. Boxelder bugs are at the stage of mapping which sensilla exist.

12.3 The practical consequence

Nobody can currently offer a pheromone-based monitoring or management tool for this species, because the pheromone is not known. Anything marketed as such should be treated with the scepticism the consumer products article in this journal recommends.

12A. Why these three and not others

A question the literature does not ask directly and which the assembled facts answer.

All three are introduced. Pollenia rudis arrived from Europe, probably with the earthworms it parasitises, and two further species followed in the 1950s and 1960s.⁴ The Asian lady beetle is native to Asia and was introduced to North America for biological control of aphids. The boxelder bug is native, but its principal host tree has been planted far beyond its original range as an ornamental.

12A.1 The common pattern

Each has been moved into a landscape where the overwintering habitat it evolved to use is scarce or absent, while an abundant substitute exists. Cluster flies would overwinter in whatever sheltered cavities a European landscape offered. Lady beetles would use rock crevices, which most of the North American plains do not provide.⁸

Buildings are the substitute in both cases, which means the problem is a consequence of introduction rather than of any change in the insects' behaviour.

12A.2 The connection to the rest of this journal

This is a third variant of the argument made elsewhere here for cockroaches, pharaoh ants and termites. Those species use heated buildings to survive a climate they could not otherwise tolerate. The lady beetle uses buildings as a structural substitute for rock. The cluster fly uses them as sheltered cavities.

Same outcome, three different mechanisms, and only the first is properly a thermal argument. We flag this categorisation as ours.

13. What harm they actually do

An honest accounting, because the answer affects what a proportionate response looks like.

Cluster flies do not bite humans or animals and are primarily a nuisance.¹ They do not cause damage to buildings, furniture or occupants.³

Lady beetles do not carry diseases or harm building materials.¹⁴

13.1 Where the lady beetle differs

Three genuine problems distinguish it from the fly.

Large numbers inside a structure can cause allergy problems in sensitive individuals, and these in particular are known to bite.¹⁴

They should not be swatted or crushed, because they release a yellow-orange liquid that can stain fabrics and walls.⁷

They are also a minor pest on ripening fruit, and if present in grape clusters at harvest they can give an off-flavour to wine known as ladybug taint.⁸

13.2 Why reflex bleeding matters more than it sounds

The yellow-orange liquid is haemolymph released from the leg joints, a defensive response found across the Coccinellidae. It carries the alkaloids that make these beetles unpalatable to predators, and it is the same chemistry responsible for the wine taint in §13.1.

We offer that connection as our own reading rather than a cited finding. What the sources establish is the practical consequence: crushing produces a stain that is difficult to remove from fabric and wall finishes,⁷ which means the instinctive response to a beetle on a wall is the one that causes the only lasting damage the species inflicts.

13.3 The parasitism footnote

One ecological detail worth recording. Average parasitism of collected beetles runs closer to 2 to 4 per cent, and fluctuations in beetle populations affect the parasite as well.¹²

Natural enemies exist and operate at a rate far too low to constitute control, which is worth knowing before anyone proposes biological management of an overwintering aggregation.

13.4 The proportionate conclusion

For cluster flies, the honest position is that this is an aesthetic problem. For lady beetles it is an aesthetic problem with a staining risk and a minor allergy and biting dimension.

Neither justifies the level of intervention that is frequently sold, and a contractor presenting either as a health hazard is overstating.

14. Why indoor treatment fails

Four independent reasons, any one of which would be sufficient.

Pesticides are generally not recommended indoors because the insects are typically overwintering in inaccessible areas such as wall voids, and because they unnecessarily expose people to pesticides.⁸

14.1 The dormancy problem

Believing the flies are breeding in your walls leads to fogging a void, which cannot reach dormant flies.¹⁰

This is the point the residual efficacy article in this journal establishes generally. A contact insecticide requires the insect to contact it. A diapausing insect in a void is, by definition, not moving across surfaces.

14.2 The no-population problem

Every control logic in this journal assumes a reproducing population, so that killing a fraction reduces the trajectory. Here there is no trajectory. A fixed cohort entered in September and is declining by attrition regardless.

Beetles that make it indoors will eventually die whatever is done.⁸

14.3 The perimeter question

The one chemical intervention with a defensible mechanism is an exterior perimeter application timed before entry, and extension guidance addresses timing considerations for perimeter pesticide applications as part of an integrated approach.¹

That is a different proposition from treating a void in January. It acts on insects that are active, on surfaces they are resting on, before they are inside.

14.4 The constraints on a perimeter application

Three conditions have to hold for it to make sense, and all three follow from material established elsewhere in this journal.

Timing. It must precede entry, which per §16 means completing before the migration rather than after the first sighting.

Substrate. The residual efficacy article in this journal established that porous surfaces absorb liquid applications below the plane an insect contacts. An exterior wall of brick, stucco or bare masonry is the difficult case, and a formulation choice that ignores it will underperform regardless of timing.

Exposure. A south-facing wall in late summer is the most ultraviolet-exposed surface on a building, and photodegradation shortens residual life on exactly the aspect where the insects aggregate.¹⁴

We raise the last two as our own analysis. The sources address timing; they do not address substrate or ultraviolet exposure for this application, and both seem material.

14.5 What a perimeter application does not do

It does not reach insects already inside, and it does not prevent the visual orientation described in §8. At best it reduces the number that successfully enter, which makes it a supplement to sealing rather than a substitute for it.

15. The carpet beetle consequence

The failure mode that turns a nuisance into an actual pest problem.

Fogging a void leaves a mass of dead insects that feeds carpet beetles.¹⁰

The same applies to removal done carelessly. Vacuum flies off windowsills and floors, but empty the vacuum canister into a sealed bag or outdoor bin so the dead flies do not attract carpet beetles.¹

15.1 Why this matters more than it sounds

Carpet beetles are a genuine fabric pest. A void containing several thousand dead flies is a substantial protein resource in a concealed, undisturbed location, which is close to ideal for dermestid development.

So the treatment that could not work against the original insect creates the conditions for a second infestation that will require real intervention.

15.2 The general principle

This is the same structure as the bat exclusion article in this journal, where sealing a maternity roost produced dead young, odour and secondary insects. Killing animals in a place you cannot reach means leaving the bodies there.

16. The window

The central operational fact, and the reason this article exists.

The best time to seal cracks and close gaps is late August through mid-September, before they begin their fall migration.¹⁵ Sealing should be done before early September.¹⁰

16.1 What this means for a service business

This is a category that has to be sold in advance of the problem, to a client who currently has no symptoms, on the strength of last year's experience.

That is commercially awkward and it is the honest structure of the situation. A contractor who accepts a January cluster fly job and treats a void is taking money for work that cannot succeed.

16.2 What can be done in January

Interior sealing of the routes in §5.2, around ceiling lights, exhaust fans, baseboards and attic hatches, reduces movement from void to living space. Vacuuming removes what appears, with the §15 caution about disposal.

Neither addresses the population in the void, and a client should be told that plainly rather than sold a treatment.

17. Where to look

The inspection is predictable because the behaviour is.

When gathering in fall, they will be found in large numbers on the sunny side of buildings and trees. Look for gaps and holes in the structure, most notably around eaves, windows and foundations, often on the south side.¹⁴

Walk the exterior and inspect every side, paying closest attention to south and west-facing walls.¹⁵

17.1 The specific measures

Exclusion is described as the safest and most effective way to reduce or prevent invasion: caulk around window frames, outdoor faucets and vents; fill holes with mesh and caulk; add rubber or brush sweeps to doors including roll-down doors; keep window screens in good repair.¹⁴

In late summer and early fall, caulk cracks, seal entry points around windows, and install screening over vents.⁷

Attic vents, chimneys and other openings should be inspected.¹¹

17.2 The shared-route efficiency

Sealing these entry points reduces access for stink bugs, boxelder bugs, and other fall invaders that use the same routes.¹⁵

One piece of work addresses the whole category, and per the rodent exclusion article in this journal it addresses rodents too, since the openings overlap.

18. The exclusion evidence gap

An admission in the literature that deserves to be better known, and which repeats a finding from elsewhere in this journal.

If overwintering pests gain access to buildings through cracks and crevices, it seems reasonable that sealing these openings will keep pests out. While no formal research has tested this hypothesis, a grant was awarded to evaluate the effectiveness of exclusion as a technique to keep stink bugs and other overwintering pests out of homes.¹⁶

18.1 The same gap as rodents

The preceding article in this journal reported that no controlled trial of rodent exclusion efficacy could be found either.

So the intervention recommended first by every extension service, every trade body and this journal, across two entirely separate pest groups, rests on mechanism and plausibility rather than on measured outcome.

18.2 Our position

We continue to recommend exclusion, because the mechanism is sound and the alternatives are demonstrably worse. But recommending something on mechanism is a weaker claim than recommending it on evidence, and the difference should be stated rather than glossed.

19. What to tell a client

The honest conversation, which is shorter than the one usually had.

They are not breeding in the house. For cluster flies this is certain.¹⁰

They will not damage anything. Neither species harms building materials or furnishings.³¹⁴

The ones inside will die whatever happens.⁸

Treating the void will not reach them and may create a carpet beetle problem.⁸¹⁰

Vacuum and dispose of the contents outside.¹

The real work is exterior sealing, and it has to happen before September.¹⁵

Do not crush the beetles.⁷

20. The Manitoba position

Three local factors.

The window is narrower here. The trigger is warm days following a significant temperature drop,⁸ and a prairie autumn compresses that sequence. A sealing programme aimed at mid-September elsewhere may need to be earlier here, though we have found no local phenology data and flag that as an inference.

The thermal argument is stronger here. A supercooling point near minus 19 degrees⁶ against a Manitoba winter means the void is not a convenience. It is the difference between survival and not.

Earthworm distribution governs cluster fly pressure. Since the fly follows introduced European earthworms,⁴ irrigated lawns and moist soil along waterways² predict where the problem concentrates, which in this province means river lots and established suburbs rather than new development on unimproved ground.

21. Limitations and open questions

The aggregation pheromone question is unresolved. Set out in §11 rather than decided.⁶⁷

No formal research tests exclusion for this group. Stated in the literature itself.¹⁶

The cold tolerance figures are from Japanese populations. Flagged in §10.2.⁶

The boxelder bug chemistry is not characterised. The published work maps sensilla in preparation for pheromone work.⁹

Several sources are commercial. The interior routes, disposal advice and timing recommendations appear in both extension and commercial material,^10111315 and we have preferred extension sources where both exist.

Sections 9.1, 11.1, 15.1 and 20 include our reasoning. The juvenile hormone research direction, the reconciliation of the pheromone disagreement, the dermestid resource argument and the Manitoba phenology inference are ours.

We sell exclusion and autumn perimeter work. This article argues against the January void treatment that is easier to sell, and in favour of work that must be booked before anyone has a problem, which is commercially harder rather than easier.

22. Conclusion

Cluster fly larvae are parasites of earthworms, with eggs laid near burrow entrances and development completing in soil,¹³ which settles the question every occupant asks. They are not breeding in the wall, because there are no earthworms in a wall.¹⁰ The flies appearing through winter are survivors of a single September cohort, dormant in voids and living off stored fat,²⁵ becoming active and entering living space on warm days.²

The Asian lady beetle overwinters in rock crevices in its native range and treats buildings as substitutes, orienting to prominent isolated objects of high visual contrast,⁸ entering diapause regulated by the corpus allatum with an empty gut and enlarged fat body, and depressing its supercooling point to around minus nineteen degrees.⁶

Indoor treatment fails on four independent grounds. The insects are in inaccessible voids, they are dormant and not crossing treated surfaces, there is no reproducing population to suppress, and the exposure to occupants buys nothing.⁸¹⁰ It also leaves a mass of carcasses that feeds carpet beetles,¹⁰ converting a cosmetic problem into a fabric pest problem.

What works is exterior sealing, and it must be done by early September.¹⁰¹⁵ Which is the awkward shape of this whole category: the only effective intervention has to be sold to someone who does not currently have a problem, four months before they will, and refused to someone who has one and wants it fixed today.

References

1. Cluster Fly, publication ENTO-136. Virginia Cooperative Extension, Virginia Tech. Source for cluster flies entering structures in large numbers in fall to overwinter; the complete life cycle with females laying eggs in soil near the entrance to earthworm burrows and larvae parasitising earthworms; the observation that they do not reduce earthworm populations enough to disrupt soil ecology; that adults do not bite and are primarily a nuisance; that they do not bite, are not associated with disease transmission and do not reproduce inside buildings; the behaviour of spinning on their sides on windowsills while buzzing loudly; the interior routes through gaps around ceiling lights and exhaust fans, cracks behind baseboards and poorly-fitting attic doors; the guidance to vacuum but empty the canister into a sealed bag or outdoor bin so dead flies do not attract carpet beetles; and the inclusion of timing considerations for perimeter pesticide applications within an integrated approach. https://ext.vt.edu/content/pubs_ext_vt_edu/en/ENTO/ENTO-136/ENTO-136.html
2. Cluster Flies. Bugwoodwiki, High Plains IPM. Source for the development of cluster flies as parasites of certain earthworm species including Eisenia rosae, an introduced European earthworm widely found in irrigated lawns and moist soil along waterways; the greater severity of problems in higher elevation communities; the behaviour of resting on sun-warm walls during late summer and early fall before moving behind walls; aggregation in loose clusters in wall voids giving rise to the name; the largest concentrations occurring in upper floors; and the dormant state through the cool season with cessation of reproduction and little movement, punctuated by activity during warm periods when some incidentally enter living areas. https://wiki.bugwood.org/HPIPM:Cluster_Flies
3. Cluster Flies fact sheet. University of New Hampshire Extension. Source for the description of cluster flies invading homes in fall and becoming wintertime pests, particularly noticeable on warm winter days; the impression created that they are breeding inside when more appear the next day; the statement that they are only using the home to spend winter and do not damage buildings, furniture or occupants; the lack of obvious characteristics distinguishing them from house flies to a layperson; and the life cycle in which females lay eggs near earthworm burrow openings, larvae move into burrows and penetrate the worms' bodies, and pupation occurs in soil. https://extension.unh.edu/resource/cluster-flies-fact-sheet
4. Pollenia (cluster flies): identification, facts and taxonomy. Source for the genus description as parasitoids or predators of earthworms and other invertebrates; approximately six species in North America with P. rudis and P. pediculata most common; species identification requiring microscopic examination of setae number, position and length; the distinguishing golden hairs on the thorax that wear off with age and absence of the bright white face of house flies; entry through small openings into wall voids, window frames and attics; higher numbers in rural areas with earthworm populations; the Palaearctic origin with P. rudis likely introduced from Europe with European earthworms and P. vagabunda and P. labialis introduced in the 1950s and 1960s; distribution across the United States and southern Canada; and up to four generations per year with peak activity March to June. https://bugswithmike.com/guide/arthropoda/hexapoda/insecta/diptera/oestroidea/polleniidae/pollenia
5. Pollenia rudis species account. Source for the statement that the fly overwinters until spring living off its own fat; the principal earthworm hosts including Aporrectodea caliginosa, A. chlorotica, Eisenia lucens, Lumbricus rubellus and Lumbricus terrestris; the immediate search for worms by newly hatched larvae; and the size range of about 6 to 10 millimetres with a dull grey body marked by black lines and dense golden-yellow hairs on the thorax. https://en.wikipedia.org/wiki/Pollenia_rudis
6. Koch, R.L. The multicolored Asian lady beetle, Harmonia axyridis: a review of its biology, uses in biological control, and non-target impacts. Journal of Insect Science, 3(1), 32. Source for the formation of mass aggregations in dark concealed locations; aggregations under south-facing rocks mid-way up mountains in mountainous areas and on buildings with southern or western exposure elsewhere; the statement citing Nalepa et al. (2000) that it seems unlikely volatile chemical cues are used for formation of overwintering aggregations; the acclimation in Japan by decreasing supercooling point and lower lethal temperature to approximately minus 19 and minus 16 degrees Celsius with the decrease correlated with increased myo-inositol content; and the passage of most of winter in diapause regulated by the corpus allatum, entered with an empty digestive tract and enlarged fat body. https://academic.oup.com/jinsectscience/article/3/1/32/854074
7. Multicolored Asian Lady Beetle. University of Maryland Extension. Source for the habit of seeking indoor sheltered overwintering sites making it a nuisance; the statement that adults apparently secrete an aggregation pheromone resulting in large numbers gathering at favourable sites; favourable sites including attics, basements and living areas; flight toward windows on warm sunny days in winter; the warning not to swat or crush because of the yellow-orange liquid that stains fabrics and walls; the minor pest status on ripening fruit; and the recommendation to caulk cracks, seal entry points around windows and install screening over vents in late summer and early fall. https://extension.umd.edu/resource/multicolored-asian-lady-beetle
8. Ladybugs everywhere: preventing and managing multicolored Asian lady beetles. Illinois Extension. Source for the statement that beetles making it indoors will eventually die; the search for overwintering sites during warm days around 64 degrees Fahrenheit after a significant drop to near-freezing temperatures; overwintering in cracks and crevices of rockfaces or cliffs in much of the native range with buildings as the next best thing where these are lacking; attraction to prominent isolated objects with high colour contrasts; the recommendation against pesticides indoors because beetles overwinter in inaccessible areas such as wall voids and because of unnecessary human exposure; and the ladybug taint in wine from beetles present in grape clusters at harvest. https://extension.illinois.edu/blogs/good-growing/2024-11-01-ladybugs-everywhere-preventing-and-managing-multicolored-asian-lady
9. Mapping Antennal Sensilla of Boxelder Bugs (Boisea trivittata) as the First Step in Understanding Overwintering Aggregation Behavior. Insects, 17(1), 6. Source for the statement that understanding overwintering behaviour is critical for preventing initial aggregation and secondary movement inside structures; that field observations suggest alarm pheromones and chemical cues mediate end-of-season aggregation; the scanning electron microscopy mapping identifying 20 subtypes across seven categories of sensilla including trichodea, basiconica, chaetica, campaniformia, coeloconica, ampullacea and bell-mouthed with distribution varying by antennal segment; and the statement that the morphological study was performed in preparation for further pheromone work. https://www.mdpi.com/2075-4450/17/1/6
10. Where Do Flies Go In Winter? Two Very Different Answers. Commercial pest control source. Used for the statement that cluster flies are not breeding in the house and physically cannot because there are no earthworms in a wall; the movement of the last generation to sun-warmed walls in late August and September and into wall voids, attics and upper storeys to overwinter as dormant adults; the warning that believing the flies are breeding leads to fogging a void which cannot reach dormant flies and leaves a mass of dead insects that feeds carpet beetles; and the recommendation to seal before early September. https://www.anchorpestservices.com/winter-pests-shift/flies
11. Cluster Flies, Pollenia rudis. Commercial pest control source. Used for the description of the life cycle with eggs laid in soil during warm months, larvae parasitising earthworms for a couple of weeks before pupating in soil; the seeking of warm places to overwinter in autumn with entry through small cracks and gaps and hibernation in attics and wall voids; the return to activity in spring seeking exits; the statement that cluster flies do not feed on garbage, human food or decaying matter; the recommendation to inspect attic vents and chimneys; and the note that although mostly a rural problem they can affect urban homes given the opportunity to enter. https://qualitypestsolutionsllc.com/cluster-flies-pollenia-rudis-2/
12. Multicolored Asian Lady Beetle Inside Houses. NC State Extension Publications. Source for the citation of Nalepa, C. et al., Orientation of Asian Lady Beetles to Buildings, American Entomologist, September 2004, and Nalepa, C. et al., Role of Visual Contrast in Alighting Behavior of Harmonia axyridis at Overwintering Sites, Environmental Entomology, 34(2), 425–431, 2005; and for the observation that average parasitism is closer to 2 to 4 per cent of beetles collected. https://content.ces.ncsu.edu/multicolored-asian-lady-beetle-inside-houses
13. What do Boxelder Bugs and Multi-Colored Asian Lady Beetles Have in Common? Commercial pest control source. Used for the observation that both species migrate to attics, basements, wall voids and garages when temperatures cool in fall, that they are usually attracted to the light reflecting off the sides and windows of buildings, and that they find cracks and crevices through which to enter overwintering sites. https://adamspestcontrol.com/blog/what-do-boxelder-bugs-and-multi-colored-asian-lady-beetles-have-in-common
14. Multicolored Asian Lady Beetle. Northeastern IPM Center. Source for the statement that they do not carry diseases or harm building materials; that large numbers inside a structure can cause allergy problems in sensitive individuals and that these in particular are known to bite; that exclusion is the safest and most effective way to reduce or prevent invasion through caulking around window frames, outdoor faucets and vents, filling holes with mesh and caulk, adding rubber or brush sweeps to doors including roll-down doors and keeping window screens in repair; the identification characters including size up to three eighths of an inch, oval shape, variable colour and spotting and the W or M on the pronotum; and the inspection guidance to look on the sunny side of buildings and trees and for gaps around eaves, windows and foundations, often on the south side. https://www.northeastipm.org/schools/pests/multicolored-asian-lady-beetle/
15. Asian Lady Beetle: identify, prevent, and control fall invaders. Commercial pest control source. Used for the recommendation that the best time to seal cracks and close gaps is late August through mid-September before the fall migration begins; the guidance to inspect every side of the building paying closest attention to south and west-facing walls; the observation that sealing these entry points reduces access for stink bugs, boxelder bugs and other fall invaders using the same routes; and the reference to integrated pest management guidance recommending exclusion as the first line of defence. https://callwaynes.com/blog/asian-lady-beetles-the-unwanted-lady-bugs-invading-this-fall/
16. Are You Ready For Fall Invaders? New York State Integrated Pest Management, Cornell University. Source for the observation that while it seems reasonable that sealing cracks and crevices will keep overwintering pests out, no formal research has tested this hypothesis, and for the note that a grant from the Pest Management Foundation was awarded to evaluate the effectiveness of exclusion as a technique to keep stink bugs and other overwintering pests out of homes. https://blogs.cornell.edu/nysipm/2015/09/10/are-you-ready-for-fall-invaders