In May, a release of aphid predators onto an infested plant usually goes well. In August, the same product from the same supplier at the same rate onto the same plant often does not, and the usual conclusion is that the batch was bad.
Almost always it was not. Everything in the equation moved: the aphids, the plant they are on, the predator you released, and — for two of the products — the length of the day.
The short version
- Why spring is easy
- Soft new growth has the amino acids aphids need. That is measured, not folklore
- Why midsummer is not
- Plant quality falls, natural enemies build up, and heat cuts aphid output directly
- The half nobody mentions
- The beneficials degrade in heat too, and at lower temperatures than the aphids
- Most heat-tolerant thing we sell
- Green lacewing. Its modeled optimum sits around 30–32 °C
- Least
- Aphidius matricariae, optimum about 25 °C — which is also what it is good for
- The autumn trap
- Aphidoletes reads daylength and shuts down under roughly 13 to 14 hours
- The autumn fix
- A dim night light. Not red, and not if the room is cold
- Indoors
- There is no season. The autumn switch does not fire above about 23 °C
Why spring is the easy season
Aphids do better on young tissue, and the reason is more specific than “it is softer.”
Aphids drink phloem sap, which is abundant in sugar and short of the amino acids they need to build bodies. How short depends on the plant’s stage. In a study on potato, aphids “performed better on developmentally young … plants than on mature … plants,” and the researchers traced it to “a developmental shift from high glutamine levels in pre-tuber-filling plants to low glutamine levels in tuber-filling plants.”
Then they did the step that turns a correlation into a mechanism: they made artificial diets matching the amino acid profiles of young and old plants, and the aphids performed differently on the diets alone. It was not the tenderness of the leaf. It was the composition of the sap.
Which is also why the fertiliser matters
UC IPM’s summary is short: “High levels of nitrogen fertilizer favor aphid reproduction.” A plant pushed hard in spring is producing exactly the tissue aphids do best on.
It is not a straight line, though. In a controlled trial on wheat, aphid fecundity rose with each level of nitrogen — and then fell significantly at the highest rate applied. More nitrogen is not indefinitely more aphids. Enough of it is.
So the spring flush is real, and a garden or a plant room coming into growth is producing aphid food. This is also the season when a release is doing the most good per dollar, because the colonies are still small and have not yet got crowded enough to start producing winged aphids.
What actually happens in August
Aphid populations commonly crash in midsummer, and the tempting explanation is that the heat kills them. That is part of it and it is not the main part, and the literature is specifically less tidy than the story.
A study of the mid-season crash attributes it to “a decline in plant nutritional quality and increased natural enemy pressure,” working through three separate mechanisms: “enhanced emigration, especially by alate (winged) aphids, depressed performance resulting in reduced birth rates, and elevated mortality caused by natural enemies.” Three things at once, only one of which is the aphids dying.
Heat does have a direct effect. In potato aphid held at 30 °C days and 20 °C nights, total offspring were “on average 43% lower than at current temperatures.” That is a real cost at an ordinary summer temperature rather than an extreme one.
And here is the part that spoils the neat version
A study of high and fluctuating temperatures on green peach aphid found its optimum for population growth at 26.7 °C, with developmental thresholds at 6.5 and 37.3 °C — and concluded that the species would benefit from a warmer Midwest summer, developing faster under fluctuating conditions than constant ones.
Melon aphid is tougher still: in one trial 57% were alive after a week at 35 °C. If something is still going strong on your plants in August, melon aphid is a good guess.
So “heat kills aphids” is not what the evidence says. Some aphids, at some temperatures, in some places.
The half that is about your predator, not your pest
This is the part that actually explains an August release going nowhere, and it is missing from most seasonal aphid advice. The organisms you buy have thermal limits too, and several of them are lower than the aphid’s.
| What it is | Published optimum | What happens above it |
|---|---|---|
| Green peach aphid | 26.7 °C for population growth | Developmental thresholds at 6.5 and 37.3 °C. It survived an hour a day above its calculated lethal point of 38.5 °C |
| Aphidius colemani | About 30 °C for development | “High mortality occurred at higher temperatures” |
| Aphidius matricariae | About 25 °C for development | Same study, same finding — and five degrees lower, which is why it is the wasp for a cool room |
| Aphidius ervi | Parasitism peaked at 25 °C in one trial | A separate study found one hour at 28 °C reduced the parents’ lifespan and impaired development and mummification in the next generation |
|
Green lacewing Chrysoperla carnea |
Modelled stage optima clustering near 30–32 °C | Modelled upper thresholds near 32–34 °C. The most heat-tolerant of the group here |
| Convergent lady beetle | 29 °C for development | Adults enter reproductive diapause when summer temperatures are too high — a bought beetle in a heatwave may not be laying at all |
Three things follow from that table, and they are all actionable.
- The lady beetle problem in August is not death, it is diapause. University of Florida: “During the summer and winter when temperatures are too high or low, and during periods when food sources are insufficient for reproduction, adult convergent lady beetles enter into reproductive diapause.” Since the eggs are the durable half of a ladybug release, a beetle that is not laying is worth much less than the same beetle in May.
- The Aphidius heat cost carries into the next generation. A brief exposure to 28 °C — an hour — was enough to impair the offspring. A wasp release into a hot greenhouse is not just working slower; the generation you were counting on to emerge from the mummies is compromised before it starts.
- Green lacewing is the one to reach for when it is genuinely hot. Its modeled optima sit around 30–32 °C, above every parasitoid here. That is a real and underused reason to switch products rather than to escalate the rate.
And late in the season, something eats the wasps
Aphidius mummies are themselves attacked by hyperparasitoids — wasps that develop inside the developing wasp. In a field study of wheat aphids, primary parasitoids made up 86.7% of the parasitoid community early in the season and 3.5% late in it; hyperparasitoids replaced them.
That is a field-crop result, and it should not be read straight across to a plant room. What it supports is the direction: a parasitoid program that has been running all season is working against something that has had all season to find it, and a late-season release into an established mummy population is not the same proposition as the first release in spring.
Autumn, and the two products that read the calendar
Two of the things we sell respond to daylength directly, which means an autumn release can fail for a reason that has nothing to do with temperature, aphid numbers or handling.
Aphidoletes shuts down under short days
The predatory midge Aphidoletes aphidimyza enters diapause as a larva when the days get short. The published thresholds are specific: one strain’s critical day length was “12.7 h at 20°C,” and a separate study found that “temperatures above 20 °C and a photoperiod greater than 14 h prevented diapause.” Call it roughly thirteen to fourteen hours, strain-dependent.
Below that, the larvae drop into the medium and stop, and what you paid for sits in the pot waiting for spring.
The fix is supplemental light at night, and a 1986 study established how little it takes — and two ways it fails:
The two things usually left out of the lighting advice
The intensity required is tiny. Diapause was prevented at radiation levels under about 5.5 lux. That is night-light territory, not grow-light territory, so this fix is cheap.
Red light did not work. In the study’s words, “larvae did not appear to respond to red light.” A red night light is not a night light for this purpose.
Cold beats the light. At a 18 °C / 10 °C thermoperiod the low light was “nullified,” and 89% went into diapause anyway. If the room is cold, adding a lamp does not rescue the midge.
There is a second constraint that pulls the other way and is easy to get wrong: Aphidoletes adults fly and lay at night. A room left on twenty-four-hour light gives them nowhere to work. What the midge needs is a long day and a dim night, not a permanent one.
Ladybugs are already in diapause when you buy them
Commercial convergent lady beetles are collected from overwintering aggregations, which means they arrive reproductively immature by design rather than by season. That is also why they arrive looking dead. The seasonal part is that summer heat and winter cold both push adults back into reproductive diapause — so the months when a beetle is most likely to lay are the shoulders, not the peak.
Aphidius, which mostly does not care
A useful contrast, because it stops the diapause point being generalized into a rule about all beneficials. In a study across nine photoperiod and temperature combinations, “historically winter-active species (A. rhopalosiphi and A. matricariae) never entered diapause,” and two others did so only at low rates — a maximum of 13.4% and 11.2%.
So a short-day failure is an Aphidoletes failure mode. If an autumn Aphidius release disappoints, look at temperature, at hyperparasitoids, or at the aphid species — not at the calendar.
Indoors, where there is no season
The autumn switch that ends an outdoor aphid population is triggered by shortening days and falling temperature together, and warmth alone cancels it. In one well-studied species, sexual females are produced below 14.5 hours of light at 15 °C — and “this response disappears above 23°C.”
A heated room sits above 23 °C most of the winter. So the population never gets the signal, never switches to eggs, and simply carries on. France’s national agricultural research institute notes that aphids “can also overwinter in the parthenogenetic form in sheltered settings like greenhouses.” Nothing is going to end it for you.
The same constancy works in your favor on the beneficial side. Under an eighteen-hour photoperiod at 22–26 °C, Aphidoletes diapause is prevented outright — above 20 °C and over fourteen hours is enough, and at 30 °C diapause was prevented even under an eleven-hour day. An indoor grow removes the seasonal failure mode entirely.
Which changes the strategy, not just the timing
Outdoors, a release is a push against a population that has a bad season coming. Indoors, there is no bad season coming, so the question changes from can I knock this down to what is going to keep it down.
That is an argument for something that reproduces on your plants rather than something you keep buying — and only one of the four aphid products does that.
What to do differently, by season
| When | What is going on | What that argues for |
|---|---|---|
| Early spring | Soft new growth, small colonies, no winged aphids yet, mild temperatures | The cheapest time to act and the best return per release. Anything works; a parasitoid gets a head start it cannot get later |
| Late spring into early summer | Populations building fast, still within everything’s comfortable range | Full rates, repeat releases at weekly intervals. Deal with ants now |
| Midsummer heat | Plant quality falling, aphids under stress but not gone, parasitoids past their optimum, lady beetles liable to be in reproductive diapause | Green lacewing, which tolerates the heat best of the group. Release at dusk. Expect less from a wasp than the same wasp gave in May |
| Autumn, outdoors | Shortening days, winged aphids on the move, hyperparasitoids established | Aphidoletes needs supplemental light or it will not work. Outdoors, this is often the season to let the weather finish it |
| Any month, indoors | No seasonal signal at all. Aphids reproduce indefinitely; beneficials do not diapause under long days and warmth | Treat it as a standing problem rather than an episode. Prevention and something that reproduces beat repeated knock-downs |
Timing a release
Read the room temperature before choosing the product. Mid-sixties Fahrenheit favors Aphidius matricariae. High seventies favors colemani. Genuinely hot favors green lacewing, whose modeled optimum is the highest of the group.
Release at dusk in summer, and mean it. It is not only about dispersal. The hour after release is the hottest exposure the shipment gets, and for Aphidius a single hour at 28 °C has been shown to cost the next generation.
If you are running Aphidoletes past September, sort the lighting first. Under roughly thirteen to fourteen hours they go into diapause. A dim night light fixes it — under about 5.5 lux is enough — but not a red one, and not if the room is cold.
Do not read a poor August result as a bad batch. Plant quality, aphid heat stress, parasitoid heat stress, lady beetle diapause and hyperparasitoids are all pointing the same way in August. The product being wrong for the month is more likely than the product being wrong.
Indoors, stop thinking in seasons. The autumn switch does not fire above about 23 °C, so nothing is going to end the population for you. Plan for a standing program rather than a campaign.
Common questions
Why did the same product work in spring and not in August?
Because more than the aphids changed. Plant quality falls, natural enemies are already present, heat cuts aphid reproduction — and the beneficial you released is past its own optimum. Aphidius optima sit at about 25–30 °C with high mortality above, one hour at 28 °C has been shown to impair the next generation of A. ervi, and convergent lady beetle adults enter reproductive diapause when summer temperatures are too high.
Do aphids die off in the heat?
Some, at some temperatures. Potato aphid produced 43% fewer offspring at 30 °C days. But green peach aphid's optimum is 26.7 °C and one study concluded it would actually benefit from warmer summers, and melon aphid was 57% alive after a week at 35 °C. “Heat kills aphids” is too broad to be useful.
Which beneficial handles heat best?
Of the ones commonly sold for aphids, green lacewing. Its modeled stage optima cluster around 30–32 °C, with upper thresholds near 32–34 °C — above every parasitoid in this article. That is a good reason to change product in a hot month rather than increase the rate.
Why did my autumn Aphidoletes release do nothing?
Most likely diapause. The midge reads daylength, and under roughly thirteen to fourteen hours the larvae drop into the medium and stop. A dim night light prevents it — under about 5.5 lux is enough — but red light does not work, and if the room is cold the light does not help: at 18 °C days and 10 °C nights, 89% went into diapause anyway.
Will my indoor aphids go away in winter?
No. The switch to overwintering eggs needs short days and cold together, and warmth alone cancels it — in one well-studied species the response “disappears above 23°C.” A heated room stays above that, so an indoor population continues indefinitely.
Is there a best month to start a biological program?
Early spring, outdoors, by a clear margin: colonies are small, nothing has produced winged aphids yet, and every organism you might release is inside its comfortable range. Indoors the answer is whenever you notice, because there is no seasonal window to hit.
Do hyperparasitoids really matter to me?
In a home collection, probably less than in a commercial greenhouse. The documented pattern — primary parasitoids early, hyperparasitoids late — comes from field wheat, and a glasshouse study found species turnover rather than a clean seasonal rise. It is worth knowing as one reason a season-long wasp program loses momentum, not as something to plan around at home.
References
- Karley, A.J., Douglas, A.E. & Parker, W.E. (2002). Amino acid composition and nutritional quality of potato leaf phloem sap for aphids. Journal of Experimental Biology 205(19): 3009–3018. The spring-flush mechanism: better performance on young plants, traced to a glutamine shift, and confirmed on chemically defined diets. doi.org
- Karley, A.J., Parker, W.E., Pitchford, J.W. & Douglas, A.E. (2004). The mid-season crash in aphid populations: why and how does it occur? Ecological Entomology 29(4): 383–388. Declining plant quality plus natural enemies, working through emigration, reduced birth rates and mortality. doi.org
- Beetge, L. & Krüger, K. (2019). Drought and heat waves associated with climate change affect performance of the potato aphid Macrosiphum euphorbiae. Scientific Reports 9: 3838. The 43% reduction in total offspring at 30 °C days and 20 °C nights. Open access. doi.org
- Davis, J.A., Radcliffe, E.B. & Ragsdale, D.W. (2006). Effects of high and fluctuating temperatures on Myzus persicae. Environmental Entomology 35(6): 1461–1468. The 26.7 °C optimum, the 6.5 and 37.3 °C developmental thresholds, and the conclusion that the species would benefit from a warmer summer — which is why this article does not claim that heat solves aphids. doi.org
- Zamani, A.A., Talebi, A., Fathipour, Y. & Baniameri, V. (2007). Effect of temperature on life history of Aphidius colemani and Aphidius matricariae. Environmental Entomology 36(2): 263–271. The two optima — about 30 °C and 25 °C — and high mortality above them. doi.org
- Ismaeil, I. et al. (2013). Trans-generational effects of mild heat stress on the life history traits of an aphid parasitoid. PLoS ONE 8(2): e54306. One hour at 28 °C, and the cost carried into the following generation of Aphidius ervi. Open access. doi.org
- Ranjbar Aghdam, H. & Nemati, Z. (2020). Modeling of the effect of temperature on developmental rate of common green lacewing, Chrysoperla carnea. Egyptian Journal of Biological Pest Control 30: 145. The stage optima near 30–32 °C and upper thresholds near 32–34 °C that make green lacewing the heat pick. Open access. doi.org
- Obrycki, J.J. & Tauber, M.J. (1982). Thermal requirements for development of Hippodamia convergens. Annals of the Entomological Society of America 75(6): 678–683. 230 degree-days over a 12 °C threshold, and a 29 °C development optimum. doi.org
- Aristizábal, L.F. & Arthurs, S.P. Convergent Lady Beetle. University of Florida IFAS Extension EENY-592. The statement that adults enter reproductive diapause when summer or winter temperatures are unsuitable, which is the August lady beetle problem. Free to read. ifas.ufl.edu
- Yamane, M. et al. (2012). Effect of photoperiod and temperature on the induction of diapause in a Japanese strain of Aphidoletes aphidimyza. Applied Entomology and Zoology 47(1): 17–26. The 12.7-hour critical day length at 20 °C, and prevention at 30 °C even under an eleven-hour day. doi.org
- Zhai, Y. et al. (2024). Molecular correlates of diapause in Aphidoletes aphidimyza. Insects 15(5): 299. Diapause prevented above 20 °C and above a 14-hour photoperiod; highest diapause below 15 °C on a 10:14 cycle. Open access. doi.org
- Gilkeson, L.A. & Hill, S.B. (1986). Diapause prevention in Aphidoletes aphidimyza by low-intensity light. Environmental Entomology 15(5): 1067–1069. The three findings this article’s lighting advice rests on: under about 5.5 lux is enough, red light did not work, and at 18 °C / 10 °C the light was nullified and 89% entered diapause anyway. doi.org
- Tougeron, K., Le Lann, C., Brodeur, J. & van Baaren, J. (2017). Are aphid parasitoids from mild winter climates losing their winter diapause? Oecologia 183(3): 619–629. A. matricariae never entered diapause across nine photoperiod and temperature combinations; A. ervi did so at a maximum of 11.2%. doi.org
- Yang, F. et al. (2017). Species composition and seasonal dynamics of aphid parasitoids and hyperparasitoids in wheat fields in northern China. Scientific Reports 7: 13989. Primary parasitoids early and hyperparasitoids late — a field-crop result, cited here for direction rather than as a greenhouse expectation. Open access. doi.org
- Ogawa, K. & Miura, T. (2014). Aphid polyphenisms: trans-generational developmental regulation through viviparity. Frontiers in Physiology 5: 1. The sexual response disappearing above 23 °C, which is why an indoor population has no autumn. Open access. doi.org
- Gash, A.F.J. (2012). Wheat nitrogen fertilisation effects on the performance of the cereal aphid Metopolophium dirhodum. Agronomy 2(1): 1–13. Fecundity rising with each level of nitrogen and falling significantly at the highest rate. Open access. doi.org
The month decides which organism is inside its own comfortable range — and in a hot one, that is not the wasp.
