Aphidius ervi is a tiny parasitic wasp, roughly 4 to 5 mm long, used widely in greenhouses and field crops to control large aphid species like the pea aphid, potato aphid, and foxglove aphid. It kills aphids from the inside out: a female wasp injects a single egg into a living aphid, and the developing larva consumes the aphid’s internal tissues over the next week or so, eventually turning the host into a hardened, puffy shell called a “mummy.” These mummies are the most visible sign that the wasp is working.
How Aphidius ervi Kills Aphids
A female wasp curls her abdomen forward beneath her body and stings an aphid in a fraction of a second, depositing one egg inside the host. She can parasitize about 100 aphid nymphs over her lifetime, targeting nymphs of any size. After hatching, the larva passes through three growth stages while feeding on the aphid’s organs. The aphid continues moving and feeding for several days, apparently unaware of what’s happening inside it.
About 7 to 10 days after the egg was laid, the mature larva kills the aphid. The aphid’s skin hardens and swells into a smooth, rounded mummy that turns brown, gold, or tan. Inside, the larva pupates and transforms into an adult wasp, which chews a small circular hole in the back of the mummy to emerge. The entire cycle from egg to reproductive adult takes about two weeks at summer temperatures, which means populations can build quickly when conditions are right.
Adult Wasp Behavior and Lifespan
Adult females live about 10 days on average. They emerge with a ready supply of 60 to 80 mature eggs, so they can begin parasitizing aphids almost immediately. Their activity follows a distinct daily rhythm: parasitism rates are very low in the morning, increase through the afternoon and evening, and peak between roughly 8 p.m. and 8 a.m., with the highest burst of activity in the early morning hours as light returns. This pattern matters if you’re releasing them in a greenhouse, since introductions timed to late afternoon give the wasps a full active cycle before the next day’s heat.
Females locate aphid colonies partly through chemical cues. Plants damaged by aphid feeding release volatile compounds that attract the wasps, and honeydew left by aphids provides additional signals. Once a female finds a colony, she moves through it rapidly, antennating individual aphids to assess whether they’ve already been parasitized by another wasp before deciding to lay an egg.
Temperature and Performance
Aphidius ervi performs best between 15°C and 25°C (59°F to 77°F). Within that range, the proportion of aphids successfully parasitized increases with temperature, rising from about 11% at 15°C to nearly 16% at 25°C. Above 25°C, effectiveness begins to decline; at 30°C, parasitism drops back to around 15%. Sustained heat above 30°C slows development, shortens adult lifespan, and reduces the overall number of aphids each female can kill.
Cold temperatures don’t kill the wasps outright, but they slow everything down. Development time inside the mummy stretches out considerably below 15°C, and adults become sluggish. For greenhouse growers, this means Aphidius ervi is most reliable during spring, early summer, and fall. In winter-heated greenhouses that stay in the 20°C to 25°C range, the wasp can function year-round.
Target Aphid Species
Aphidius ervi specializes in larger aphid species. Its most common targets include the pea aphid, potato aphid, foxglove aphid, and green peach aphid in certain contexts. It is not effective against all aphids equally. The bird cherry-oat aphid, for example, is too small to support the wasp’s larval development, so growers dealing with small-bodied aphid species need a different parasitoid, such as Aphidius colemani or Aphidius matricariae.
This size requirement is important when setting up biological control programs. Identifying which aphid species you’re dealing with determines which wasp to order. Using the wrong parasitoid wastes money and time while the aphid population continues growing.
Using Banker Plants to Sustain Populations
Rather than making repeated wasp purchases, many greenhouse growers use “banker plant” systems to maintain a self-sustaining population of Aphidius ervi. The concept is straightforward: you grow a non-crop plant infested with a non-pest aphid species that serves as a continuous breeding host for the wasps. The wasps reproduce on the banker plant and then fly out into the crop to find pest aphids.
The choice of banker plant and aphid pairing matters. Because Aphidius ervi needs larger aphid hosts, standard cereal-based banker systems using bird cherry-oat aphids won’t work for this species. Growers targeting larger pest aphids with A. ervi typically need banker systems using appropriately sized alternative aphid hosts that won’t infest the cash crop. Getting this pairing wrong is a common reason banker plant systems underperform.
Hyperparasitoids: The Main Threat
The biggest biological limitation on Aphidius ervi isn’t temperature or aphid availability. It’s hyperparasitoids, tiny wasps that parasitize the parasitoid itself. At least six genera of hyperparasitoids attack Aphidius ervi mummies, including species from the families Figitidae, Pteromalidae, Megaspilidae, and Encyrtidae. These secondary parasites lay their eggs inside or on the developing Aphidius larva within the mummy, killing it before it can emerge.
Hyperparasitism rates can be strikingly high. In a Belgian study of greenhouse sweet pepper crops, 42% of Aphidius mummies in banker plants were hyperparasitized. Raspberry crops grown in open tunnels had a much lower rate of about 10%, likely because wider temperature swings and more escape routes for insects slowed hyperparasitoid population buildup. Hyperparasitoids appear early in the growing season and persist through the end, so they’re not just a late-season problem.
Enclosed, temperature-stable greenhouses are especially vulnerable because the warm, consistent conditions that favor Aphidius ervi also favor its enemies. Growers can reduce the impact by monitoring mummies closely. If you start seeing mummies with ragged or off-center exit holes (rather than the clean, round hole left by Aphidius), hyperparasitoids may be present, and replacing banker plants or making fresh wasp releases can help reset the balance.
Pesticide Compatibility
Aphidius ervi is highly sensitive to many conventional insecticides, which makes spray timing critical in programs that combine chemical and biological control. Among commonly used aphid products tested at half their recommended field rates, the organophosphate dimethoate was extremely toxic, killing wasps rapidly and eliminating their ability to reproduce. Pirimicarb and spinosad showed moderate toxicity. Imidacloprid was the least damaging to the adult wasps in laboratory trials.
In practice, any broad-spectrum insecticide applied while Aphidius ervi is active in a crop will set the biological control program back significantly. If chemical intervention is unavoidable, spot-treating aphid hotspots rather than blanket-spraying preserves wasp populations in untreated areas. Allowing adequate time between a spray application and a new wasp release, typically one to several weeks depending on the product’s residual activity, gives the parasitoids a better chance of surviving.
Identifying Parasitism in Your Crop
The easiest way to confirm Aphidius ervi is working is to look for mummies. Healthy, unparasitized aphids are soft-bodied and green, pink, or dark-colored depending on the species. A mummified aphid is distinctly different: swollen, rigid, and tan to golden-brown, often stuck to the underside of a leaf. Within a few days of mummy formation, a small round exit hole appears on the back where the adult wasp chewed its way out.
If you’re scouting a greenhouse or field, finding mummies with exit holes means wasps have already completed at least one generation and new adults are actively searching for more aphids to parasitize. Finding intact mummies without holes means wasps are still developing inside and will emerge soon. A rising ratio of mummies to live aphids over successive weeks is the clearest indicator that biological control is gaining the upper hand.