BIOLOGICAL CONTROL AGENTS FOR APHIDS

Aphidius species (adult female pictured right, attacking an aphid)

Aphidius wasps are similar in size to the aphids that they attack. The adults feed on aphid honeydew and/or floral nectar, and the females lay their eggs singly inside individual aphids. On hatching, the wasp larva gradually consumes the aphid's internal tissues, eventually killing it. At that point, the wasp larva spins a silken cocoon inside the dead body of the aphid (forming a golden brown 'mummy'), inside which the wasp pupates. After several days, the newly formed adult wasp cuts a neat circular hole in the back of the dead aphid, through which it emerges to mate and begin the life-cycle again. Different Aphidius species differ somewhat in the aphid species that they will attack, but they are all quite strong fliers, and will disperse readily from their release point.

Please note that the immature stages of both A. ervi and A. colemani can be attacked by other naturally occurring parasitic wasps while developing inside host aphids; even in greenhouses, these 'secondary' or 'hyperparasitoids' can gradually increase over the growing season and may eventually reduce the effectiveness of Aphidius. If this occurs, it may be necessary to introduce another aphid biological control agent (such as green lacewings (Chrysoperla rufilabris), or the aphid midge (Aphidoletes aphidimyza).

Aphidius ervi

Aphidius ervi tends to prefer larger aphid species (Tribe: Macrosiphini), including the potato aphid (Macrosiphum euphorbiae), foxglove aphid (Aulacorthum solani) and cannabis aphid (Phorodon cannabis). However, its host range also includes some smaller aphid species, and adults that emerge from these hosts will be somewhat smaller than those emerging from larger hosts. The largest adults are typically about 2.5–3 mm long, with dark bodies and legs. The life-cycle (egg to adult) takes approximately 12 days at 25 °C (77 °F), with males generally emerging slightly earlier than females.

The optimal temperature for this species is around 25 °C/77 °F; it can still be effective at 20 °C/68 °F), but at lower temperatures (15 °C/59 °F), both survival and parasitism levels are reduced. Survival is also reduced at temperatures of (30 °C/86 °F) or higher. 

Aphidius ervi is available in vials containing 250, 500, or 1000 mummies (parasitized aphids in which the adult wasps are at, or close to, the point of emergence).

Aphidius colemani

Adults of Aphidius colemani iare very similar in appearance those of Aphidius ervi. However, A. colemani attacks a wider range of aphid species than does A. ervi, including the cotton-melon aphid (Aphis gossypii), and green peach aphid (Myzus persicae). Most of the aphid species attacked by A. colemani are smaller than those preferred by A. ervi (mainly in the Tribe Aphidini), and the adults of A. colemani are usually slightly smaller than adults of A. ervi (2 mm or less in length). 

The life-cycle of A. colemani is similar to that described for A. ervi. The optimal temperature for this species is also around 25 °C/77 °F, and survival and parasitism levels are reduced at temperatures at or below 15 °C/59 °F, or above 30 °C/86 °F. At 25 °C (77 °F), the life cycle (egg to adult) is completed in approximately 9-10 days. 

Like other Aphidius species, A. colemani is a strong flier, and is very good at finding aphid colonies: in one study, the adults dispersed more than 50 ft (16 meters) from the release point within 24 hours. 

Aphidius colemani is available in vials containing 500, 1000, or 5000 mummies (parasitized aphids in which the adult wasps are at, or close to, the point of emergence). 

Aphelinus abdominalis

Aphelinus abdominalis is slightly smaller than the Aphidius wasps (approximately 1–1.5 mm long), with a dark head and thorax and a yellow abdomen. This species has quite a broad host range, but in glasshouses is primarily used for potato aphid (Macrosiphum euphorbiae) and foxglove aphid (Aulacorthum solani); it will also attack the green peach aphid (Myzus persicae). 

As in Aphidius species, the females insert a single egg inside the host aphid, and the developing wasp larva feeds on the aphid's internal tissues until the host dies and the wasp pupates. In this case, however, the dead, 'mummified' aphids turn black rather than golden brown. When pupation is complete, the newly formed adult wasp emerges by chewing an exit hole in the back of the dead aphid. Mummies may be found at some distance from active aphid colonies, as aphids parasitized by A. abdominalis have a tendency to disperse before the wasp within pupates.

Aphelinus differs from the Aphidius species in that the adult females will not only kill aphids by parasitizing them,  but also through the process of 'host-feeding'; this involves puncturing the host with their ovipositor (egg-laying structure) and then feeding on the fluids leaking from the wound. With larger aphid species (such as the potato aphid and foxglove aphid), the youngest aphids (first and second instars) are usually used for host-feeding, while the second and third instars are the main stages that are actually parasitized.

Aphelinus abdominalis has a longer adult lifespan than the Aphidius wasps (15–27 days, depending on temperature), during which an adult female may parasitize approximately 200 aphids and kill an additional 30–50 through host-feeding. This species performs best at temperatures of 20 °C (68 °F) or higher. At 20 °C, it takes approximately 7 days for the first mummies to form, and an additional 14 days before the new adults emerge.

Aphelinus species are not considered strong fliers and are unlikely to disperse as far from their release points as the Aphidius species. They also tend to lay fewer eggs per day than do the Aphidius species. However, because they usually live longer, they are a useful complement to Aphidius.

Aphelinus abdominalis is available in a vial containing 250 adults, or a bottle containing 1000 adults.

Chrysoperla rufilabris (Green lacewing)  

Adults of the lacewing Chrysoperla rufilabris are bright green with delicate, transparent wings with many veins. The adults are approximately 2 cm in length, with a wingspan of 2–3 cm. Chrysoperla adults feed on aphid honeydew, nectar and pollen, and lay their eggs on leaves or stems close to aphid colonies. 

Each egg is supported on a thin 'stalk' (approx. 4–5 mm long), and is initially bright green, gradually becoming darker as the larva develops inside. All three larval stages are predaceous and will feed on a variety of soft-bodied arthropods, including aphids, mites, immature whiteflies, some insect eggs and the smaller larvae of several moth and beetle species. 

Approximately 70–80% of the total lifetime prey consumption occurs when the insect is in its third (final) larval stage, and each larva may consume approximately 150 final instar aphids during its development. Lacewing larvae have pronounced curving mandibles which are used to impale their prey and suck out their internal fluids. Mature larvae are about 6–7 mm long and pupation takes place within a white silken cocoon in sheltered sites on the plant, under pot rims, etc.

The optimal temperature for this species is around 25 °C (77 °F), with a relative humidity of 75% or higher. Survival and reproduction are reduced at lower relative humidities (55% or less), and by temperatures below 20 °C (68°F). At 25 °C and 75% relative humidity, development time (egg to adult) take approximately 20–25 days. Under such conditions, adults may live for 4 weeks or more and lay approximately 250 eggs.

Chrysoperla rufilabris is available as eggs, larvae or adults. Keep in mind that, while the larvae are good for addressing aphid 'hot spots', their lack of wings limits their dispersal ability and hence they must be released close to prey patches. Adults are better able to locate prey patches in which to lay their eggs, but prey populations will not be impacted until the eggs hatch and the larvae start to feed.

Chrysoperla rufilabris is available in units of 1000 or 5000 larvae, or as eggs on hanging cards (5000 eggs in total). 

Aphidoletes aphidimyza (Aphid midge)

Aphidoletes aphidimyza is a small, delicate midge (fly), approximately 2.5 mm long, with relatively long legs and antennae. The adults feed on aphid honeydew and are mainly nocturnal, becoming active at dusk; during the day they tend to rest amongst the leaves. Despite their delicate appearance, adults can disperse nearly150 ft (45 m) from their release site.

Adult females lay their minute orange eggs on the leaf surface amongst aphid colonies, and all of the subsequent larval stages prey on aphids. The larvae feed by injecting a paralyzing toxin into the leg of the target aphid, and then use their hook-like mouthparts to suck out the aphid's body fluids, eventually leaving little more than a husk (as pictured here). 

When fully mature, the larvae drop to the soil surface, where they form a cocoon within the top 1–2 cm (0.5"), in which they pupate. This aspect of their behavior may affect population establishment in areas where much of the floor area is covered with impermeable materials such as concrete or weed-suppressing fabric.
 

Worldwide, Aphidoletes aphidimyza has been recorded attacking more than 70 different species of aphids (including most of those commonly found in glasshouse crops), with the cotton-melon aphid (Aphis gossypii) being a highly preferred species. Larvae will also attack nymphs of the cannabis aphid (Phorodon cannabis). 

Adult emergence from pupal cocoons typically begins at or around dusk (i.e., when the light intensity gradually declines), and is largely complete by midnight. Most adults will mate during their first night, when they seek spider webs or similar fine supporting structures to which they can cling. Egg-laying also takes place mainly at night. 

Optimal conditions for this species are temperatures of 20–25 °C (68–77 °F), with a relative humidity of 70% or higher (the pupae in particular are very sensitive to low humidity). Note that, in autumn, larvae exposed to short days and reduced temperatures may enter diapause (a type of resting state), resulting in delayed or reduced emergence from the pupal stage. 

The critical day-length at which this occurs depends on temperature and geographical location, but typically varies from 14–16 hours when night-time temperatures are about 20 °C (68 °F). However, raising the night-time temperature or adding even quite low levels of supplemental light can prevent or reduce the onset of diapause. If using supplemental lighting for this purpose, it is best to increase the day-length by adding extra light in the morning rather than the evening, so as to preserve the natural 'dusk' that regulates adult activity.

Aphidoletes aphidimyza is available as pupae in units of 250, 1000, 2000, or 10,000.

Orius insidiosus (Pirate bug)

This species is a generalist predator with a preference for thrips, but will also feed on aphids and other small insects. More details are shown here.