HortFACT
Identification
The adult is a small, black wasp with orange legs and mouthparts. It has a body length of 3mm (excluding the antennae). Two pairs of wings are held folded back on top of the body and are clear, except for a characteristic yellow, triangular stigma on the leading edge of the forewings. The adult is a weak flyer and is normally seen walking over the upper parts of leguminous plants searching for its host, the pea aphid (Acyrthosiphon pisum Harris).
The female is distinguished by having a pointed abdomen and slightly shorter antennae than the male. The female uses its antennae to locate the host aphid. It then bends its abdomen forward between its legs and extends the ovipositor. A single egg is inserted into the body of the aphid.
The egg is very small and translucent. Each female may produce 300-400 eggs.
The larva develops through four stages (instars) inside the host. The first three instars feed by ingesting liquid food. They cause no injury to the vital organs of the aphid, which shows no external indication of being parasitised. However, the fourth instar larva has mandibles which it uses to completely consume the inside of the aphid. The dying aphid usually moves to the uppermost leaves of its host plant, making it easily visible. The fourth instar larva then cuts a small hole ventrally in the aphid's cuticle and attaches the aphid to the plant or other substrate using secretions from its silk glands. The larva spins a cocoon inside the aphid's cuticle, which takes on a distinctive pale brown, swollen, mummified appearance.
The pupa which forms the inside of the cocoon is initially whitish, gradually yellowing and then darkening to the typical adult coloration. The adult parasite cuts a circular hole dorsally at the postenor end of the aphid's cuticle and emerges ready to mate and oviposit almost immediately. The adult will attack pea aphids of any size but has most success ovipositing into nymphs in the first to third instar.
Host insect
Aphidius eadyi is a specific parasite, which attacks only the pea aphid successfully. It will also attack the blue green aphid (Acyrthosiphom kondoi Shinji), but does not develop successfully. Pea aphid is a pest of many legumes, sucking sap from host plants and can transmit plant virus diseases. It is a serious pest of lucerne and can severely damage peas and broad beans.
Beneficial effects
Aphids that are parasitised keep on feeding for almost a week but are eventually killed by the final instar larva. Generally no offspring are produced by parasitised adult aphids. Pea aphid populations can build up to very high levels in summer, but A. eadyi has been shown to reduce these populations.
Distribution
This beneficial insect was introduced into New Zealand in November 1977 at Auckland and has since dispersed over most of the North Island. Dispersal is continuing.
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Days |
At 20oC the egg hatches 3 days after oviposition. The four larval stages develop over 7 days, after which the cocoon is formed and the aphid is "mummified". The pupal stage lasts 2 days before the adult forms, still inside the aphid cuticle. After cutting its way out, the adult may survive and oviposit for 11 days.
The pea aphid parasite has a number of overlapping generations each year. Development from egg to adult takes about 2 weeks during summer, but during winter proceeds at a very much slower rate. The parasite is common from November to April and rare from June to August.
G.P. WALKER