Parasitic Wasp of Aphids
Aphidius colemani is a slender, black insect with brown legs, long antennae and conspicuous wing venation. Its size depends on the size of the parasitized aphid, but is usually about 2 mm. The female has a pointed abdomen, while the male’s abdomen is round-shaped.
The female Aphidius deposits an egg in an aphid. She bends her abdomen under her legs and injects an egg in the aphid with her ovipositor. This takes only a fraction of a second. The wasp parasitizes adults and nymphs. During the egg stage of the wasp (the first 3 days after parasitation), the aphid even eats more than normal and secretes more honeydew. Parasitized aphid adults or 4th instars keep on producing progeny.
Then, the Aphidius larva starts eating the aphid from inside, starting with the non-vital parts. Seven days after parasitation it fixes the aphid onto the leaf, and forms a silken cocoon which makes the aphid swell. The outside of the aphid becomes golden-brown and leather-like, and is then called a mummy. Four days after the beginning of the mummification (at 21°C or 70°F) an adult Aphidius leaves the mummy through a round hole.
The total development of Aphidius colemani takes 14 days at 21°C (70°F), which is longer than aphid development in optimal circumstances (9 days). However, this is largely compensated by the hundreds of eggsAphidius lays. Most of these eggs are laid during the first 4 days. An adult Aphidius lives for 2-3 weeks.
The parasitic wasp finds aphid colonies from a long distance by “alarm signals” produced by an infected plant. At shorter distance it smells the honeydew. In the presence of an Aphidius in an aphid colony, aphids often secrete ‘alarm pheromones’. The other aphids start to panic, and often let themselves fall down, and usually die on the ground.
Male Aphidius emerge from unfertilized eggs. These are deposited soon after mating or at the end of the female’s life. The ratio females-males is usually about 2:1.
- Applicable in many crops
- Also controls well the cotton aphid and “red aphid”
- Preventative introduction possible
- Good searching ability
- High number of eggs per female
- Parasitization is easy to recognize (mummies)
- Population maintains also at low infestations