Parasitic wasp

Aphidius colemani

Aphidius species are small, dark brown parasitic wasps that attack aphids_ The adult wasps are approximately 2 mm long and feed on aphid honeydew and floral nectar. The females lay their eggs singly inside individual aphids and when they hatch, the wasp larvae gradually consume
the host aphid’s internal tissues, eventually killing them. At that point, the wasp larva spins a silken cocoon inside the dead body of the aphid (forrming 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 seek a mate and begin the life-cycle again.

Different Aphidius species differ somewhat in the aphid species that they will attack: Aphidius colemani attacks a wider range of aphid species than does A.orvi, including the cotton-melon aphid (Aphis gossypii, and the green peach aphid (Myzus persicae). Most of the aphid species attacked by A. colemani are smaller than those preferred by A. ervi, and the adults of A colemani are usually slightly smaller than adults of A. ervi.

The optimal temperature for A colemani is approximately 25  °C (77 °F). Survival and  parasitism levels are reduced at temperatures of 15°C  (59 °F) or less, or above 30 °C (86 °F). At 25 °C (77 °F), the life cycle (egg to adult) is completed in approximately 9—10 days.  Aphidius wasps are quite strong fliers, and will disperse readily from their release point: in field studies, A. colemani was found to move at least 16 m (52 ft) within 24 hours of release.

Note that the immature stages of both A colemani and A. ervi 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 species. If this occurs, it may be necessary to introduce another aphid biological control agent (such as Chrysoperla rufillabris or Aphidoletes aphidimyza).