Cardiochiles Wasp (tobacco budworm parasitoid)
Blake Layton (Mississippi State University) and Scott Stewart (University of Tennessee), Department of Entomology and Plant Pathology
Most of these small parasitic insects do not have common names, and we are forced to use scientific names. However, this little wasp is commonly referred to as “the red-tailed wasp” by scouts. In general, boll weevil eradication and Bt cotton have had a positive effect on the populations of most beneficial insects, but this is not true for Cardiochiles nigriceps. The primary host of this parasitic wasp is the tobacco budworm. Because most of the state’s cotton acreage is now planted to Bt varieties, and because tobacco budworm does not survive on Bt cotton, this is one beneficial insect that we see less of than in the past.
Although most species of parasitic wasps are small and inconspicuous, this is not true of Cardiochiles. The adult wasp is larger than most other parasitic wasps, approximately 1/4 inch in length, and has black wings and a red abdomen. The wasps are commonly seen flying at terminal height along the rows searching for larvae. These wasps are frequently observed in non-Bt cotton fields during mid and late summer when their hosts are more common. If you take the time to observe them, you will be impressed by their searching ability. If you see the wasp change course, land and begin searching a terminal, the odds are good that it has, or at least had, a larva present.
Cardiochiles is an important biological control agent of tobacco budworm. The primary hosts of this parasitoid are the tobacco budworm (Heliothis virescens) and a related species (H. subflexa) that is found mostly on ground cherry. If eggs are laid in bollworm larvae, they will not hatch because bollworm has a defense mechanism that causes the eggs to become encapsulated. Extremely high populations of these wasps are often observed when tobacco budworms are common.
Adult wasps will “sting” or lay eggs in tobacco budworm larvae of all sizes, but they prefer newly hatched larvae. Older larvae are better able to “fight off” the wasp and keep her from successfully laying eggs. Try tickling a large budworm on the back with a blade of grass and see how it reacts. The wasp larva hatches inside the caterpillar and begins feeding, but the wasp larva grows slowly until the budworm larva enters the ground to pupate. The wasp larva then begins to develop more rapidly, consuming much of its host in the process. Parasitized caterpillar larvae grow more slowly and cause less damage than normal. Cardiochiles wasps overwinter as pupae in a pupal cell inside their host. Overall development rate closely matches that of tobacco budworm, so that generations of wasp and host coincide.