Identification

Trichopoda pennipes

Tachinid parasitoid

Appearance The adult fly is about the size of a house fly and may hover above or be found sitting on squash plants in search of prey. It has a bright orange abdomen, velvety black head and thorax, and a fringe of short black hairs on the hind legs. The wings of male flies have a dark spot. The tip of the abdomen of female flies is black (Fig 1). They can be seen throughout the production period of squash and pumpkins.

Pests Attacked Primarily squash bug and southern green stinkbug. Although Trichopoda pennipes is a parasitoid of several true bugs, there appear to be different biotypes across the country. Limited field studies suggest that different populations attack different species of true bugs. For example, in California, a population of T. pennipes has been reported attacking the bordered plant bug. However the same fly was never found attacking the squash bug.

In one study, Trichopoda pennipes was collected from fields of squash in New York and shipped to California and released. Two years later finds that this population of flies has permanently established itself near these California farms growing summer and winter squash. In the past, nymphs of squash bug in this area were never attacked by parasitoids. Now, one can find 50% or more of these nymphs with fly eggs (Fig 2).

Life Cycle T. pennipes overwinters as a larva in its host and emerges in late spring. The female usually lays one but also sometimes many small, white or gray, oval eggs on large nymphs or adult bugs (Fig 2). T. pennipes is highly attracted by an aggregation pheromone produced by male southern green stink bugs, which results in the males being parasitized at a consistently higher rate than females. Each female lays an average of 100 eggs. Most of the eggs are placed on the underside of the thorax or abdomen, but they can occur on almost any part of the bug. Many eggs may be laid on the same host, but only one larva will survive in each bug. The young larva that hatches from the egg bores directly into the host body. The maggot feeds on the body fluids of the host for about two weeks, during which time it increases to a size almost equal to that of the body cavity of its host. When it has completed its development, the third instar maggot emerges from the bug between the posterior abdominal segments. The bug dies after emergence of the fly, not from the feeding, but from the mechanical injury to its body. The maggot pupates about an inch down in the soil in a dark reddish-brown puparium formed from the last larval skin, and an adult fly emerges about two weeks later. There are three generations per year in our location. Adult flies feed on nectar, especially from plants such as wild carrot (Queen Anne's lace) and meadowsweet (Spiraea salicifolia).

Fig. 1 Adult T. pennipes female.

Photo by J. Clark

Fig. 2 T. pennipes eggs on squash bug

Photo by J. Brust

Relative Effectiveness The rate of parasitism can be as high as 90% on southern green stink bug and up to 95% on squash bug, but because the bugs continue to feed after parasitization, T. pennipes will not always prevent crop damage. However, if parasitized early in squash bug development the reproductive organs of the host bug begin to atrophy when the parasitoid reaches the second instar, so pest population increase will be reduced. The fly is most effective when it parasitizes nymphs because 50% die before becoming adults and 65% of the remaining population that become adults will die before laying eggs. The literature states that the life history of the parasitoid and squash bug are not well synchronized, and host finding by the fly is not efficient, making T. pennipes unable to maintain pest species at low densities. However, in my studies by mid to late season parasitoid numbers can be great enough (95% of adults and 3^rd instar nymphs are parasitized) that squash bug populations can be reduced by 75%. By using other management tactics to further reduce squash bug numbers it is possible to achieve a high level of control of this pest, but only later in the season at this point. My hope is to better understand the life history of the parasitoid to increase its effectiveness early in the season.

For more information, contact Dr. Gerald Brust

Last updated: 11/2/2006