Competition between Gonatocerus ashmeadi and G. triguttatus for glassy winged sharpshooter (Homalodisca coagulata) egg masses

The introduction of a biological control agent can have significant effects on natural enemies that use the same host. Interspecific competition between natural enemies can impact the efficacy of control and, consequently, is the subject of increasing research scrutiny. Three experimental approaches were used to evaluate competitive outcomes between Gonatocerus ashmeadi and G. triguttatus parasitizing Homalodisca coagulata egg masses in the laboratory. (1) When both species were introduced to high densities of host eggs 1, 3 and 5 days of age, mean percentage offspring was significantly higher for G. ashmeadi offspring (23.2% greater than G. triguttatus). (2) When a female parasitoid of each species was offered a single egg mass, offspring production was statistically similar for the two species. Gonatocerus triguttatus showed aggressive behavior, although this only accounted for 0.8% of the female's total foraging time and did not lead to proportionately higher offspring production. (3) Regardless of order, more G. triguttatus offspring (up to 96%) emerged than G. ashmeadi offspring when one female was introduced sequentially to one egg mass. The relative success in offspring production was affected primarily by the sequence in which the parasitoids were introduced, and to a lesser extent by the interval between successive parasitoid introductions, and the age of the egg mass. These results illustrate the importance of experimental design in the assessment of competitive superiority between two species of parasitoids. Improper experimental design can lead to contradictory outcomes in laboratory-based competition studies due to the interplay of extrinsic and intrinsic competitive behavior. Biological control practitioners need to be aware of the complexity of competitive behavior when designing pre-introduction laboratory tests to determine a priori which natural enemy from several candidate species is likely to be the most effective agent at controlling the target.