Intra- and interspecific competition in two congeneric parasitoids of the diamondback moth

Abstract

In solitary parasitoids, a host can only support the complete development of a single parasitoid. For this reason, intraspecific or interspecific competition takes place whenever two or more individuals of the same or different species parasitize the same host. The exotic Diadegma semiclausum (Hellen) (Hymenoptera: Ichneumonidae) was introduced into Kenya in 2002 for the control of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), and led to the competitive displacement of its indigenous congeneric species Diadegma mollipla (Holmgren) in some areas but not in others. In an attempt to understand this process, superparasitism in D. mollipla and multiparasitism involving both species were investigated in the laboratory through exposure of non-parasitized and parasitized host larvae to D. mollipla. Hosts were exposed to the two congeners both simultaneously and sequentially. The study revealed that D. mollipla minimizes parasitism and superparasitism in the presence of hosts containing a larva of a conspecific female. When the two parasitoid species competed for non-parasitized hosts simultaneously, there were no significant differences in the total progeny produced under intraspecific vs. interspecific competition. Under simultaneous interspecific competition, D. semiclausum produced 74% of total progeny. Under sequential exposure, the species that attacked first always dominated with 85–96% of the resultant progeny, irrespective of the exposure interval. Offspring sex ratio was significantly more female-biased when the larvae were exposed to D. mollipla first. Overall, our results indicate that D. semiclausum dominates under simultaneous competition and D. mollipla dominates in terms of female progeny, but under delayed exposure to the other species, the first attacker is always the winner. On the basis of our results, D. semiclausum may not completely displace D. mollipla from diamondback moth in the field.