Patterns of Host Exploitation by the Parasitoid Wasp Trichogramma minutum (Hymenoptera: Trichogrammatidae) when Attacking Eggs of the Spruce Budworm (Lepidoptera: Tortricidae) in Canadian Forests

Abstract

We examined patterns of host exploitation by natural populations of the parasitoid Trichogramma minutum Riley parasitizing eggs of the spruce budworm, Choristoneura fumiferana (Clemens), in Canadian boreal forests in an effort to identify aspects to improve inundative releases. Egg masses of spruce budworm were sampled from eight different sites in Ontario and New Brunswick during the period of oviposition in 1992. Parasitoids did not necessarily fully exploit a discovered egg mass; 35% of parasitized egg masses were completely parasitized by naturally occurring T. minutum, but the remaining 65% were only partially parasitized. Not all parasitized host eggs produced viable parasitoid offspring. Natural mortality of immature T. minutum was as high as 52% in one site. This high mortality was not correlated with the degree of superparasitism. The size of adult T. minutum as estimated by wing length varied among different localities, but females were always larger than males from the same site. Females were usually mated before they discovered spruce budworm eggs and produced a female-biased sex ratio when allocating progeny in the field. Field patterns of host exploitation can help us to predict what to expect from inundative release programs and, more important, to identify aspects that need to be enhanced so that a better level of control of spruce budworms can be achieved.

Assays with Commercially Produced Trichogramma (Hymenoptera: Trichogrammatidae) To Determine Suitability for Obliquebanded Leafroller (Lepidoptera: Tortricidae) Control

“…Therefore, in tlle ramaining studies, we chose to use the number of eggs parasitized per egg mass as a measure of parasitism because counting the number of parasitized eggs is easier than determining percentage parasitism in each egg mass. Similar observations were made for the relation between the number of T. minutumparasitized spruce budworm eggs and the number of eggs per egg mass (Bai and Smith 1994).…”

The relationship between egg load and fecundity among Trichogramma parasitoids

“…Lepidopteran eggs, the hosts of Trichogramma species, however, are laid either singly or in clusters that can be so large as to exceed the egg load of the parasitoids (e.g. Bai & Smith, 1994), and thus selection to avoid egg depletion appears be the most likely explanation for the variation in potential fecundity of Trichogramma species. To what extent Trichogramma species differ in host range and whether species with greater fecundities are more frequently associated with hosts that lay eggs in clusters is unknown, but may have important implications for the use of Trichogramma species in inundative biological control.…”

Biological characteristics of threeTrichogrammaspecies on the eggs of diamondback moth (Plutella xylostellaL.)

“…In all three Trichogramma species, oviposition rate was gradually decreased with increasing age of female wasps. According to the opinion of Bai and Smith (1994), maximum oviposition of female wasps will occur in the first day, if Trichogramma wasps possess so many hosts. Similarly, in the current research, the oviposition of wasps and subsequently the rate of parasitism (on 50 host eggs) were recorded on the first day and was found that the average number of eggs of T. pintoi was more than T. embryophagum and T. brassicae.…”