Biological control: Implications of the analogy between the trophic interactions of insect pest-parasitoid and snail-trematode systems

Abstract

An analogy between the tropic interactions of insect pest-hymenopterous parasitoid and snail-larval trematode systems is proposed. The goal of most agricultural pest management programs is increase in production of a plant crop, the deleterious agent is an herbivorous insect pest, the controlling agent, a parasitoid. Other parasitoid species may or may not have a significant effect on the control of the host by the key mortality factor. In snail—schistosome systems the goal is reduction of worm burdens in human populations, removed in space and time from the snail-sporocyst interaction; the deleterious agent is the schistosome, trophically equivalent to the hymenopterous parasitoid. Therefore, control may be achieved through competitive displacement of the schistosomes by other larval trematodes having superior intrinsic competitive abilities and better searching efficiencies at low host densities. Hyperparasites, sciomyzid Diptera and Daubaylia may also play a role in this view of schistosomiasis control. The Hassell-Varley parasite quest theory is applied to larval trematodes. The inversely proportional relationship between the area of discovery and miracidial density when logarithmically transformed is further evidence for the dynamic similarity of snail-trematode and insect-parasitoid systems. Other applications of the generally accepted principles of insect pest biological control to a medically important trematode indicate that (1) schistosome population control, not eradication, is the appropriate goal of public health programs, (2) the criteria for implementation and success of such programs must be in terms of the relationship between medical injury and the frequency distribution of worms in humans, and (3) a good biological control agent is likely to have a high searching efficiency at low host densities, kill more than one snail host, be highly host specific, be scarce when successful, reside in regions were schistosomiasis is negligible as a medical problem.