Searching Success of Predators in Artificial Leaf Litter

Abstract

To study the effects on searching success of some of the purely geometrical properties of leaf litter, the author constructed artificial litter components leaves, twigs, and in the laboratory, and allowed a parasitoid wasp (Trichogramma brasiliaense) and a gamasine mite (Blattisocius tarsalis) to search among these in the dark for eggs of the flour moth (Sitotroga cerealella). The distribution of parasitoid wasps and gamasine mites in natural habitats suggests that the wasps would be able to search more effectively among the leaves, and the mite among the twigs and chunks. This prediction was confirmed. Two different sizes for each type of structure were used; the wasp was in every case less efficient when searching among smaller pieces, whereas the mite showed no such differences. This suggests that the mite may be better able to search among small crevices, which fact perhaps also explains its superior ability to search among the chunks. INTRODUCTION A small arthropod searching for prey in leaf litter finds itself in a habitat of bewildering structural complexity. There are large flat surfaces (leaves), cylindrical ones (twigs) and polyhedral chunks (dried fruits, etc.). Disturbance from wind and mice tends to produce stratification, with the leaves more on top and the solid pieces nearer the soil. What are the opportunities for specialized searching behavior in such a habitat? There will, of course, be more light towards the top and more moisture towards the bottom, with consequent gradients in fungal and other growth. But to what extent are the opportunities for specialization related to enviromental features of a purely geometrical character? E.g., does the locomotory pattern best suited for searching along sticks differ from that best suited for searching the surface of a leaf? As a preliminary approach to this problem, I set up a laboratory model, using eggs of the flour moth Sitotroga cerealella (Olivier) as prey and comparing the performances of two predatorsa chalcidoid parasitoid wasp (Trichogramma brasiliaense Ashmead) and a phytoseiid gamasine mite (Blattisocius tarsalis (Berlese) ). These can beregarded as representatives of whole groups of arthropods that one sees distributed from the top to the bottom of leaf litter. The wasps are found abundantly on the broad surfaces of growing vegetation and to some extent on the surface of litter, but are conspicuously uncommon in the deeper parts of leaf litter. Exactly the opposite holds true for gamasine mites. In the laboratory model, all non-geometrical variables are eliminated. There are no light gradients: the animals are forced to search I M.A. dissertation, University of California, Los Angeles. 2 Present address: Dpto. de Zoologia Agricola, Universidad Central de Venezuela, Maracay.