Host plant and natural enemy influences on survivorship in the milkweed leaf miner Liriomyza asclepiadis

Abstract

Biologists have been arguing the importance of bottom-up versus top-down interactions in driving the direction of evolution for years. The argument is based on the question of which trophic level determines how communities are structured; is it a lower trophic level in the form of primary producers, or the top level in the form of predators and parasites? The top-down model was first proposed by Hairston et al. (1960) in their well known paper that attempted to describe why the world is green. The top-down argument holds that in a three-level trophic model secondary predators drive interactions between the levels because they are the most limited in their food supply. This being true, secondary predators maintain the populations of the trophic level below them (the primary predators or grazers) at low enough numbers that they release the primary (producer) level from predatory pressures, allowing it to flourish (Hairston et al., 1960). The bottom-up model developed as a response to Hairston et al (1960). It champions the role of producers as the driving force for interactions between all levels of every trophic system (Powers 1992). This model stresses the interactions between producers and their primary predators as determinant of the structure of trophic systems. Hunter and Price (1992) argue that at times trophic levels can be difficult to determine due to the complexity of interactions. They therefore proposed a model that synthesized the top-down and bottomup models but allowed for one model to dominate the other in systems for which one interacting level seemed to produce a stronger driving force than the other (Fig. 1). In this study, I explored the relationships between Liriomyza asclepiadis, a species of leaf miner fly, its host plant Asclepias syriaca, and any predators that L. syriaca might have. I explored the way this system fits into the model proposed by Hunter and Price (1992). I hypothesized that within the mine, larvae of this fly are relatively safe from predation and parasitism and therefore face direct danger only from the host plant itself.