Bottom‐up, top‐down, and within‐trophic level pressures on a cactus‐feeding insect

Abstract

Abstract 1. The relative importance of host‐plants and predators in the population dynamics of herbivorous insects, and the frequency and intensity of inter‐specific competition among herbivores, have both been intensively studied and debated. The joint effects of bottom‐up, top‐down, and within‐trophic level interactions, however, have rarely been integrated in a single system.2. I studied the dynamics of the cactus bug (Narnia pallidicornis), a specialist feeder on tree cholla cactus (Opuntia imbricata), in response to variable host‐plant quality, spider predation, and interactions with cactus‐feeding beetles (Moneilema appressum). Previous work suggests that cactus reproductive effort (the proportion of meristems allocated to reproduction) is an important component of host‐plant quality for Narnia. I conducted a 2‐year field experiment to test the hypotheses that Narnia abundance is positively related to host‐plant reproductive effort, and that interactions with predators and putative competitors alter the shape of this relationship.3. I found strong support for the first prediction (positive Narnia–plant quality relationship) in both years, but neither predator removal nor beetle exclusion had detectable effects on this relationship in either year. I conclude that the dynamics of this insect herbivore are driven predominantly from the bottom‐up, and that available data from this work and from previous studies are too variable to permit broad generalisations for the combined effects of host‐plants, predation, and competition on herbivore dynamics.