Model Selection For A Subterranean Trophic Cascad: Root-Feeding Caterpillars And Entomopathogenic Nematodes

Ecology, Dec, 1999 by D. R. Strong, A. V. Whipple, A. L. Child, B. Dennis

Ecology of an entomopathogenic nematode, Heterorhabditis hepialus

Adults, reproduction, and feeding of entomopathogenic nematodes are restricted to the interior of host insects (Kaya 1990). The nonfeeding "infective juveniles," or third-instar, dauer larvae, live in the soil, search for hosts, and disperse. H. hepialus infective juveniles are -0.55 mm in length. Attracted to [CO.sub.2] and other waste gases, they move up to 6 cm/d through moist soil toward an insect (Strong et al. 1996). An infective juvenile enters the host through a spiracle or other orifice, punctures a membrane, then regurgitates the symbiotic bacterium Photorhabdus luminescens, which kills the host within 48 h (Forst and Nealson 1996, Bowen et al. 1998). A burgeoning bacterial population then digests the cadaver and provides food for the exponentially growing adult nematode population inside. Although several infective juveniles can invade a host insect, nematodes in the family Heterorhabditidae are hermaphrodites; a single infective juvenile can kill a host and give rise to a full complement of offsp ring. The symbiotic bacteria produce antibiotics and other antimicrobial substances that protect the host cadaver and the adult nematodes inside from invasion of alien bacteria and fungi from the soil. When the insect cadaver is exhausted of resources, reproduction shunts to infective juveniles, which break the host integument and disperse into the soil. As many as 410 000 H. hepialus infective juveniles are produced in a large ghost moth caterpillar (Strong et al. 1996).

We conducted a field experiment that tested interactions in the effects of young ghost moth caterpillars and entomopathogenic nematodes on survival of lupine seedlings. The focus was upon plant protection during the crucial first spring of growth, when seedling bush lupine grow a tap root and are attacked by small caterpillars. "Nematode" refers to the entomopathogenic nematode H. hepialus.

Model-centered statistical approach

The binary survival data generated by this experiment had nonstandard features that were not well described by the statistical models in conventional statistical packages. We adapted the one-hit dose-response model (Turner 1975, Janardan 1986) for the statistical analysis. The model has seen extensive use in carcinogenesis and toxicology studies (Krewski and Brown 1981), but has been seldom employed in ecology (Costantino et al. 1998). The model is based on a plausible biological mechanism and might find useful applications in other systems. We accordingly describe the statistical methods here in some detail. Our model-centered approach emphasized customized model construction, model selection, parameter estimation, and model evaluation, in addition to the more traditional pairwise hypothesis tests. The use of an Akaike-type model selection index (Sakamoto et al. 1986) played a crucial role in sorting out the various hypotheses about the interactions between nematodes and caterpillars. Finally, we contrasted this approach with conventional logistic regression in statistics packages.