Tritrophic Effects

Heterorhabditis marelatus (Heterorhabditidae: Rhabditida) attacks soil-inhabiting insects, and upon entering the host, releases a symbiotic bacterium that kills the host insect. Nematode feeds on the bacterial/host slurry, developing through several generations before exiting the host as an environmentally-resistant infective juvenile. And thus provides highly effective control of pupating Colorado potato beetle. The nematode enters the beetle, and the beetle dies within two days. However, the nematode is not known to reproduce in the beetle. The nematode and/or its bacteria may be inhibited by the presence of alkaloids that are found in high levels in the potato plants upon which the beetle feeds. Alkaloids inhibit acetyl cholinesterase and so affect the nervous system, as well as upsetting cell membranes in some organisms. The beetles may be sequestering alkaloids from their host plants as a defense against natural enemies. Effects of these alkaloids on the symbiotic bacteria and the nematode individually were examined and documented;the location of alkaloids can affect natural enemies.

1. Barbercheck-ME; Wang-J; Hirsh-IS.1995. Host plant effects on entomopathogenic nematodes. Journal-of-Invertebrate-Pathology, 66: 2, 169-177.
AB:Assays were conducted to determine the effects of host plant and a plant secondary metabolite, cucurbitacin D, on the mortality of Diabrotica undecimpunctata howardi from infection by entomopathogenic nematodes and on nematode progeny production. Rootworms were produced on maize, groundnut, and two squash varieties, one continuing cucurbitacin D and the other lacking this secondary metabolite. Rootworms were exposed to the NC, HP88 and Lewiston strains of Heterorhabditis bacteriophora and the All, Mexican and Agriotos strains of Steinernema carpocapsae. The plant on which the rootworms had fed significantly affected rootworm mortality and nematode progeny production. In general, rootworms which had fed on maize suffered significantly lower mortality than those reared on groundnuts or either of the squash varieties. Rootworms that had fed on the squash varieties suffered greater mortality. Nematode progeny production was highest from rootworms that had fed on maize, lower for groundnut, and lowest on squash. Progeny production from rootworms that had fed on bitter squash was lower than from non-bitter squash for all nematode strains tested. Possible effects of cucurbitacins and plant primary metabolites on rootworms and entomopathogenic nematodes are discussed.

2. Jaworska-M; Ropek-D. 1994. Influence of host-plant on the susceptibility of Sitona lineatus L. (Col., Curculionidae) to Steinernema carpocapsae Weiser. Journal-of-Invertebrate-Pathology, 64: 2, 96-99.
AB: Laboratory experiments were conducted to investigate the effects of food plant on the susceptibility of Sitona lineatus to infection by the entomophilic nematode S. carpocapsae. Larvae collected from early, late and field cultivars of peas and faba beans grown in the field near Krakow were exposed to S. carpocapsae for 6 days. Results are tabulated according to the cultivar which show that mortality of S. lineatus was significantly greater for larvae originally from peas than for those collected from faba beans. Young adults of this pest from pea-fed larvae were also more susceptible to the nematodes. However, larvae of S. lineatus from beans appeared more favourable hosts for nematode multiplication than larvae from peas because greater numbers of juveniles of S. carpocapsae emerged from bean-fed S. lineatus.

Epsky-ND; Capinera-JL. 1994. Influence of herbivore diet on the pathogenesis of Steinernema carpocapsae (Nematoda: STEINERNEMAtidae). Environmental-Entomology. , 23: 2, 487-491.
AB:The relationship of insect diet to pathogenicity and reproduction of an entomophilic nematode, S. carpocapsae All strain, were investigated in Agrotis ipsilon. Larvae fed on collard [kale] foliage or on artificial diet were equally infected and killed by S. carpocapsae. However, nematode progeny production was lower in kale-fed larvae. The difference in progeny production corresponded to difference in lipid content of larvae; 210nematodes/mg were produced from kale-fed larvae, which contained 8% lipids, while 390 nematodes/mg were produced from artificial diet-fed larvae, which contained 16% lipids. Addition of commercially obtained sinigrin, a glucosinolate allelochemical found in collards, to artificial diet at biologically relevant levels also demonstrated suppression of nematode progeny production. It appears that host diet is a salient characteristic in progeny production of entomophilic nematodes.