| الملخص: |
Perennial ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinacea Schreb.) contain symbiotic fungi (Neotyphodium spp.) that provide several ecological advantages to host plants: enhanced resistance to drought, disease, and insect herbivory. The resistance to insect herbivory is the result of fungus-produced alkaloids that are toxic to most herbivores. However, black cutworm, Agrotis ipsilon, is a generalist herbivore that is able to feed and develop on endophytic perennial ryegrass. As some insects can use plant secondary compounds to defend themselves against predators, I hypothesized that the cutworms fed on endophytic grasses would exhibit greater defense against a lethal endoparasitic nematode, Steinernema carpocapsae. To test this hypothesis, I developed a method to remove the fungal endophytes from the grass seed through a heat treatment. Laboratory experiments involving 4-5th instars support the hypothesis that A. ipsilon feeding on plants with high (>90%) incidence of endophyte are less susceptible to entomopathogenic nematodes than those feeding on plants with no or low incidence of endophyte. Field studies show decreased susceptibility to S. carpocapsae when larvae were confined to areas of endophytic grass (>75% infected). Early (2-3rd) instars were equally susceptible to nematode attack regardless of host plant. Endophytic grass fed to cutworm larvae did not influence nematode attachment behavior, or their ability to penetrate and successfully develop into adults. I examined the effects of ergot alkaloids that are produced by N. lolii such as, ergotamine, ergonovine, ergocryptine, ergocristine, and seed extract on nematode viability and infectivity. Ergonovine malate increased and ergocristine decreased the rates of nematode infectivity, whereas other treatments had no significant effect. I investigated the effects of ergocristine on Xenorhabdus nematophila, the symbiotic bacterium of S. carpocapsae. Bacterial growth and pathogenicity were significantly reduced when bacteria were grown in nutrient broth containing 200 ìg/ml concentration of ergocristine. Further research demonstrated that several fungal alkaloids, including ergocristine, persist in or are sequestered by black cutworm and another noctuid, the fall armyworm. I conclude that A. ipsilon developing on endophytic grasses may acquire some level of resistance against entomopathogenic nematodes. This resistance is mediated through the effects of alkaloids; thus our results underscore the ability of N. lolii to affect trophic interactions through the production of alkaloids. |
