Fleahopper Induction of Stress Ethylene Production in Cotton

P.W. Morgan, P.D. Powell, M.P. Grisham, W.L. Sterling, and L. Cooper


 
ABSTRACT

Fleahopper damage to cotton includes inhibition of internode elongation, promotion of young flower bud abscission, and damage to unexpanded leaves. Much early season flower bud abscission in the field has been shown to be due to plant bugs, a group which includes tie fleahopper. Duffey and Powell (Ann. Ento. Soc. Amer. 72:599-601, 1970) found that fleahopper feeding induces stress ethylene production in detached apical buds of cotton. They also found that the insects apparently inoculated the plant tissue with microorganisms and that the microorganisms can themselves induce stress ethylene production.

Currently our efforts are directed at understanding the mechanism of stress ethylene production in the cotton plant-fleahopper-microorganism system. We have examined the microflora of the insect in detail. Fleahopper eggs were exicised from Croton stems and microbial contamination was determined for whole eggs or eggs which were sterilized and then ruptured. All whole eggs were contaminated with one or more microorganisms. Less than 20% of the sterilized and subsequently ruptured eggs (a procedure to selectively test for internal contamination) did not produce microbial growth. First instar nymphs just hatched from Croton stems in the laboratory were uniformly contaminated. We then examined nymphs just after moulting during the night. After each moult from instar 1 through 6 insects remained essentially 100% contaminated. Insects collected in the field at College Station and Temple were essentially 100%, contaminated. The most frequently occurring microorganisms are in four genera: Fusarium, Altenaria, Xanthomonas, and Penicillium. Among isolates from the four genera, mechanical inoculation of buds with Fusarium produces the largest stimulation of stress ethylene release. Since ethylene production consistently falls to low levels as the growth of the fungus appears on the plant tissue surface, it appears likely that the tissue its self is producing the ethylene.

Mechanical inoculation of cotton buds with Fusarium produces a peak release of ethylene more rapidly than exposure of buds to insects. The % hatch and vigor of fleahoppers from eggs in 1981 Croton stems decreased with time. During the fall of 1992 we found that insects from this less vigorous population had a significantly less diverse and less vigorous microflora. The degree of contamination with Fusarium decreased markedly. These insects induced the production of only a moderate level of stress ethylene when feeding on excised cotton terminal buds. The insects only doubled the amount of ethylene released over that resulting from physical wounding alone. Previously, insects induced about a 10-fold increase in ethylene production. The decline in the capacity of insects from "old" eggs to induce stress ethylene production is not due to a failure in the assay system. Inoculation of cotton buds with Fusarium isolated from cultures of insects consistently produces a typical increase in ethylene release. Therefore, the inability of the insects from "old" eggs to induce stress ethylene production appears to be due either to a loss in vigor of feeding or to a change in the intenal microflora. If the latter explanation is true, then such insects will be a valuable tool in further studies to establish unequivocally how fleahoppers induce stress ethylene production. Such information may lead to control strategies that block the damage mechanism without killing the insects. Taken together, our findings up to this point indicate that microorganisms appear to be involved in stess ethylene induction by fleahopper.



Reprinted from Proceedings of the 1983 Beltwide Cotton Production Research Conference pp. 54 - 55
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998