THE CHEMOTACTIC RESPONSES THAT DETERMINE HOST SPECIFICITY IN AN OLIGOPHAGOUS INSECT (PLUTELLA MACULIPENNIS(CURT.) LEPIDOPTERA)

Abstract

The mustard oil glucosides, sinigrin, sinalbin, and glucocheirolin and the musard oil, allyl isothiocyanate, as well as the enzyme myrosin, were prepared from the seeds of cruciferous plants. Pieris brassicae (L.) and Plutella maculipennis (Curt.) were induced to feed on leaves of plant species which they normally refuse by painting the leaves with a solution of sinigrin or sinalbin but would not feed on such leaves treated with allyl mustard oil. Feeding responses of P. maculipennis larvae were tested on agar gels containing the powdered, dehydrated leaves of various plants alone and in combinations with glucosides, mustard oil, and myrosin. The amount of feeding was estimated by counts of frass pellets produced by the larvae during the test period. It was shown that sinigrin, sinalbin, and glucocheirolin are feeding stimulants for P. maculipennis larvae. Since the larvae feed readily on at least 40 plant species which are reported to contain mustard oil glucosides it was concluded that the mustard oil glucosides are specific feeding stimulants for this insect. The threshold concentrations for P. maculipennis of gustatory perception of sinigrin under these conditions are of the order of 2 p.p.m. for sinigrin and about 20 p.p.m. for sinalbin. Optimum feeding responses were obtained only when the glucoside is offered in a medium containing other nutrients in the form of powdered, dehydrated leaves or artificial mixtures. In some experiments the addition of allyl mustard oils slightly increased feeding on media containing sinigrin. The addition of viable myrosin to diets containing sinigrin decreased the feeding responses. Since heat-killed myrosin has no repellent effect, it appears that the action of myrosin is due to the depletion by hydrolysis of the quantity of sinigrin in the medium. Apparently the fission products of the hydrolysis including mustard oil are less attractive than the parent glucosides. Since hydrolysis of glucoside will release only minute amounts of mustard oil in the short space of time intervening between biting and swallowing it is unlikely that the gustatory receptors will be exposed to appreciable concentrations of mustard oil as compared with glucoside. On the other hand it is possible that infinitesimal amounts of mustard oil vapor emanating from leaves may stimulate the olfactory sense which is characteristically extremely sensitive in insects. While hunger induces sustained feeding in the absence of mustard oil provided a gustatory stimulant is present, such an olfactory stimulus might conceivably initiate feeding more promptly. This would account for observations in some experiments that larvae produced more frass when feeding on media containing a little mustard oil as well as sinigrin. The power of sinigrin to induce P. maculipennis to feed on nutrient media makes possible the development of an artificial medium for studies of the nutrition of this phytophagous insect.