Oviposition responses to natural and artificial substrates were investigated. More eggs were deposited on plants that contain isothiocyanates (natural hosts). Depletion of isothiocyanate content by sulfur-deficient plant nutrition appears to reduce attractiveness of host plants as egg substrates. Eggs are laid preferentially in concavities of leaves or stems. Significantly more eggs are laid on dimpled than smooth polyvinyl plastic surfaces. On both types of plastic substrate more eggs are laid if allyl isothiocynate or mustard leaf juice is added. Mustard leaf juice contains allyl isothiocyanate but is more stimulating to oviposition indicating that other factors contribute to the total effect.Although the coumarin in Melilotus sp. tends to inhibit oviposition, Plutella may be induced to increase egg deposition on such plants by treating them with allyl isothiocyanate. Unknown inhibitory constituents in tomato leaves prevent egg laying even in the presence of allyl isothiocyanate. It was concluded that oviposition is regulated significantly by external stimuli and that inhibitory chemical influences as well as positive chemical stimuli and texture of the surface are all significant factors.The chemical influence of plant constituents on acceptance or rejection of plants as food by the larva of the diamond-back moth, Plutella maculiDennis (Curt.) has been considered previously (GuPTA & THORSTEINSON, 1960; THOR-STEINSON, 1953). The mechanisms of food plant specificity as expressed by this insect in nature must also take into account the regulation of oviposition behaviour of the adult female inasmuch as this determines what plant the newly hatched larva must accept or reject. The object of this study is, therefore, to determine what attributes of the plant are most significant as oviposition stimulants and to investigate interactions among these influences.
MATERIALS AND METHODSAdults for the experiments were taken from a culture that had been maintained in our laboratory for three years. Newly emerged adults were isolated in new wooden cages or paper cartons provided with brass wire net for ventilation. The insects were fed on sugar solution. Plants of Brassica juncea (Mustard), Lycopersicon esculentum (Tomato), Pisum sativum (Garden Pea), Melilotus officinalis (Sweet Clover) and Melilotus alba (var. Coumino) used in various experiments were grown in the green house. Allyl isothiocyanate (Eastman Organic Chemicals) *) On visit from the University of Lucknow, India.
The exclusion of botanical species from the food plant range of the diamond‐back moth larva is determined not only by the infrequency of adequate stimulants for feeding but also by a high frequency of occurrence of feeding inhibitors. A few plant species lack feeding inhibitors but are excluded by a deficiency of feeding stimulants. Some plants are devoid of decisive feeding inhibitors and contain effective feeding stimulants but yet are excluded as host plants by the possession of toxic principles. At least one plant species, Pisum sativum, devoid of mustard oil glucosides, is sufficiently palatable, nutritious and free of toxicants to support successive generations of Plutella larvae in the laboratory. The free mustard oils in the natural host plants apparently tend to inhibit dispersal of the larvae.
Resume
L'exclusion d'espèces botaniques comme plantes pouvant servir de nourriture à la larve de Plutella maculipennis est detérminée non seulement par la rǎreté de stimulants nécessaires pour que l'insecte se nourrisse mais aussi par la présence fréquente de substances qui empěchent l'insecte de se nourrir. Quelques espèces de plantes qui n'ont pas de substances qui empěchent l'insecte de se nourrir, sont excluées par une déficience de stimulants. Certaines plantes n'ont pas de substances qui empěchent l'insecte de se nourrir et contiennent des stimulants efficaces, mais cependant elles sont excluées comme plantes nourricières par la possession de principes toxiques. Au moins une espèce de plante, Pisurn sativum, qui ne possède pas les glucosides d'huile de moutarde, est assez agréable au goǔt, nourrissante et privée de toxines pour pouvoir soutenir des générations successives de larves de P. maculipennis dans la laboratoire. Les huiles de moutarde des plantes nourricières naturelles semblent empěcher la dispersion des larves.
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