EFFECT OF HERBICIDE TREATMENT ON HOST-PLANT QUALITY FOR A LEAF-EATINGBEETLE

The effect of sub-lethal chlorsulfuron treatment of the annual weed Po lygonum convolvulus L. on growth and survival of the folivorous beetle Gastrophysa polygoni L. was studied in a controlled environment. Trea ted and untreated plants were used in whole plant bioassays and in fee ding experiments with excised leaves. Direct insect toxicity of the he rbicide was studied by use of a Potter Spray Tower. The herbicide was non-toxic to the larvae when applied topically. Larvae fed excised lea ves from treated plants did not differ from larvae fed leaves from con trol plants in terms of developmental time, weight or survival. This s uggests that the treatment did not affect the nutritional value of the plant or at least that the beetles could compensate for any such chan ges. Chlorsulfuron reduced survival of larvae living on whole plants t reated with sub-lethal dosages (8 to 67% of the recommended field rate ). The mortality of larvae in whole plant experiments was dependent on herbicide dosage and herbivore load. Mortality ranged from approximat ely 20% on control plants to a maximum of 80% on plants sustaining the highest dosages and herbivore loads. It is argued that the beetles' r esponse to chlorsulfuron-treated plants could be caused by a herbivore -induced plant response enhanced by the action of chlorsulfuron.