EFFECTS OF DIFFERENTIAL HOST-PLANT CONSUMPTION BY SPODOPTERA-EXIGUA (LEPIDOPTERA, NOCTUIDAE) ON BACILLUS-THURINGIENSIS EFFICACY

Bacillus thuringiensis Berliner toxicity is a dose-dependent phenomeno n, and the dose acquired by an individual is directly related to the q uantity of treated food consumed. A bioassay was conducted using two A pium graveolens var. rapaceum (L.) cultivars to determine if the B. th uringiensis dose acquired by Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) larvae was influenced by host plant-induced variation in co nsumption rates. S. exigua fed the cultivar more suitable for larval g rowth and development consumed significantly greater leaf surface area s, survived longer, and grew faster than larvae fed the less suitable cultivar. S. exigua fed B. thuringiensis-treated foliage consumed sign ificantly smaller leaf surface areas, bad significantly shorter surviv al times, and grew slower than larvae fed untreated foliage. When leaf surface area consumption was adjusted to reflect the B. thuringiensis consumed, larvae fed the more suitable cultivar received significantl y higher cumulative B. thuringiensis doses, but this difference did no t explain the observed differences in insect performance. The fact tha t larvae fed the more suitable cultivar received a greater B. thuringi ensis dose than larvae fed the less suitable cultivar demonstrates tha t host plant-induced variation in consumption rates can affect the B. thuringiensis dose a herbivore receives. In this particular case, larv ae that received the overall greater B. thuringiensis doses performed better than larvae that received the overall lower doses, indicating t hat other factors associated with the host plant play an important rol e in mediating B. thuringiensis efficacy.