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

Bacillus thuringiensis Berliner toxicity is a dose-dependent phenomenon, and the dose acquired by an individual is directly related to the quantity of treated food consumed. A bioassay was conducted using two Apium graveolens var. rapaceum (L.) cultivars to determine if the B. thuringiensis dose acquired by Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) larvae was influenced by host plant-induced variation in consumption rates. S. exigua fed the cultivar more suitable for larval growth and development consumed significantly greater leaf surface areas, survived longer, and grew faster than larvae fed the less suitable cultivar. S. exigua fed B. thuringiensis-treated foliage consumed significantly smaller leaf surface areas, bad significantly shorter survival 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 significantly higher cumulative B. thuringiensis doses, but this difference did not explain the observed differences in insect performance. The fact that larvae fed the more suitable cultivar received a greater B. thuringiensis dose than larvae fed the less suitable cultivar demonstrates that host plant-induced variation in consumption rates can affect the B. thuringiensis dose a herbivore receives. In this particular case, larvae that received the overall greater B. thuringiensis doses performed better than larvae that received the overall lower doses, indicating that other factors associated with the host plant play an important role in mediating B. thuringiensis efficacy.