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The ability of naturally occurring levels of theFusarium spp. fungal metabolite fusaric acid to synergize the toxicity of the allelochemicals gossypol, a saponin, and 6-methoxy-2-benzoxazolinone to larvae ofHeliothis zea (Boddie) was tested. Levels of fusaric acid comparable to those found near the fungus increased mortality ofH. zea to gossypol, the saponin, and 6-methoxy-2-benzoxazolinone, and decreased the development rate of surviving larvae exposed to gossypol and 6-methoxy-2-benzoxazolinone. Some effect was also noted for levels of fusaric acid found generally distributed throughout infected plants. The chemical properties of fusaric acid suggest that it synergizes the toxicity of the allelochemicals by inhibiting oxidative enzymes responsible for detoxification. Production of the biosynthetically simple fusaric acid may be a fungal strategy for conserving resources as compared to those fungi that produce biosynthetically complex toxins of their own.

First-instar larvae of the false tobacco budworm (corn earworm, Helicoverpa zea) that fed on either intact plants, leaf disks from undamaged plants, or leaf disks from insect-damaged plants of Nicotiana sylvestrisand N. tabacumCoker plants overproducing a tobacco anionic peroxidase generally caused significantly less damage than those caged with corresponding material from wild-type plants. In some cases mortality was significantly higher and weights significantly less for caterpillars feeding on leaf material from overproducing vs. wild-type plants. First-instar H. zeafed on the same type of leaf material from N. tabacumXanthi underproducing tobacco anionic peroxidase generally caused significantly more damage than those fed leaf material from wild-type plants. However, first-instar H. zeafed on underexpressing leaf material from N. sylvestrisdid not cause significantly greater damage compared to wild-type material. In cases where peroxidase enzyme activity was determined, significantly higher mean peroxidase activity was seen in leaves of plant types that also had significantly less mean feeding ratings. This information suggests that peroxidase activity can contribute to leaf resistance to chewing insects. However, the context of the peroxidase (cooccurring substrates, additional inducible factors) can mediate the degree of influence seen by changes in individual peroxidase isozyme levels.