Compound interactions effects of plant antioxidants in combination with carbaryl on performance ofTrichoplusia ni...

Plant chemicals naturally exist in complex mixtures, which can interact either additively, synergistically, or antagonistically. We investigated the potential interactions of three naturally occurring antioxidants- nordihydroguaiaretic acid (NDGA), safrole, and α-tocopherol-with the general insecticide carbarayl to affect the performance of cabbage looper larvae (Trichoplusia ni). The cabbage looper is known to produce a mixed-function oxidase enzyme system in response to the presence of carbaryl. We proposed that plant antioxidants would interfere with enzymatic oxidation, enhancing the susceptibility of this insect to carbaryl. Insects were fed artificial diets containing each antioxidant alone or in pairwise combinations with the insecticide carbaryl to test for their effects on the insect's nutritional measurement indices. The three antioxidants tested were not equally effective individually against insect survivorship and interacted differentially in combination with the insecticide. The nutritional indices were measured on insects fed diets containing the chemicals at nonlethal doses. Insects fed 0.001 % wet wt NDGA diets grew 1.62 times less, and had gross and net conversion efficiencies reduced 3.20 and 3.63 times, respectively, compared to the control larvae. Carbaryl (0.002% wet wt) in combination with NDGA acts as an antagonist to the effects mentioned above, while safrole (1 × 10(-4) wet wt) had an additive effect when combined with the insecticide, reducing 1.76 times larval relative growth rate and efficiency of conversion of ingested food in respect to the control. The larvae fed significantly more (1.2 times) on both insecticide and safrole diets than on the controls or their combined diets. Larvae fed α-tocopherol alone or in combination with carbaryl had similar growth and conversion efficiencies as controls. We conclude that the effects of different combinations of compounds cannot be predicted a priori and must be determined experimentally.