Direct and interactive effects of planting date, irrigation, and corn earworm (Lepidoptera: Noctuidae) damage on aflatoxin production in preharvest field corn

The influence of planting date, irrigation, and insect damage on aflatoxin production in field corn, Zea mays L., was evaluated in 1988 and 1989 at the Macon Ridge Branch of the Northeast Research Station, Winnsboro, La. Severe early season drought conditions were experienced in 1988, and four irrigations were necessary. The plots received excessive amounts of precipitation in 1989, and no irrigation was needed. The most damaging insect pest in both years was the corn earworm, Helicoverpa zea (Boddie). A significant interaction between effects of planting date and irrigation on corn earworm damage was detected in 1988. A significant interaction between effects of planting date and insecticide treatment on corn earworm damage occurred in 1988 and 1989. Mean corn earworm damage to ears in plots that received insecticide applications was significantly lower compared with that in plots that did not receive insecticide applications in 1988 and 1989. A significant interaction between effects of irrigation and insecticide treatment on percentage of ears that displayed bright greenish-yellow fluorescence (BGYF) and aflatoxin B1 concentration occurred in 1988. The plots that received irrigation and were treated with insecticide had the lowest BGYF (1.1%) and geometric mean aflatoxin B1 concentration (0.06 parts per billion [ppb]). In the plots that did not receive irrigation and insecticide applications, a drastic increase in BGYF (43.8%) and aflatoxin B1 concentration (191.5 ppb) was observed. Geometric mean aflatoxin B1 concentrations for planting dates 1 and 2 (9.1 and 11.9 ppb, respectively) were significantly higher than the aflatoxin concentration in planting date 3 (2.4 ppb) in 1988. Only 5 of 720 ears observed exhibited BGYF in 1989, and no aflatoxin B1 was detected in any of the samples. Results of this experiment suggest that drought stress and corn earworm damage had a synergistic effect in enhancing aflatoxin levels in preharvest field corn.