Effects of Gamma Radiation and a Chemosterilant Applied to Late Nymphal Instars of the Large Milkweed Bug (Hemiptera: Lygaeidae)

Biological effects of gamma radiation and apholate, a chemosterilant, were evaluated when fifth-instar nymphs of Oncopeltus fasciatus (Dallas) were treated. Irradiation of older fifth instars revealed less sensitivity than younger ones. Dosages 2 and 1.2 times greater were necessary on older nymphs for mortality and wing deformities, respectively, in 50% of the insects. Compared with mortality, fecundity and fertility were affected at lower dosages of radiation. When both sexes were treated and mated, fecundity was reduced by 75% at 2.2 krad; 75% reduction in fertility occurred at about two-thirds of that dose, or 1.4 krad. When males alone were treated and mated, ED75 values were higher (3.5 krad for fecundity and 3.0 for fertility). Thus, males were less sensitive with respect to these two criteria. Adult longevity of nonirradiated females and males was 32.0 to 33.5 d, respectively. Longevity was reduced by 14% in females and 18% in males after treatment of late fifth Instars with 6 krad. Topical applications of apholate resulted in wing deformities and mortality similar to the pattern described for radiation. Compared with late fifth instars, greater sensitivity occurred in fifth instars treated early. Lethal effects occurred at lower doses than wing deformities. This result contrasted with radiation treatments in which doses were similar with respect to lethality and wing deformities. Weight losses also occurred following treatment with apholate but were not significant. Treatments of early or late fifth instars with gamma radiation or apholate showed no differences between sexes in mortality or wing deformities.