How low can we go? Minimum radiation doses for preventing reproduction of the cabbage looper, Triciplusia ni

Phytosanitary irradiation is used as an alternative to chemical fumigants for controlling invasive species associated with international trade of fresh fruits and vegetables. A dose of 250 Gy is applied to commodities when they are suspected of containing lepidopteran larvae has been proposed as a generic dose to prevent emergence of normal-looking adults of Lepidoptera. However, much lower doses of radiation may be effectively used in phytosanitary treatments if those lower doses induce complete sterility. Irradiation of larvae can generate developmental defects in adults, including wings that prevent flight. Malformed adults produced from irradiated larvae are often considered to be inviable, but this cannot always be assumed. Here, we examined how irradiation of larvae of a model lepidopteran, Trichoplusia ni (Hubner), at two low doses that induce a substantial proportion of both normal-looking and malformed adults, 40 Gy and 60 Gy, affect reproduction of both normal-looking and malformed adults. We show that fecundity was decreased when unirradiated females or males were mated with either 40 Gy or 60 Gy irradiated males or females. Fertility was eliminated in both normal-looking and malformed females treated at 60 Gy. These data suggest that much lower doses than the current generic dose of 250 Gy may be applied to larvae in commodities to preventing establishment of invasive lepidopteran pests. However, additional work with a greater dose range and greater sample sizes will be needed to substantiate these early observations.