Conditions to Terminate Reproductive Diapause of a Univoltine Insect: Ceratapion basicorne (Coleoptera: Apionidae), a Biological Control Agent of Yellow Starthistle.

Ceratapion basicorne (Illiger) is a recently approved univoltine biological control agent that develops inside the rosette of yellow starthistle (Centaurea solstitialis L.), an invasive annual plant. Adult weevils normally emerge in early summer, and females are thought to be in reproductive diapause until the following spring, when they oviposit in rosettes. The long period of reproductive diapause constrains mass-rearing this weevil because only one generation per year can be produced. Determining the environmental conditions that regulate diapause termination may enable shortening diapause under laboratory conditions to increase production of adults to release. We tested three hibernating conditions (greenhouse [ambient temperature and photoperiod], glass door refrigerator [5°C and ambient photoperiod], and growth chamber [5°C and 24 h dark]) for three durations (4, 8, 11 wk). The highest proportion of females laying eggs came from the growth chamber, with 40% terminating diapause after 4 wk, 80% after 8 wk, and 95% after 11 wk of exposure. Our study demonstrates that duration of cold temperature is an important stimulus to terminate reproductive diapause of C. basicorne, and that exposure to ambient light had no effect at 5°C. However, 47% of females held at ambient greenhouse conditions, without any chilling period, completed diapause within 11 wk. Thus, a cold period can accelerate diapause development, but it is not necessary for its completion. Reducing the winter diapause period from about 6 mo to 11 wk should enable the production of multiple generations per year to increase the number of insects available to release.