Megacerus discoidus, a potential biological control agent of hedge bindweed in southwestern Virginia

Megacerus discoidus (Say)(Coleoptera: Bruchidae), a native North American seedfeeder, has 4 larval instars and one generation a year. Overwintering occurred in the 4th instar. Larvae fed on 40-50% of the seed content before diapause and the rest of the seed in the following spring. When more than one larvae attacked a seed, only one adult emerged because of cannibalism, which was most common among 2nd and 3rd instars. Adults oviposited mainly under the bracts of host seedpods. The life cycle of the beetle was well synchronized with the host phenology.

When diapause of the insect was experimentally terminated under 20°C and 28°C, there was increased mortality and prolonged adult emergence. No adult emergence occurred when infested seeds were maintained in the dark under 10°C for 200 days. However, mortality of the larvae was significantly increased. Humidity and soil coverage of host seeds had no significant impact on the beetle mortality during diapause under field conditions.

Of 19 plant species tested, only hedge bindweed and wild sweet potato, Ipomoea pandurata, supported larval development to adults. Oviposition occurred only on seedpods of hedge bindweed and heavenly blue morningglory, Ipomoea tricolor. The latter did not support larval development.

An infestation rate of 5 M. discoidus eggs per seedpod of hedge bindweed in the field cages was achieved by releasing the adults at a relatively high adult/plant ratio (1.2 pairs/plant) plus inoculation of eggs on all seedpods in the cages. This resulted in a high rate (ca. 70%) of seed infestation, but did not destroy all of the seeds in the field cages. Release of lower number of adults led to lower seed infestation. Seed consumption did not cause any change in reproduction and vegetative growth of hedge bindweed. I consider the use of M. discoidus to be safe but it must be combined with other stress factors to achieve effective control.