DEVELOPMENT, SURVIVAL AND PHENOLOGY OF THE SWEETCLOVER WEEVIL PARASITOID, PYGOSTOLUS-FALCATUS (HYMENOPTERA, BRACONIDAE)

Biennial sweetclovers were widely used for soil improvement and as for ages in the first half of the 1900s. The introduction of the sweetclov er weevil, Sitona cylindricollis, caused a drastic decline in sweetclo ver acreage. In North Dakota, yellow sweetclover, Melilotus officinali s, is still the legume of choice on organic farms. In an effort to con trol the weevil, the thelyotokous parasitoid Pygostolus falcatus was i mported. Parasitoids were studied for temperature-dependent developmen t, and adult longevity as influenced by temperature and availability o f provisions. Development from egg to adult at 15, 20, 25 and 30 degre es C was 58, 28, 22 and 21 d, respectively. No parasitoids were reared out at 10 degrees C, although diapausing first instars were present. Longevity of adult parasitoids provided honey, water, sweetclover and sweetclover weevils at 15, 20, 25 and 30 degrees C was 29, 22, 12 and 6 d, respectively. Adults provided the following combinations of provi sions at 25 degrees C survived for: nothing-2 d; water-2 d; honey-4 d; honey and water-6 d; honey, water and sweetclover-11 d; honey, water, sweetclover and hosts-12 d. Field cage releases and a degree-day mode l developed for the parasitoid demonstrated that poor synchrony betwee n PI falcatus and the sweetclover weevil hinders its usefulness as a b iological control agent.