There are three generations of hemlock woolly adelgid, Adelges tsugae Annand, that develop on the secondary host Tsuga spp. Two of these generations, the progrediens and sexupara, are present concurrently in the spring. Sistens are present from early summer until the following spring. Constant temperature studies were conducted to quantify the influence of temperature on hemlock woolly adelgid progrediens development and to develop a degree-day model. Progrediens development in the field also was sampled to test the accuracy of the model. Hemlock woolly adelgid progrediens developed at temperatures ranging from 4 to 22°C. Duration of first-instar progrediens was less dependent on temperature than in the other nymphal stages and may have been a result of the difficulty in determining the median point of settled first-instar nymphs and the inability to distinguish progrediens from sexuparae at this stage. Duration of development from second stadium to adult was highly dependent on temperature. Using this range of progrediens stages, it was determined that the low temperature threshold was 3.9°C and requires 222 degree-days to reach the adult stage. The number of degree-days needed to complete development in the field ranged from 110 to 123% of degree-days required to complete development in the laboratory.
3Canadian Forestry Service.