DEVELOPMENTAL THRESHOLD, DAY-DEGREE REQUIREMENTS AND VOLTINISM OF THEAPHID PREDATOR EPISYRPHUS-BALTEATUS (DIPTERA, SYRPHIDAE)

Episyrphus balteatus is the dominant syrphid predator of aphids in the UK; reestablishment of populations each season is mainly by migration from warmer dimes, as overwintering in this country is minimal with a n uncertain level of success. The effects of temperature on the develo pmental rate of E. balteatus were investigated to determine the potent ial voltinism of this species in the UK. Estimates of the developmenta l threshold temperature from oviposition to adult emergence varied bet ween 6.0 degrees C and 7.1 degrees C with a day-degree (DD) requiremen t of between 256 and 280 depending on the method of analysis used. Usi ng the DD requirement derived from a weighted linear regression (261 D D above a threshold of 6.8 degrees C) in combination with the required value from adult emergence to first oviposition, suggests a total req uirement of approximately 374 DD above 6.8 degrees C per generation. T hese data, together with climate records, indicate that one to three g enerations are possible each year, depending on the available DD and o rigin of the population. The advantages and limitations of different a pproaches and models used to estimate developmental thresholds and DD requirements are discussed.