Biology of Laricobius nigrinus Fender (Coleoptera: Derodontidae) and its potential as a biological control agent of the hemlock woolly adelgid, Adelges tsugae Annand (Homoptera: Adelgidae) in the eastern United States

The biology of Laricobius nigrinus Fender (Coleoptera: Derodontidae) and its potential for control of hemlock woolly adelgid (HWA), Adelges tsugae Annand (Homoptera: Adelgidae) in the eastern United States were investigated. Laricobius nigrinus completed development on HWA in laboratory studies. There are four larval instars. Mean larval consumption was 225.9 and 252.3 HWA eggs at 12 and 18 °C, respectively. Post-aestivation activity period was 36.6 and 30.8 weeks for males and females at 13 °C, respectively. Mean lifetime fecundity was 100.8 eggs over a mean ovipositional period of 13.2 weeks.

Laricobius nigrinus is host specific. In paired-choice and no-choice oviposition tests, L. nigrinus laid more eggs in HWA ovisacs than other test species. Laricobius nigrinus consumed more eggs of HWA than eggs of Adelges piceae (Ratzeburg) and Pineus strobi (Hartig) but not of Adelges abietis (L.). In larval development tests, L. nigrinus only completed development on HWA.

A two-year field study conducted in British Columbia showed that: 1) the life cycle of HWA in British Columbia is similar to that previously reported in Virginia and Connecticut; 2) L. nigrinus adults undergo an aestival diapause that coincides with the diapause of the first instar HWA sistens. Adult activity beginning in autumn coincides with resumption of development of the sistens generation; 3) oviposition and subsequent larval development of L. nigrinus coincide with oviposition by HWA sistens adults.

Temperature-dependent development was determined for egg, larval, pre-pupal and pupal stages of L. nigrinus at five constant temperatures (9, 12, 15, 18 and 21 °C). Development time was inversely proportional to temperature between 9 and 18 °C. Laricobius nigrinus did not complete development at 21 °C. Minimum developmental temperatures for eggs (6.5 °C), larvae (5.1 °C), pre-pupae (3.8 °C), and pupae (3.1 °C) were determined by extrapolation of linear regression equations to the x-intercept. Degree-days calculated for eggs, larvae, pre-pupae, pupae, and egg to adult were 54.6, 161.3, 196.1, 212.8, and 666.7, respectively. The laboratory derived degree-day model for predicting egg hatch was validated with field data collected in British Columbia. Observed median egg hatch at three of four sites over two years was within 15% of the predicted DD value (54.6).