Survival of male click beetles, Agriotes obscurus L., (Coleoptera: Elateridae) during and after storage at different temperatures

Journal of the Entomological Society of British Columbia, Dec 2008 by Van Herk, Willem G, Kabaluk, J Todd, Lam, Viola W M, Vernon, Robert S

The dusky wireworm, Agrietes obscurus L. (Coleoptera: Elateridae) is a significant pest of vegetable and field crops in the Fraser Valley of BC (Vernon et al. 2001). Adults emerge from the soil from late March through May and die soon after mating (males) or egg laying (Brian 1947), restricting their availability for research and necessitating storage methods that prolong survival. Here we compare beetle survival during and after storage at various temperatures for various durations.

Male A. obscurus beetles were collected at the Pacific Agri-Food Research Centre (Agassiz, BC) during their peak emergence (1st wk of May, 2005) and placed in groups of 10 beetles into 150 ml plastic containers with a freshly-cut apple piece (approx. 2 x 2 x 0.5cm) placed on 70 g of moist sandy clay-loam soil. Containers were put in growth chambers (15 per chamber) (Controlled Environments Ltd., Winnipeg, MB) set at 5, 8.5, 12, and 20 °C (± 0.1 °C), and dead beetles removed, and apple pieces replaced, biweekly. Three containers were removed from each chamber after 2, 4, 6, 8, and 10 wks to determine subsequent survival at room temperature (RT) (21 ±1 ° C), and beetles transferred to 10cm Petri dishes (one per container) placed on 5cm high racks inside Styrofoam boxes. Each box (36 x 26 x 9cm deep) contained 2.5ml water to prevent desiccation; a 1cm gap between the box and its lid permitted air exchange. Beetle feeding and observation continued (as above) for up to 12 wks.

Beetle mortality during storage was highest at 20 °C, and similar at 5, 8.5, and 12° C for the first 6 wks but considerably lower at 8.5 °C thereafter. Beetle mortality was rapid within the first 2 wks of storage, and increased with duration for all temperatures except 8.5 °C (Table 1). Regression (stepwise, backward; Proc REG, SAS 9.1; SAS Institute 2002) of the proportion dead per container during storage (m) to temperature (t) and duration (d) yielded the following: m = 0.003r^sup 2^ - 0.022t 0.050d (SE slopes: 0.0005, 0.012, 0.009 respectively; P

For post-storage survival analysis, each beetle was considered an experimental unit, and storage temperatures were compared with Kaplan-Meier survival analysis (Proc LIFETEST, SAS 9.1), with strata duration (Cox and Oakes 1984). The survival time of 50% of beetles (ST50) was subsequently estimated by modelling survivorship for each storage temperature-duration combination. Survivorship curves were compared with log-rank tests; ST50 values were compared using 95% confidence intervals.

Post-storage beetle mortality was rapid regardless of previous storage temperature. Beetles stored at 12 °C died more quickly at RT than those stored at 8.5 or 5 °C (χ^sup 2^ = 12.64, P = 0.0004; χ^sup 2^ = 19.27, P 0.05, Table 1). Surprisingly, beetles stored for 2 wks at 5, 8.5, or 12 °C survived longer at RT than those stored at 20 °C (Table 1).

These results indicate that storage at lower temperatures prolongs male click beetle survival, and that storage at 8.5 °C caused highest overall survival. Future research should investigate how cold storage conditions affects beetle physiology.

We thank Drs. D. Gray, C. Stevenson, D. Raworth and R. Bennett, and sundry reviewers for valuable advice.

REFERENCES

Brian, M.V. 1947. On the ecology of beetles of the genus Agrietes with special reference to A. obscurus. Journal of Animal Ecology 16: 210-224.

Cox, D.R., and D. Oakes. 1984. Analysis of Survival Data. London: Chapman and Hall.

SAS Institute Inc. 2002. SAS/STAT user's guide, Version 9.1. SAS Institute Inc. Cary, North Carolina.

Vernon, R.S., E. LaGasa, and H. Philip. 2001. Geographic and temporal distribution of Agriotes obscurus and A. lineatus (Coleoptera: Elateridae) in British Columbia and Washington as determined by pheromone trap surveys. Journal of the Entomological Society of British Columbia 98: 257-265.

WILLEM G. VAN HERK1, J. TODD KABALUK1, VIOLA W.M. LAM1 and ROBERT S. VERNON1

1 Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, P.O. Box 1000, Agassiz, British Columbia, Canada VOM 1A0