Submitted to: Journal of British Columbia Entomological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 6, 2006
Publication Date: December 4, 2006
Citation: Horton, D.R. 2006. Quantitative relationship between potato tuber damage and counts of Pacific Coast wireworm (Coleoptera: Elateridae) in baits: seasonal effects. Journal of British Columbia Entomological Society, 103:37-48. Interpretive Summary: Potato growers are experiencing increasing difficulties in managing wireworms. These subterranean pests are difficult to sample, which means that growers who treat their fields with insecticides to control wireworms in general do so without knowing whether there is a need for treatment. Attempts at using baits composed of germinating seed or rolled oats have been inconsistent in the past for predicting wireworm numbers or damage potential. Researchers at the USDA-ARS, Yakima Agricultural Research Laboratory, Wapato, WA explored whether these inconsistencies could in part be related to wireworm movement in the soil. They showed that baiting during spring before and at planting time could be used to quantitatively predict end-of-the-season tuber damage, but that the exact relationship between numbers in baits and predicted damage changed week-to-week. A second study showed that wireworm depth in the soil changed during the baiting interval in spring, and results suggest that this movement by wireworms led to inconsistencies week-to-week in predicting damage with use of baits. The studies reported here will assist scientists in better understanding early season damage potential of wireworms, and should help to develop improved monitoring methods for early spring populations in potato fields.
Technical Abstract: Plots were baited with rolled oats in spring to assess the relationship between counts of Pacific coast wireworm, Limonius canus (Coleoptera: Elateridae) and damage to potato tubers. Baiting was done at 7 intervals beginning before planting of potatoes and ending following germination. Injury (percentage of tubers damaged or number of holes per tuber) showed a curvilinear relationship with increasing wireworm counts. Damage increased rapidly with increasing wireworm numbers at lower densities, eventually flattening out at very high wireworm counts. Wireworm counts in baits fluctuated seasonally, increasing from lows obtained during pre-planting samples to a peak just before potato germination, followed thereafter by declines in counts. Thus, baiting efficiency varied seasonally. This variation in efficiency led to date-to-date differences in predicted damage for a given wireworm count. I assessed depth of wireworms in the soil profile between late-March and mid-May, and found that a relatively large percentage of wireworms occurred deep in the soil (61-91 cm) until soil temperatures at 31 cm approached 17 oC in early- to mid-May. These results are consistent with the idea that low counts of wireworms in baits observed during the pre-planting samples occurred because a proportion of the population was deep in the soil. Low counts in baits following plant germination may have been due to the presence of competing food sources (i.e., the decaying seed piece and developing potato plant).