Quantitative relationship between potato tuber damage and counts of Pacific coast wireworm (Coleoptera: Elateridae) in baits: seasonal effects

Abstract

Experimental plots of potatoes were baited with rolled oats in spring to assess the relationship between counts of Pacific coast wireworm, Limonius canus (Coleoptera: Elateridae), and end-of-the-season damage to potato tubers. Baiting was done at seven intervals beginning before planting of potatoes and ending following plant emergence. Injury (percentage of tubers damaged or number of holes per tuber) showed a curvilinear relationship with increasing wireworm counts in baits. Damage increased rapidly with increasing wireworrn numbers at lower densities, eventually flattening out at very high counts. Wireworm counts in baits fluctuated seasonally, increasing from lows obtained during pre-planting samples to a peak just before plant emergence, followed thereafter by declines in counts. Thus, baiting efficiency varied seasonally. Low counts in baits during the pre-planting interval may have been due primarily to low soil temperatures, while declining counts following plant emergence may have been due to the presence of competing food sources (i.e., the seed piece and developing potato plant). I also assessed depth of wireworms in the soil profile between late-March and mid-May, and found that a relatively large percentage (approaching 25% on two dates) of wireworms occurred very deep in the soil (61-91 cm) until soil temperatures at 31 cm approached 17 °C in early- to mid-May. Thus, low counts in baits during the pre-planting samples may also have occurred in part because a proportion of the population was deep in the soil during this time interval. Seasonal variation in baiting efficiency led to date-to-date differences in predicted damage for a given wireworm count. Low efficiency during the preplanting interval would complicate efforts to use pre-planting baiting as a means to predict end-of-the-season tuber damage.