Mortality of Metarhizium anisopliae -infected wireworms (Coleoptera: Elateridae) and feeding on wheat seedlings are affected by wireworm weight

As some wireworm species are notorious pests of common wheat, Triticum aestivum (Vernon et al. 2009), a small study was conducted at the Pacific Agri-Food Research Centre (PARC) in Agassiz, BC in August 2009, to determine whether the number of germinating wheat seedlings (cv. AC Barrie) killed by the dusky wireworm, Agriotes obscurus, is affected by the number and size of the wireworms that seedlings are exposed to. An unexpected factor appeared at the end of the study in that many wireworms were infected with Metarhizium anisopliae, resulting in considerable mortality. This factor precluded us from meeting some of the initial objectives of the experiment, but still allowed us to determine if mortality of seedlings was affected by wireworm weight and number, and if mortality of the wireworms from M. anisopliae infection was affected by their weight. We filled 160 500 ml circular (dia = 11 cm, height = 8 cm) plastic containers (Plastipak Industries, Inc., La Prairie, QC) with 500.0 (+/0.2) g of soil collected from a field at PARC, Agassiz in 2009. The soil was sieved through a 2 mm x 2 mm screen to remove rocks and organic material, made up to 20% moisture by weight, and homogenized. Containers with soil were placed in a walk-in cooler set at 15.0 +/-0.5°C to mimic soil temperature conditions in spring when wheat is normally planted. Wireworms were weighed individually on August 7, and 0-4 wireworms were placed in each container (Table 1). All wireworms were collected at PARC in April 2009 and stored in 40l Rubbermaid tubs without food, at 8-10°C, until 1 wk before the study, when tubs were brought up to room temperature and food baits (a cup of 100 ml moist vermiculite mixed with 10 ml wheat seed) placed inside. Only mobile wireworms, appearing healthy (Vernon et al. 2008) and actively feeding 2 to 3 d prior to placement in the containers were used for this study. Wireworms selected ranged considerably in weight (range: 6.6 to 46.4 mg; Table 1), but all wireworms in individual containers were similar in weight (within 5.0 mg), and an attempt was made to have an equal number of similar sized wireworms for each of the 1, 2, and 3 or 4 wireworm densities to determine the effect of wireworm weight on the number of wheat seedlings killed. Two days after wireworms were placed in containers, 21 untreated wheat seeds were planted 2 cm deep in small pre-made holes. Seeds were spaced at equal distance (1.75 cm) from each other, in a 3, 5, 5, 5, 3-grid pattern. After planting, groups of eight containers were placed in 26 cm x 47 cm x 6 cm deep nursery flats (Eddi’s Wholesale Garden Supplies, Ltd., Surrey, BC), 1.0l cold water added between the containers in the flat, and flats covered with 14 cm high transparent plastic domes (Eddi’s Wholesale) to prevent desiccation of the upper layer of soil. After planting, containers were subjected to a 12:12 light:dark regimen. Seedling emergence was first observed 5 d after planting, stand counts were conducted 8 d (when domes were permanently removed due to the length of plant shoots) and 15 d after planting. Wireworms were removed 25 d after planting and their health evaluated (Vernon et al. 2008). This revealed that only 123 of 255 larvae were alive, the rest having died from Metarhizium infection, most likely within the first two weeks of the study as evident from the extent of mould formation on the surface of the cadavers. Analysis of the proportion of wireworms dead in each 38 J. ENTOMOL. SOC. BRIT. COLUMBIA 108, DECEMBER 2011