Estimation of alfalfa weevil, Hypera postica (Coleoptera: Curculionidae) economic damage based on the larval laboratory feeding rate

The alfalfa weevil (AW), Hypera postica (Gyllenhal) (Coleoptera: Curculionidae), is a major pest of alfalfa Medicago sativa L. (Fabaceae). Both adults and larvae feed on alfalfa foliage, damage and reduce hay yield and quality, especially in spring. Larval feeding rate was estimated based on alfalfa leaflet consumption under laboratory conditions (18 ± 2 °C, 60–70% RH). Adults did not consume after emergence, apparently due to diapause. The first instar larvae of AW (n=150) were removed immediately after hatching in the laboratory and were confined with alfalfa leaves. The leaf weights (g) were measured prior to and each 24h after larval feeding. The larvae fed for 33-35 days in the laboratory and underwent four instars. The results revealed that average larval consumption is 8.178±0.153 g leaf tissue during the whole larval stages. Our estimates show that insecticide treatment will cost 6,700,000-14,700,000 IRR/ha depending on market values. One kg alfalfa feed appreciates 40000 IRR. Hence, 167.5-367.5 kg of feed corresponds to control costs. If all larvae survive to pupate (whole lifetime consumption), 2.0-4.5 larvae m-2 are tolerable and need no action. However, if a total mortality of 25, 50, or 75% occurs with a constant rate, tolerable densities will increase to (2.5-5.4), (3.2-7.0), and (4.8-10.6) m-2, respectively. Assuming that an insecticide kills 90- 99% of the pest, 7.9-17.4 to 21.1-46.3 larvae m-2 will cause no loss if farmers intervene on time.