July 23, 2012 17:51
BY MICHEL TREMBLAY, PAG.
PROVINCIAL SPECIALIST, FORAGE CROPS
The 2012 growing season has been characterized by significant rainfall across the agricultural zone of Saskatchewan, following a dry, warm winter. In Saskatchewan, spring precipitation is the largest single determinant of yield of cool season forage species. With favorable soil moisture present in nearly all areas of the province, a good forage crop should be expected. Some producers have noticed that their alfalfa fields are not yielding, considering the soil moisture present. The following factors may be contributing to decreased alfalfa vigor and yield.
The alfalfa weevil (Hypera postica) is a pest of alfalfa crops, and is increasing in occurrence in Saskatchewan. Alfalfa weevils have been observed predominately in the southeastern and east-central parts of the province in alfalfa hay and seed fields. Adult weevils are approximately 5 mm in length, brown in colour, with a darker brown stripe from the head running down the back. The alfalfa weevil is a snout beetle, with a pronounced hook shaped proboscis at its anterior end. The larvae, when newly hatched, are yellowish green. At maturity, larvae are approximately 8 mm in length, and have a black head and a white stripe down the centre of its back. Adult weevils overwinter under plant debris and soil in and around alfalfa fields. Weevils emerge in spring and begin feeding on alfalfa leaves, creating round holes in the leaves. Females, when ready to lay eggs, chew a hole in the stem of the alfalfa plant and deposit from one to 40 eggs per stem. The bright yellow eggs can be seen with the naked eye if the stem is cut open. Eggs hatch one to two weeks after being laid, and the emerging larvae initially feed within the stem before moving to the developing buds, then newest leaves.
Alfalfa weevil larvae leaf damage.
Source: Saskatchewan Agriculture
Damage begins as pinholes and progresses to extensive feeding damage to leaf surfaces between veins, resulting in a ragged, skeletonised leaf. Heavily infested fields may not have flowers present, as the larvae will remove developing inflorescences. Often the first sign of weevil damage is the discoloration of the crop as the larvae feed. Evident from the field edge, the crop will develop a whitish sheen, or frosted appearance, due to foliar damage.
Alfalfa weevil larvae.
Source: Saskatchewan Agriculture
Larvae feeding occurs predominantly early in the season, in mid-June to mid-July. Mature larvae move down to the base of the plant or onto the soil and spin a lace-like cocoon. The adults emerge from the cocoon in one to two weeks. The larvae represent the most destructive stage of the alfalfa weevil life cycle, and most weevil damage occurs on the first cut. Usually a single generation of the weevil occurs per season in northern climates.
The most cost effective control can be cultural. Cutting when the potential for significant weevil damage becomes apparent will stop yield losses. If the infestation is severe and early cutting is not feasible, alfalfa weevils can be controlled by using insecticides as per economic thresholds indicated below.
Economic thresholds for alfalfa weevil pesticide application
Foliage: 35-50 per cent of foliage tips show feeding damage.
Larvae: 20-30 3rd/4th instar larvae per 90o sweep of insect sweep net.
30 cm crop height and one larva per stem.
40 cm crop height and two larvae per stem.
Three larvae per stem requires immediate action regardless of height of crop.
Two or more active larvae per crown (four to eight larvae per sq. ft) on regrowth after the first cut.