The alfalfa weevil (Hypera postica) is a pest of alfalfa crops, and is increasing in occurrence in Saskatchewan, predominately in the south east and east central parts of the province in alfalfa hay and seed fields. Alfalfa weevil was first reported in North America in 1904, and quickly became a major pest on alfalfa throughout much of the United States. Although it has been present in eastern Canada and Manitoba for some time, weevil damage has been on the increase in Saskatchewan in recent years and can now be found in all regions of the province.
How to Identify the Alfalfa Weevil
Adult weevils are:
- Approximately 5 mm long;
- Brown; and
- Have a dark brown stripe from the head running down the back.
The alfalfa weevil is a snout beetle, with a pronounced hook-shaped proboscis. 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 their back.
Alfalfa Weevil Life Cycle
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. The bright yellow eggs can be seen with the naked eye if the stem is cut open.
Eggs hatch one to two weeks after laying. Larvae initially feed within the stem before moving to developing buds and newest leaves. Damage begins as pinholes and progresses to extensive feeding damage to leaf surfaces between veins, resulting in a ragged, skeletonized leaf. Often the first sign of weevil damage is discoloration of the crop as larvae feed. The crop will develop a whitish sheen or frosted appearance, due to foliar damage.
Larvae feeding occurs in May and June. In late June and early July, larvae move down to the base of the plant or onto the soil and spin a lace-like cocoon. Adults emerge from cocoons 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 weevil occurs per season in northern climates.
Field Scouting and Control of Alfalfa Weevil
Fields that are being monitored should be walked in a 'w' shaped pattern to efficiently and thoroughly check for the pest. Collect 30 stems while walking the field and place them into a white pail. Beat the stems against the side of the pail to dislodge the larvae. The economic thresholds for chemical control of alfalfa weevil are:
- Foliage: 35 to 50 per cent of foliage tips show feeding damage; and
- Larvae: 20-30 3rd/4th instar larvae per 180-degree sweep of insect sweep net.
- 30 cm crop height and one larvae/stem;
- 40 cm crop height and two larvae/stem; and
- Three larvae/stem requires immediate action regardless of height of crop.
Control of Alfalfa Weevil
Biological control can occur due to predation by other insects or fungal disease. Various wasps, lady bugs, lacewings and damsel bugs are known to attack weevil larvae. A fungal pathogen can also infect weevil larvae. Infected larvae are yellow or tan, and slow moving.
The most cost effective control can be cultural. Onset of maximum feeding damage by alfalfa weevil coincides with early bloom. Early bloom harvesting of alfalfa results in the best compromise between yield and quality during haymaking. Cutting when the potential for significant weevil damage becomes apparent will generally stop yield losses.
In the case of severe infestations and early cutting is not feasible, chemical control of alfalfa weevil can be accomplished by using insecticides. For further information, please see The Guide to Crop Protection.
Field surveillance for alfalfa seed weevil in areas reported to have the pest, and assessing the economic threshold of affected fields will determine the most effective control option and help minimize the economic impact of the pest.