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.
May 25, 2012 12:58
Nadia Mori, MSc, PAg, Regional Forage Specialist
Watrous Regional Services Office
Saskatchewan Ministry of Agriculture
Integrated pest management (IPM) is a well established practice in crop protection and can be a valuable approach in forage stand management. IPM means to have a well rounded weed and pest control plan which considers at all options from prevention to control methods available. The following components should be part of an IPM approach:
1. Monitor Weeds
Monitoring is the process of regularly inspecting pastures to determine if any undesirable plants are present. Scouting also identifies conditions which could favour the development of a weed infestation. For example a recently flooded area on a slightly saline soil may start to convert to foxtail barley.
2. Pest Identification and Biology
Correct pest identification is necessary in order to select appropriate and effective control measures. Consult with an agrologist or biologist if you are unsure about the identification of a weed or insect found in your pasture. Some basic understanding of the biology of the pest is also critical to effective control and prevention. For example, since annual weeds reproduce by seeds, control measures will be more effective if done before seeds are produced.
3. Weed Control
Weed control measures must be evaluated in order to select the most appropriate control measures and combine control methods effectively. Herbicide application is one form of control but other alternatives like providing rest during the growing season, mowing, targeted grazing, burning, biological controls and even hand rouging should all be considered. Each control method will have associated costs and make some solutions more economical. For example, cost of weed control procedure, cost of lost production, and cost of damage to non-target plants are some costs to be considered.
4. Evaluate Weed Control
Control measures must be evaluated to verify the degree of effectiveness. If adequate control has not been achieved, the reasons for the lack of effectiveness should be identified and corrected. Effects on non-target plants and impacts away from the target area must also be identified.
5. Recordkeeping and Program Management
A complete and accurate set of records is basic to any pest control program. Records will assist in identifying key information such as: which pests have been a problem; where the infestations occurred; how successful different control options proofed to be; what the actual cost of the chosen control option was; during which conditions control options worked or not; which conditions allow certain pests to become a problem (for example, site disturbance, drought conditions, or overgrazing).
For more information, please contact:
Watrous Regional Services Office (306-946-3220),
Agriculture Knowledge Centre (1-866-457-2377) or
Visit our website at www.agriculture.gov.sk.ca.