Alfalfa weevil is a major alfalfa pest in Ontario. Larvae feeding on alfalfa leaves can reduce both yield and forage quality. 

The simple answer

As alfalfa gets taller it can tolerate more weevils before action is necessary to protect forage yield and quality. Action thresholds for managing alfalfa weevil are as follows: 

Stem Height	Number of larvae per stem
Up to 30 cm (12 in.)	More than 1
Up to 40 cm (16 in.)	2 or more
Any height	More than 3

• Cutting alfalfa early can reduce alfalfa weevil populations. 
• If 40% of plant tips show signs of weevil feeding, and it is more than 7-10 days until the preferred harvest date, consider spraying an insecticide.  
• Check the label first: some insecticides offer suppression rather than control. 
• With severe infestations, monitor weevil activity on stubble regrowth for 5 to 7 days. The characteristic symptom is the alfalfa plant does not “green up” due to larvae and newly hatched adults feeding on the developing crown buds. The presence of 2 or more active larvae per crown, or 4 to 8 larvae per square foot indicates a need to spray the stubble with insecticide. 

A little more information

Alfalfa weevil larvae begin feeding in mid- to late-May, beginning with the leaf buds. The larvae then move to leaves near the stem tips, where damage starts off as pinholes (Figure 1) and progresses until no leaf tissue remains between the veins. This is called “skeletonization”. Heavy infestations may look frosted from a distance, as the shredded leaves appear greyish white.

When alfalfa is harvested, alfalfa weevil larvae are removed from the field, and remaining larvae either dry out or starve. This reduces the population and usually gives alfalfa regrowth a chance to get ahead of feeding pressure. 

The Ontario Crop Protection Hub is OMAFA’s official crop protection resource. Insecticides labelled for use on alfalfa against alfalfa weevil can be found here. 

The full story

There is only one generation of alfalfa weevils per year. Adults overwinter in plant debris, so mild winters may increase survival. Eggs are laid in alfalfa stems in May. When larvae hatch, they climb up the stems to feed on leaf and flower buds. Hot, dry springs slow the development of pathogens that keep alfalfa weevil populations in check. Fields located in areas with frequent insecticide use are more likely to have alfalfa weevil problems, since parasitoid wasp populations are suppressed. 

Scout fields twice a week from mid-May until 10 days after first cut. To scout, check out several areas throughout the field, especially areas with shallow soils or southerly slopes. Walk an M-shaped pattern while collecting 30 stems. Hold the stems inside a clean white bucket and beat them against the sides to knock off the weevil larvae. Count how many 3^rd and 4^th stage instar larvae are in the bucket (Figure 2). Do not include any larvae less than 3 mm long, or diseased larvae of any size, which are slow-moving and yellow or tan (Figure 3). Divide the total of healthy 3^rd and 4^th stage larvae by 30 to get an average per stem. 

Biological control is the main way alfalfa weevil populations are kept in check in Ontario. Parasitoid wasps (Microctonus aethiopoides and M. colesi) attack adult and larval weevils and lay their eggs inside them. When the eggs hatch, the wasp larvae feed on their host, which eventually kills it. A fungus (Zoophthora phythonomi) also infects alfalfa weevil larvae. The larvae will curl around the leaves at the top of the plant and turn brown. These biological control organisms are widespread throughout the province. It is only when weather conditions allow alfalfa weevil development to outpace the biological control that other interventions become necessary. 

Cutting alfalfa early can reduce alfalfa weevil populations. However, damage from larval feeding is generally not apparent until the crop reaches bud stage, which coincides with peak forage quality and the earliest first cut harvest dates. In years where the larvae reach threshold before the alfalfa reaches bud stage, cutting early can reduce second cut yield and vigour. These high larval populations may also be above threshold on the stubble after cutting, and feed faster than the alfalfa can regrow. 

As a result of the Pest Management Regulatory Agency re-evaluation, all feed uses are cancelled for lambda-cyhalothrin products, effective April 29, 2023. Any crops treated with lambda-cyhalothrin products cannot be used directly or as by-products in feed to livestock. As alfalfa is grown in Ontario exclusively for livestock feed, lambda-cyhalothrin products (Matador, Silencer, and Labamba) cannot be used on this crop.  

Parasitoid wasps are affected by insecticides. Spraying for alfalfa weevils increases the risk of future outbreaks, because wasp populations are reduced.