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Alfalfa weevil resistance investigated

Widespread deltamethrin resistance found in southern Alberta.

April 18, 2024  By Bruce Barker


Unsprayed plots had severe alfalfa weevil damage with yield loss of approximately 90 per cent. Photo courtesy of Jennifer Retzalff.

These alfalfa weevils wobble but they don’t fall down. That’s the finding from southern Alberta research that looked into whether alfalfa weevils were resistant to the insecticide deltamethrin (Decis; Group 3). 

“Alfalfa weevil can cause damage, particularly in seed fields, by stripping the plant and leaving skeleton-like leaves that result in a huge reduction in photosynthesis and resulting yield,” says entomologist Boyd Mori with the University of Alberta.

The alfalfa weevil overwinters as an adult under plant material along field margins. The adult is described as four to five mm long (approximately 3/8 inch) with a weevil snout and dark brown stripe from the top of the head down the middle of two-thirds of the body. Eggs are laid in May and hatch in four to 21 days. The green larvae, typically up to 10 mm long (about 3/8 inch) with a black head and white stripe down the body, begin to feed on stems, leaves and buds for three to four weeks, peaking in mid-June to mid-July. The larvae then pupate, and new adults emerge later in the year before overwintering.

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“Fortunately, there is only one generation per year,” says Mori.

The first suspected insecticide resistance was noted in Alberta and other parts of the Prairies in 2015. The suspicion was warranted since alfalfa weevil insecticide resistance was first noted in the USA in the 1960s, followed by confirmation of pyrethroid (Group 3) resistance in California in the 2010s. In 2015, Alberta Agriculture entomologist Scott Meers conducted a rapid test on a population in southern Alberta and concluded that pyrethroid resistance existed.

By 2018, there seemed to be a resurgence of alfalfa weevil on the Prairies, leading to research into whether insecticide resistance could be the cause. In 2018, two alfalfa weevil populations with suspected resistance were collected near Rosemary, Alta., along with a population at Lethbridge, Alta. The Lethbridge population had not been exposed to an insecticide and was assumed to be susceptible to insecticides.

Using a protocol established by the Insecticide Resistance Action Committee, 10 adult weevils were added to a vial and replicated with six to seven vials. The weevils were exposed to the recommended high rate of deltamethrin, one-half rate, one-quarter rate and a control without insecticide. The one-half and full rate of deltamethrin never achieved more than 60 per cent control with the two Rosemary populations. One hundred per cent control was achieved with the Lethbridge population. 

Alfalfa weevil larvae feeding on alfalfa.
Photo courtesy of Larry Grenkow.

“If mortality is less than 90 per cent, the population is considered to have insecticide resistance,” says Mori.

Another simplified method was tested on the three populations. Alfalfa plants were treated with deltamethrin and then exposed to alfalfa weevils. The Rosemary populations caused feeding damage, but the Lethbridge population did not.

In 2019, alfalfa weevil populations were again collected from Lethbridge and one site at Rosemary. Using the same vial test method, the populations were exposed to 0.1X, 1X, 10X and 100X of the recommended rate of deltamethrin. The purpose was to see how much insecticide was needed to control the populations. At Lethbridge, the 1X rate achieved over 90 per cent control, but the best control of the Rosemary population never reached 40 per cent even at the 100X rate.

In the spring of 2022, 15 grower fields in southern Alberta were sampled for deltamethrin resistance testing by Michelle Reid, an MSc student supervised by Mori. Mortality was on average less than 60 per cent, indicating widespread resistance in the southern Alberta alfalfa seed production region.

Management options
Scouting for alfalfa weevil should occur during the May and June timeframe when larvae emerge and start to feed on alfalfa plants. In hay crops, collect 30 stems in an M-shaped pattern and beat the stems in a pail to knock off the larvae. The economic threshold is one larvae per stem for plants less than 12 inches (30 cm) tall and two larvae per stem if the plant is less than 16 inches (40 cm) tall. But if three larvae per stem are found, action is required regardless of the height of the crop.

Alfalfa weevil adult.
Photo courtesy of Ian Grettenberger.

Economic thresholds for alfalfa seed crops are different than forage crops. Twenty to 25 larvae per 90-degree sweep or 35 to 50 per cent of leaf tips showing damage meets the economic threshold level.

Several parasitoids can provide biological control of alfalfa weevil. Bathyplectes curculionis and Oomyzus incertus are two parasitic wasps that can parasitize alfalfa weevil larvae. Reid and Mori collected parasitoids from southern Alberta in 2020 and 2021 to see what levels of control they were able to achieve. The parasitism rates varied widely across the sites ranging from zero to 90 per cent.

Based on the research, Mori says insecticide resistance to deltamethrin is well established in the Rosemary area, and that pyrethroid insecticides should be avoided in this region. He says preliminary research has found that organophosphates are still effective. Further research is needed to find out how widespread resistance is in Western Canada, and there is a need for a field-based quick test to rapidly detect resistance. 

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