Irrigated, non-dormant alfalfa hay dominates the cropping system in the western United States. Alfalfa weevil (AW), Hypera postica, is the major yield reducing insect pest attacking alfalfa in the region (Fig. 1). Our economic threshold data showed that AW larvae can reduce the yield by half a ton per acre or more (Mostafa, 2019). Because of this economic consequence, growers use control measures against this serious pest. There are a variety of insecticides that can combat AW; however, almost all these insecticides are broad-spectrum. Our surveys of growers and pest control advisors showed that the efficacy of current broad-spectrum insecticides against alfalfa pests, particularly alfalfa weevil, is declining. We have evaluated the efficacy of an alternative selective chemical that could be used to replace broad-spectrum pesticides; however, the pace of registrations of novel insecticides is dilatory.

Insects, including AW, are known for their ability to develop resistance to insecticides. Resistance is a genetic phenomenon. In a population of an insect species, there may be a few individuals that carry the genes for resistance to a chemical. Upon exposure to insecticides, insects that do not carry the resistance genes die, thus allowing the individuals with the resistance genes to survive and reproduce, creating more resistant insects. With every subsequent generation and continued selection, the number of resistant insects increases; therefore, the insecticide becomes less effective in controlling the pest population.

We investigated lambda-cyhalothrin (Warrior II™) against AW to test for any resistance by comparing the dose response of field populations of AW from seven states using feeding assays. The results of these experiments will be helpful to growers and the agriculture industry in the western region and around the nation as they decide which insecticides to use on their crops and which ones are most effective.

Insect

Alfalfa weevil larvae were collected from alfalfa fields from 14 locations in seven states. These locations were near: Maricopa, AZ; Scottsdale, AZ; Tonopah, AZ; Wellton, AZ; Poston, AZ; Blythe, CA; Holtville, CA; Yreka, CA; Tulelake, CA; Radersburg, MT; Los Lunas, NM; Stillwater, OK; Muleshoe, TX; and Logan, UT. The sweep net collected samples were put into brown paper bags with some alfalfa plants, stored in a cooler, and shipped to our laboratory in Phoenix. Larvae of AW were collected and sorted from samples. Third-instar larvae were placed into cages overnight, for conditioning, with alfalfa plants as food.

Insecticide

The formulated insecticide product of lambda-cyhalothrin was diluted with distilled water. A series of dilutions was made to yield concentrations of 1500, 1000, 500, 250, 50, 10 ppm (parts per million), along with a control check of distilled water.

Procedures

The substrate used to hold the insecticide solution was alfalfa leaflets. Alfalfa leaflets were dipped individually in containers with the different insecticide concentrations. Each leaflet was then inserted into 1-oz plastic cup that contained a thin layer of agar. One, third-instar, AW larva was added to each cup and then capped. Six small holes were punctured into each cap. A total of 25 insects were evaluated at each concentration for each population. Insects were kept at room temperature (Fig 2). Results were recorded at 24 and 48 hours after treatment. Insect responses were assessed using four categories: alive, dead, lethargic and moribund. Insects described as dead showed no movement when probed and no legs kicking when flipped over. Insects described as moribund were unresponsive to probing, but legs were kicking and mouthparts or their body were still contracting. Insects described as lethargic showed slow, sluggish, and clumsy movements; they did not speed up when probed. Insects described as alive showed normal evasive tendencies when probed; quick, alert movements. For these experiments, dead and moribund were categorized as mortality, while lethargic and alive were categorized as alive.