Lygus Bug and Alfalfa Weevil Insecticide Resistance in the Pacific Northwest

Lygus Bugs 2020

Lygus bugs were collected by sweep net and placed in a bug dorm (BioQuip, Inc.) and transported to the Environmental and Agricultural Entomology Laboratory located at Washington State University’s (WSU) Irrigated Agriculture Research and Extension Center near Prosser, WA. The Lygus bugs were provided with organically produced green beans for sustenance and held overnight. They were then aspirated into small snap‐top plastic vials in aliquots of approximately 10 adults and held for approximately 15 minutes. The Lygus bugs were then anesthetized with CO₂ and placed in a petri dish with a piece of filter paper on the base. These individual cohorts of approximately 10 Lygus bugs per petri dish were then subjected to increasing doses of each insecticide in our Potter precision spray tower in dilutions equivalent to 20 gal/ac. 

The rates for bifenthrin were 0, 1, 5, 10, and 50% of the maximum labeled field rate of 0.1 lb of active ingredient per acre for bifenthin. The rates for Lambda‐cyhalothrin were 0, 1, 10, 20, and 50% of the labeled field rate of 0.03 lb of active ingredient for lambda‐cyhalothrin. The rates for sulfoxaflor were 0, 1, 5, 10, and 20% of the labeled field rate of 0.071 lb of active ingredient per acre for sulfoxaflor. Each dilution rate for all three insecticides was replicated three times on cohorts of approximately 10 Lygus bugs per Petri dish. The Lygus bugs were provided with a 3 cm piece of green bean and held overnight and assessed for mortality at 24 hours after treatment. For analysis in PoloPLus, dose is converted into parts per million. Mortality is corrected in the serial dilutions against the water control treatment by Abbot’s formula, and the data are run through PoloPlus to calculate LD50 and LD90 values. These values represent the concentration of each insecticide required to kill 50 and 90% of each tested population, respectively.

Alfalfa Weevils 2021

To quantify the dose response of alfalfa weevil populations in Washington State, alfalfa weevil larva were collected at four locations in Washington State and transported back to the Environmental and Agricultural Entomology Laboratory at WSU Prosser. These weevils were then subjected to dose response bioassay via our Potter precision spray tower. Larva populations were collected from sites including an alfalfa forage field in Goldendale, an alfalfa field at WSU Othello, an alfalfa field near Gardena, and an alfalfa field at WSU IAREC. The four insecticides tested included the registered products Warrior II (lambda‐cyhalothrin), Mustang Maxx (zeta‐cypermethrin), Baythroid (beta‐cyfluthrin), and Lorsban Advanced (chlorpyrifos).

Serial dilutions were completed for each insecticide in a dilution equivalent to 20 gal/ac at 100, 75, 50, 25, 10, 5, and 0% of the maximum field rate for Lorsban Advanced and Warrior II and 75, 50, 25, 10, 5, and 0% of the maximum field rate for Mustang Maxx and Baythroid. Each treatment was applied to four replicates of five weevil grubs in a Petri dish with a filter paper bottom in 2 mL of solution in our Potter precision spray tower. The weevil larva were evaluated at 24 and 48 hours after treatment for mortality and survivorship. Subsequently, our data evaluations were completed on the weevil morality after 48 hours of exposure due to greater consistency of results. Weevil larva were considered dead when they failed to respond to being touched with a fine camel hair brush.

Lygus Bugs 2020

Table 1 provides the geographic region, GPS coordinates, host plant (Crop) the Lygus population was captured from, and the day the Lygus were exposed to bifenthrin. Also detailed are the PoloPlus‐calculated LD50 and LD90 concentrations for each Lygus population. Empty cells in the data for LD50 and LD90 are where PoloPlus failed to calculate values where the tested rates were either too high or low. Similar information is provided in Tables 2 and 3 for the insecticides lambda‐cyhalothrin and sulfoxaflor, respectively.

Note. LD50 and LD90 values represent the concentration of each insecticide required to kill 50 and 90% of each tested population, respectively.

Note. LD50 and LD90 values represent the concentration of each insecticide required to kill 50 and 90% of each tested population, respectively.

Note. LD50 and LD90 values represent the concentration of each insecticide required to kill 50 and 90% of each tested population, respectively.

The field rate of bifenthrin at 0.10 lb of active ingredient diluted in the equivalent of 20 gal/ac converts to 599 ppm. The greatest value for a LD90 was observed in an unmanaged alfalfa field in North Pasco. The LD90 was 752.2 ppm. The Lygus in this site were certainly tolerant to bifenthrin, but we do not think that this population was truly resistant to bifenthrin. Many of the populations were very susceptible to being killed by bifenthrin. Several of the locations/populations were sampled multiple times. The forage alfalfa location in Gardena was in close proximity to alfalfa produced for seed. The LD90 values in this location were fairly consistent among the sampling dates and dropped down quite a bit in September. Overall, the Lygus bugs that were tested among multiple crops/host plants across a large geography in the Columbia Basin of Washington State were not resistant to bifenthrin.

The field rate of lambda‐cyhalothrin at 0.03 lb of active ingredient diluted in the equivalent of 20 gal/ac converts to 1,875 ppm. The greatest value for a LD90 (2,365 ppm) was observed in a unmanaged alfalfa field in North Pasco. However, there were a greater number of locations including the alfalfa forage field near Gardena and the alfalfa forage field on the Roza site that exhibited greater tolerance in early August than on any other dates that these sites were sampled. The LD90 values were around 1,900 to 2,000 ppm. There were many sites in which the Lygus bugs were very susceptible to being killed by lambda‐cyhalothrin at recommended field rates. However, it appears that between the two pyrethroids, bifenthrin and lambda‐cyhalothrin, the Lygus in general are more tolerant to lambda‐cyhalothrin. Lambda‐cyhalothrin has been used extensively in the past on forage alfalfa and alfalfa grown for seed. Bifenthrin is not registered for use on forage alfalfa. Rather it is registered under a 24C Special Local Need registration in most western alfalfa seed growing states. This may be the reason we observed differences in the susceptibilities in several Lygus populations to lambda‐cyhalothrin versus bifenthrin.

The field rate of sulfoxaflor at 0.071 lb of active ingredient diluted in the equivalent of 20 gal/ac converts to 406 ppm. The greatest LD90, 253.4, was observed in a Lygus bug population collected from forage alfalfa on the WSU Roza site. This was an anomaly because the next greatest LD90 was in Gardena from a field of alfalfa produced for seed at 60.61. This seed field had been treated one to two times per growing season for the past five years with sulfoxaflor. These results provide a good snapshot for the relative susceptibility of Lygus bugs in south‐central Washington State to sulfoxaflor at an early period of its registration. We can check periodically in future years to quantify whether Lygus are gaining tolerance to sulfoxaflor.

Alfalfa Weevils 2021

Among all the four populations tested against the organophosphate chlorpyrifos and the three synthetic pyrethroids, only the Goldendale population exhibited resistance to lambda‐cyhalothrin, and the Roza population exhibited some resistance to chlorpyrifos (Figures 1–4). The revocation of chlorpyrifos for alfalfa weevil control will only put increased pressure on the synthetic pyrethroids and increase the likelihood of resistance developing to the pyrethroids.