Oklahoma’s continued mild, extremely dry weather has led to the rise of a significant pest in the state’s south central to southwestern alfalfa fields—the spotted alfalfa aphid.
This pest is potentially more devastating than other aphids that occur in Oklahoma alfalfa fields and populations are expanding rapidly, said Phil Mulder, head of the Oklahoma State University Department of Entomology and Plant Pathology.
“We began the season with severe limitations in soil moisture and then cowpea aphid populations exploded,” he said. “Many growers elected to control these pests early and hoped that alfalfa weevils would be simultaneously managed.”
Unfortunately, growers who made multiple insecticide applications for aphids and weevils may also have eliminated beneficial organisms such as ladybird beetles and parasitic wasps that can help control pest populations naturally.
“The question at this point concerns the grower’s approach to these dangerous spotted aphid populations and what if anything should be done about adult alfalfa weevils,” Mulder said. “I feel strongly that multiple applications should be avoided.”
Mulder suggests that alfalfa producers who have managed to preserve their first cutting and who see damaging spotted aphid populations increasing should consider an early harvest.
“Once the hay is baled and removed from the field then serious consideration should be (given) for a stubble treatment with Lorsban, Dimethoate or Lannate insecticides,” he said. “Don’t assume that the harvest operation is going to control the aphids.”
Producers who were fortunate enough to avoid the spotted aphid problem but now have adult weevils emerging also might consider an early harvest and stubble treatment.
In most years, adult weevils will emerge and go into a summer aestivation or “summer sleep” period where they are not laying eggs or harming the alfalfa; however, this inactive period is triggered by more than warm temperatures.
“The day length that the earlier larval stage was exposed to can affect how soon the adults go into aestivation,” Mulder said. “Many of our weevil populations this year emerged early because of the relatively mild fall and an equally mild late January and February, (usually) the peak time for laying eggs.”
Adult weevils can be expected to feed on the remaining forage in alfalfa fields, thereby decreasing not only the quantity but the quality of the plants. They will feed on the epidermal tissue of stems, creating a very woody final product.
“Oklahoma applicators and alfalfa growers are certainly exploring some new territory in alfalfa insect management this year,” Mulder said. “Though significant spring rains could make current challenges a bit less formidable, everyone needs to be planning for the worst-case scenario. Hope is great, but it’s not a plan.”
A sampling of Oklahoma alfalfa fields indicates that growers who made a single, thorough application – 20 gallons per acre or more at slower speeds – earlier this year may have saved an application or two, but their alfalfa took a serious hit from heavy weevil and aphid feeding.
Similarly, growers who made two applications about 14 days apart took a less serious hit to their alfalfa crop and now beneficial organisms are starting to rebound.
In the Chickasha area, with two applications, there are noticeable spots of stunted alfalfa but relatively light aphid populations. However, adult weevils are emerging.
In the Pauls Valley area, where multiple applications have been made, beneficial organisms are less common and spotted aphids are increasing rapidly.