Be Proactive About Agronomic Crop Pests
Mites in Corn
The warmer weather and expected drought conditions in the West could make for a suitable environment for outbreaks of mites this season. In the last several years, a number of Utah counties have had to deal with Bank’s grass mite and sometimes two-spotted spider mite in corn. Warm and dry conditions allow mites to build up rapidly because their generation times are shortened, they feed more, road dust builds up on corn leaves creating favorable mite habitat, and their natural predators are less effective under these conditions. In addition, drought-stressed plants are thought to have increased availability of amino acids allowing mites to thrive. With this in mind, proper irrigation is key for avoiding mite outbreaks and monitoring mite populations is a useful tool to decide whether treatment is warranted.
Start with monitoring drought-stressed sections of a field for mites. For established mite infestations, the general guideline for mite treatment is if damage on the lower third of the plant is visible and mite colonies are present on the middle third of the corn plants. Note that mite treatments when corn has reached the hard-dough stage are too late to provide an economic benefit.
Using a 15-in diameter sweep net, alfalfa weevil larvae can be calculated on a per sweep basis to determine whether management is recommended.
Early season monitoring of alfalfa weevil is key for successful suppression and improving the bottom line. Weevil damage occurs early in the season around the first cutting. Therefore the best success for management is monitoring weevil larvae with a sweep net (or stem sampling) regularly before the first cutting when stems are at least 10 inches tall. In 2008, USU county faculty conducted a study investigating commercial alfalfa fields where insecticides were used to treat weevil versus fields where weevil was not treated. They found that yield was not affected by treatment of weevil because weevils did not reach economic thresholds that year (<10 weevil larvae per sweep) - treatment would not have been recommended. Sampling in this case would have helped growers' bottom line. Unnecessary treatment of weevil, particularly with pyrethroid insecticides, can also lead to outbreaks of other pests like aphids that would require additional treatment costs.
Unfortunately, weevils are most problematic following a warm spring because weevils are active early in the season as alfalfa starts to grow. In cool springs (consistent daytime temperatures of 40-48°F) alfalfa is able to get a head-start and sometimes “outgrow” weevils which remain inactive at these cool temperatures. In a warm spring it is likely that monitoring will reveal that weevil numbers have exceeded an economic threshold (more than 15-20 weevil larvae per sweep) and require treatment.
If the threshold is exceeded, what are the options? If the threshold is reached within 1 to 2 weeks of anticipated harvest, consider harvesting hay early and quickly removing bales from the field as an as an alternative to insecticides. Although some growers find it useful to run a spring-tooth harrow through the field following a cutting to mechanically crush weevil stages, it is generally not recommended because of the damage it can cause to alfalfa crowns and allow entrance for disease pathogens. The limited research on harrow use for weevil suppression does not support a measurable benefit. Finally, monitoring early can allow for better timing of any necessary insecticide treatments.
-Ricardo Ramirez, Extension Entomologist
Evans, E.W. 1989. The alfalfa weevil in Utah. Utah State University Extension Publication No. 58.
Higgins, R.A., S.L. Blodgett, and A.W. Lenssen. 1989. Alfalfa weevil management in Kansas: Non-chemical controls. Kansas State University Extension Publication No. 115.
Peairs, F.B. 2010. Spider mites in corn. Colorado State University Extension Publication No. 5.555.