Insecticide For Pastures

Treatment Threshold. The decision to treat for fall armyworms depends on the stage of the armyworms and the intended use of the forage. A population of 2 to 3 or more fall armyworms per square foot is a reasonable treatment threshold. As with other pests, timing is important. If infestations are detected too late, the damage may already have been done.

If necessary, treat with insecticides at the right time. Small fall armyworms are much easier to kill than larger ones. Some products will not control large larvae at all. If you check an area properly, you can determine the extent of an infestation and spot-treat.

Frequently, mowing is the best option for salvaging a hay crop. When this approach is taken, it may be possible to avoid using an insecticide. The harvesting operation kills some caterpillars directly, and others die from exposure to the high soil surface temperatures that occur after harvest. Fall armyworms that survive the harvest operation can continue to feed on the cut hay until it dries down. The caterpillars are most likely to survive if we have relatively cool weather conditions after harvest. The faster hay can be raked and baled, the better. If extremely high numbers of caterpillars are found or cool weather is expected, harvesting alone may not be enough to stop the armyworms.

Insecticidal Control. There are numerous insecticides that can be used for controlling fall armyworm caterpillars in perennial grass forages (table 1). Some, but not all of these, can also be used in annual grass forages. It is important to read the directions for use (the label) of an insecticide carefully to make sure that the product can be applied to a particular type of forage. The label also contains information on how to mix and apply the product for maximum efficacy. Insecticides have two names. Most of us are more familiar with the trade name of the insecticide, because that is the marketing name. However, it is important to be familiar with the insecticide’s common name, or active ingredient, which can be found in the active ingredient section of the product label. The common name is typically written in lowercase letters to distinguish it from the trade name. The same insecticide may be sold under several trade names. The most up-to-date list of what insecticides are registered can be found in Extension publication IPM-0028, Pastures and Forage Crops IPM Guide.

The insecticides vary in price; grazing and harvesting intervals; mode of action; speed of action; and safety to the environment and to the applicator. The insecticide label usually recommends a range of application rates. In general, use higher rates of insecticides (and higher volumes of water) when the grass is thick, when fall armyworm populations are high, when caterpillars are large, and to get the longest residual effect. Some insecticides will kill only smaller caterpillars, so check the comments about each insecticide. Pay close attention to the number of times a particular insecticide can be applied per hay cutting and per season. Within a season, rotate between different modes of action, if possible.