How do you determine if it is economical to spray for alfalfa pests? Use of the proper chemical for the pest in question, at the appropriate time, is critical to pest control. You may want to simply cut your hay early and then monitor closely for pests, rather than treat the stand with an insecticide then have to wait for the required interval before you can harvest. This article by Glenn Shewmaker, Extension Forage Specialist at the University of Idaho, outlines 7 points to consider in alfalfa pest control. His points include the following:
1. If you can see insect damage from your pickup, you are at least two weeks late in scouting for problems.
2. If you start spraying early in the season, you will probably need to continue throughout the season because you likely have taken out many of the beneficial insects that help reduce population surges of damaging insects.
3. Insect damage on alfalfa seedlings can reduce the stand for its life, so pay close attention to new stands.
4. It is usually best to cut hay early then monitor for resurgence of insect pests than to treat with an insecticide and wait for the preharvest interval.
5. An IPM strategy will optimize economic returns in alfalfa.
6. The more you use a specific insecticide, the less effective it may become because these applications can select for insects with resistant qualities to those insecticides.
7. Check extension information sources. Just because a crop adviser wants you to spray, it doesn’t mean it is economical. For example, there is no evidence that pesticide application for western flower thrips is warranted, despite the leaf distortion they cause.
Alfalfa weevil may be the first insect pest to appear in early spring. A common recommendation is to cut or spray when 40 percent of terminal buds show chewing damage.
Most thresholds are established based on sweep net data, so invest in one just as you would a soil probe for soil sampling. The standard sweep net is 15 inches in diameter with a 26-inch handle. The net should be swung in a 180-degree arc, such that the net rim strikes the top 6 inches of alfalfa growth. Sweep right to left, take a step and take a second sweep from left to right. Repeat until the desired number of sweeps is taken for the pest management guide you are using. After the last sweep, quickly pull up the net to force the insects into the bottom of the net bag and grasp the net bag about half way up. Slowly invert the bag to count insects. Some slow-moving insects, such as caterpillar larvae, can be emptied into a white bucket to be counted. Large numbers of active insects may require chilling the entire net bag to count highly active insects.
It is important that the proper chemical is used to target the proper insect and stage of development, and that the pesticide is registered for that use in your area. Pesticide registrations change frequently, so you must follow the label and beware of “internet solutions.” Most state extension services have web resources with reliable information. Industry representatives and websites are also excellent resources of information. Beware of “spray and pray” salesmen who think there is a silver bullet for every problem.
We must be good stewards of the technology for pest management, or it will become rapidly ineffective or illegal to use. Think of the long-term effect of applying pesticides. The most practical and economic strategy may be to cut early and monitor closely after harvest.