The past couple years spider mites have put pressure on our area’s corn and the mites are at it again in 2013. There are two species we run into commonly, most often we find the Banks grass mite, and occasionally the two spotted spider mite. The two spotted spider mite has dark spots on either side of the body and can be more damaging than the more solidly colored around the its body edges Banks grass mite. Hot and dry conditions favor spider mites, and their populations can boom in these conditions.
Spider mites can be difficult to control, and often beneficials can provide control if enough are present in your field. Fields that have been treated to control Lepidopteron pests, such as southwestern corn borer and western bean cutworm, are often more likely to develop mite problems. Insecticide applications to control caterpillars in a field can actually knock back predator populations and cause mite populations to flare. This summer we have been working on a miticide trial in Hale County, and we actually had to flare the mites by spraying a pyrethroid before the mites started becoming problematic. We are comparing all possible products labeled for mite control: Portal, Oberon, Onager, and Zeal for best mite control, some at variable rates. We will be sharing the data from the trial as soon as it becomes available. We are as eager as you are to see the miticides compared to one another, hopefully before they cause any economic damage for you. At 3 DAT we had no statistically significant differences between any treatments and the untreated check, but noted several dead mites in all treatments. This is not uncommon due to the nature of our usable miticides.
A corn plant can tolerate reasonable mite damage to the lower leaves before mites become economic. Mites begin to be an economic threat when they near the ear leaf. When this leaf, the zero leaf, is nearing 50% coverage by mite colonies and the leaves below have heavy colonies and you note some mite movement above, it’s time to seriously consider a miticide treatment. This is a lower threshold then many of us utilized in years past, but remember all of our newer miticides do not have a very quick knockdown but rather take time to work and rely upon support from beneficials for absolute control. Many of the older chemistries might still be labeled for mites, but the mites have proven to be resistant to them. Meanwhile, these newer miticides are much more predator friendly, but if mites do reach ET, treatment needs to be swift. Only if your corn has reached a solid dent stage, and has near threshold mite populations, you may be able to consider holding off a treatment. When the corn is starting to dry down, mites do not cause as much economic damage. However, if the mite populations remain too high, they can cause lodging in the drying crop. Be sure to consider your options and take a good look at mite populations, plant stage, and predators before treating, but treat quickly if needed.