With the dry weather we have had recently, spider mites are starting to show up in a lot of cotton fields. Spider mites can be one of the most difficult pests to manage in cotton for several reasons. One of the main problems is that spider mites are rarely, if ever, uniformly distributed across a field. They begin to show up in clumps mostly scattered along field edges, but can be scattered throughout fields. This creates a lot of stress among managers when trying to determine how many “mite hotspots” are enough to justify a spray. This is compounded by the fact that the thresholds published in the insect control guide are vague and not very helpful.
We have had a couple of graduate students working on spider mite projects over the last few years to try to address this. We also had a regional project across the Mid-South to address the impact of infestation timing on cotton yields. The research showed that when spider mite infestations persist for longer than 14 days, even at low levels, significant yield losses are likely to occur in those areas. Keep in mind that the point when you start seeing symptoms in the field is not the start of an infestation. The mites are likely to have been there for a few days before the symptoms show up. As a result, the time from detection until yield losses occur is probably a few days shorter than the 14 days previously mentioned.
The big question is: When do I spray? There is no simple answer! The best advice that we can give is to watch areas where symptomology is occurring or mites have been observed and monitor whether or not these areas are increasing at each sample date. Several other factors can impact the severity of spider mites and should be considered when making management decisions. For instance, sprays going out for other pests such as tarnished plant bugs will likely flare spider mites, especially if mites are present at the time of the application. In these instances, it may be advisable to add a miticide into the tank to prevent the mites from getting worse. An alternative is to select insecticides that will not flare mites. The new tarnished plant bug insecticide, Transform, is less likely to flare mites than the organophosphates or neonicotinoids.
Weather is another important factor that should be considered. Spider mites prefer hot, dry conditions and are usually negatively affected by rainfall. However, rainfall has not consistently reduced spider mite applications over the last few years. In situations where we have gotten rain on fields with an active infestation of spider mites, the rainfall will knock the mites back for a few days, but they seem to blow right back up when it dries up. In other words, we rarely have situations where rainfall will eliminate a spray. The spray seems inevitable, but the rainfall will delay the application for a few days.
The next big question is what to use. Fortunately there are a lot of good miticides available to control spider mites. Unfortunately, most of those options can be fairly pricey. Abamectin seems to be the first choice in most situations because it has provided adequate control and is relatively inexpensive. However, we have been seeing less consistent control with abamectin over the last couple years in some areas. It is becoming less and less common to manage a spider mite infestation with a single application of abamectin, even at higher rates. The older miticide, Comite II, has been less consistent in recent years, but multiple applications can provide adequate control in some situations. Some of the “newer” miticides like Oberon, Zeal, and Portal should provide good consistent control, but they also have some limitations. They are some of the more expensive miticides, they will only control mites, and some of them can be difficult to find in Mississippi. The higher price tag may not be as much of a limiting factor, because one application with those products will usually do as good or better than multiple applications of the other products.
The bottom line is that spider mites can be an important pest and can cause significant yield losses if left untreated. Even low levels of mites that persist for a short period of time can reduce cotton yields and should be treated accordingly. We will cover termination for spider mite applications in another article later in the season.