: Spider mites beginning to appear on soybeans, can corn be far behind?

Doug Johnson, Extension Entomologist

I warned of this pest some time back (See KPN No. 1308 June 26, 2012. I Hate Crying Wolf…). In fact, I am mildly surprised that it has taken this long for field scouts to come across the pest. Reports of spider mites have alerted us of their activity and samples sent to the Plant Diagnostic Lab have confirmed that it is the two-spotted spider mite causing the problems. At present the damage appears to be on a small scale and very localized in fields. Somewhat surprisingly some of the infestations were noted in irrigated fields.

For as long as we have drought stressed plants, and in particular LOW humidity, we are increasingly likely to be plagued by this pest. In soybeans, the most important time is during the reproductive stages of R1 (beginning bloom) - R5 (beginning seed) and it is even more likely in fields where a synthetic pyrethroid insecticide has been used. In our current corn production situation, it may very well be too late to worry about mites. In either case, one of the first and perhaps most important questions is: Am I likely to be able to make a crop? If you are or are planning to convert your corn to silage, then pay attention to the “days to harvest” restrictions on the product you choose to use.

To sample in soybeans, shake plants over a piece of white paper and look for tiny, black, moving specks. You will need a hand lens to determine if the specks are actually mites, but if they are crawling across the paper you probably know the answer. In corn, scouting is much more difficult and less is known about making a control decision. The mites are no harder to find, simply follow the instructions for soybean but use corn leaves. One simply wishes to determine if the infestation is only on the outer edges or occurs across the field. You will need to sample several areas of the field. Mites are notorious for being spotty in their distribution. They also have a very strong “edge effect” which would include waterways, etc., that may cross the field. If a treatable infestation is found, it may very well be controlled by a border application.

Scouting should start well before R1 (soybean) and in younger corn. Cooler temperatures and high humidity allow a natural fungus to control spider mites. Rainfall will help the plant tolerate the infestation but will not reduce the mite population. Application of synthetic pyrethroid insecticides and fungicides may worsen the infestation because these insecticides don’t work well on mites and the fungicide may hamper the natural occurring fungus that kills the mites.

In soybeans, if direct control is required, consider using an organophosphate insecticide like chlorpyrifos or dimethoate. If you must use a pyrethroid, consider bifenthrin. An application is warranted when most plants are infested with spider mites and leaf speckling and discoloration are apparent. Treating after R7 is not recommended

In corn, chlorpyrifos is not labeled for spider mites; dimethoate may not be applied during pollen shed. Bifenthrin may be used. See labels for rates and specific directions.

Information on insecticides is supplied for the readers convenience. Use of a trade name does not constitute a recommendation. These products are not ranked in preference or efficacy. Always read and follow the label.

Photo: Frank Peairs, Colorado State Univ., bugwood.org

Drought Resources for 2012

Chad Lee, Extension Agronomist, University of Kentucky

We have put together a new website to house the 2012 drought information for corn and soybeans. This drought is widespread and we have included articles from other states as well as our resources here in Kentucky.

Websites that monitor the drought conditions and include some other information about weather are near the top of the page. Timely articles, including date published, from Kentucky are listed below the websites. Articles from other states come next. States as far north as Wisconsin are reporting drought concerns. Finally, there are some links to websites about irrigation and crop water use.

If you know of other resources that should be on this 2012 drought website, please let me know. Thanks.

2012 Drought Resources.

Drought Stress and Corn Pollination

Chad Lee, Extension Agronomist, University of Kentucky

Drought-stress corn in western Kentucky. Photo: June 25, 2012. 

Most of western Kentucky is in a severe drought status and that severity is likely to spread to central Kentucky on the next report. Most of the corn is going through tassel and pollination, which is well ahead of the five-year average, and the worst possible time to experience temperatures in the high 90's F (35 C or more). The high temperatures and low humidity dry out pollen and silks faster, both of which need moisture for successful fertilization of the ovules.

In addition to the need for moisture for pollination, the corn plant simply needs more water. Corn around tasseling and silking demands about 0.3 inches (7.7 mm) per day. Most of our fields were at a water deficit prior to tasseling and the water deficit is increasing. The lack of water could lead to the abortion of kernels.

Dry conditions prior to tasseling often result in the tassel emergence and pollen drop before silk emergence. In many cases, the resulting ears may have a few kernels develop near the base of the ear but very few to no kernels near the tip of the ear.

Could Sugar help Drought Stressed Corn?

Chad Lee, Extension Agronomist, University of Kentucky

The dry weather has farmers looking for any options to alleviate drought stress. Some are tempted to try sugar. Plants make glucose sugar through the process of photosynthesis. Crop scientists estimate that about 78 pounds of glucose is needed to produce one bushel of corn and 119 pounds of glucose is needed to produce one bushel of soybean.

To extend those calculations, 50 bushels of soybeans require about 5,965 pounds of glucose and 200 bushels of corn require about 15,580 pounds of glucose. In comparison, some of the products being sold are putting on 16 fluid ounces of product. Those products contain 34% sugar, which comes out to 5.44 ounces of sugar per acre. That is a very small amount compared to what the crops require.