This article was originally published on June 11, 2012 and expired on June 18, 2012. It is provided here for archival purposes and may contain dated information.
Spider mites, not insects but insect relatives, are present, and if dry weather continues, they will cause yield losses in some area bean fields. They should be a scouting priority in 2012.
Spider mites lay eggs during the early summer on the leaves of soybean plants and weeds. Immature spider mites emerge from those eggs. These "hatchlings" are very similar in appearance to their parents except for the fact that first-stage immature mites have six legs as opposed to eight. After the first molt, immature spider mites "grow" another set of legs. Three immature stages of the mite occur over a few to several days. The entire "egg-to-adult" portion of the lifecycle takes about 1 ½ to 2 weeks. Female spider mites overwinter in sheltered areas, such as field margins, and given a mild winter, this likely means plenty of mites entered the 2012 season. They move into fields either via crawling from overwintering sites into the field or passively moving into fields with equipment. Intense populations may also produce webbing which allows individual spider mites to "balloon" or "parachute" into interior portions of the field.
A few spider mites are not a problem, but when populations are high and dry weather prevails, these "pin-point sized" pests can inflict serious yield penalties. A needle-like mouthpart is used by the spider mite to suck up plant juices. Symptoms of mite injury to beans and other plants often resemble a yellow speckling of the leaflets as if the leaflets had been vandalized by a "spritz" of yellow paint. Since this arthropod moves into the field from overwintering sites along field margins, symptoms of spider mite injury often are first noted along the edges of the field.
Scouting for spider mites involves using a sheet of white paper as the primary scouting tool. The scout shakes the leaves of a suspect plant over the paper. Mites will at first "play possum", but will soon start scurrying about.
Control of spider mites in soybean is recommended when ¼ of the plants appear to be discolored. After pod set, 10 % discoloration is the threshold.
Spider mites are a dry weather pest. These arthropods rely on ingested liquid within plant cells to survive and dry weather concentrates sugars in that liquid. The end result is better nutrition for the mite, which causes a subsequent "surge" in the spider mite population. Spider mites also fair well in dry weather because fungal pathogens are inhibited in dry weather. High humidity promotes the development of such pathogens.
Source: Matt Montgomery, Extension Educator, Local Food Systems and Small Farms, firstname.lastname@example.org
Pull date: June 18, 2012