Soybean growers be aware of possible damage from two-spotted spider mites

By DALE HILDEBRANT, Farm & Ranch Guide
Friday, March 12, 2010 12:36 PM CST

MOORHEAD, Minn. - The past two cool, damp growing seasons have lessened the population numbers of the two-spotted spider mite, which can be a threat to soybean stands under certain conditions, according to Phil Glogoza, University of Minnesota Extension educator for crops.

Glogoza presented an information session at the recent “Best of the Best” session in Moorhead, Minn.

But, sooner or later the region will again see a warm and dry growing season and those are the right conditions under which spider mites thrive. In addition, the chemical control of soybean aphids can have a drastic impact on the spider mite population in that field, and usually in an adverse way, Glogoza noted.

The two-spotted spider mites attack not only soybeans, but a wide variety of crops, ornamental plants and even trees and they overwinter as eggs in permanent vegetation. Newly hatched eggs rapidly develop into adults in a 5 to 19-day period, with the process happening faster at warmer temperatures. Once hatched, the spider mites are easily dispersed by the wind.

“Temperature is very important in how fast the population numbers reach critical mass,” Glogoza said. “Spider mites produce silk - when they want to move they put out a little strand of silk and that strand of silk gets caught in a breeze and that will pick up the little spider mite and move it along in a process that is known as ‘ballooning'.”

Spider mites have piercing, sucking mouthparts. They work on the underside of the soybean plant's leaves feeding on the plant sap. Early damage to the leaf causes yellow spotting, which is called stipling. Early on this is a speckled pattern, but as the damage increases the entire leaf will take on a bronze color, according to Glogoza.

This damage reduces the photosynthesis ability of the plant and eventually the plant loses its ability to keep growing and start the flowering and seed set process. The damage usually starts on the lower canopy and then moves upward as the spider mite numbers increase.

It is important to determine the extent of spider mite injury before embarking on a control program, Glogoza noted. The injury levels are rated from 0 to 5 and are determined as follows:

0 - No spider mites or injury to the plants observed.

1. Minor stipling on the lower leaves, no premature yellowing observed.

2. Stipling common on lower leaves, small areas or scattered plants with yellowing.

3. Heavy stipling on the lower leaves with some stipling extending into the middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common, small areas with lower leaf loss. (This is the spray threshold stage.)

4. Lower leaf yellowing readily apparent. Leaf drop common. Stipling webbing and mites common in middle canopy. Mites and minor stipling present in upper canopy. (Economic loss is occurring in this stage.)

5. Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mite present in high levels in middle and upper canopy.

When scouting for spider mites, he suggested examining plants at the field edge first, especially for those fields adjacent to drainage ditches, alfalfa or corn fields. The soybean plants need to be pulled and the bottom side of the leaves examined for webbing and stipling. The leaves should be tapped over a sheet of white paper, which will make the mites easier to see.

“If you see little black specks moving those are probably spider mites,” he said. “To make sure you can crush them with your finger tip and if it's a spider mite you will leave a gray blotch on the paper when it's crushed.”

When selecting insecticides to use on a soybean field for pests such as the soybean aphid, the grower must be very careful to use a product that won't increase the number of spider mites, while controlling other insect numbers.

Pyrethroid compounds actually can cause the population of spider mites to explode for several reasons, he noted. First it increases the movement of the spider mites within the field sprayed. It also changes the sex ration of the eggs favoring the creation of more females.

The number of eggs being laid is also increased which, in turn, increases the reproduction rate. Instead, producers should use an organophosphate insecticide to avoid mite population explosions.

Glogoza also noted there are predaceous mites that feed on the spider mites and help to control their numbers and care should be taken to keep the predaceous mite population high. In addition, there are several fungal diseases that prey on spider mites, which warrants the careful use of fungicides on soybean fields.

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