2012 is proving to be a very dry year after a very wet spring. Most areas in our region experienced 10 to 14 inches of rain during the month of May. The faucet pretty well turned off after the first 10 days or so of June. The month of July saw 1.5 to 2.5 inches of rain around the area for most locations. Much of this came in small amounts, perhaps two tenths or so at a time.
As the summer continues it appears that our very dry year is extending into an actual drought. Prolonged drought raises the threat of two-spotted spider mite outbreaks in soybeans and corn. As we progress into August, infestations are reaching treatable levels. Spider mite outbreaks are rare but have occurred more frequently in recent years notably 2007, 2009 and 2010. The worst year was also a very dry year 1988.
First of all, what are two-spotted spider mites? They are minute (less than 0.002 inch) greenish to yellowish to orange arachnids with two large spots on their abdomen. Note that they have eight legs and not six as insects have. Spider mite adults are half the size of less of the smallest soybean aphid nymph. They can attack a wide variety of plants, including crops such as alfalfa, corn, soybeans and other vegetable plants.
Tina LeBrun & Wayne Schoper
These mites overwinter on permanent vegetation and hatch during the year. Look closely at the underside of leaves on an infested plant and you will see the characteristic "webbing" that earns them the name "spider mites." They have a straightforward life cycle, progressing through three stages between egg and adult. Depending on temperature, development takes five to 19 days.
Hotter temperatures (over 90) speed reproduction, while cooler temperatures slow it sown. Recent hot temps have greatly accelerated reproduction. With females producing up to 100 eggs each, it's easy to understand how populations can explode, increasing up to 70 times in as little as seven to ten days.
Why are problems worse in drought? Spider mite populations are held in balance by natural enemies, weather, and host quality. Drought triggers spider mite outbreaks in corn and soybeans by upsetting this balance in four ways:
Acceleration of spider mite movement to soybean and corn from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Cutting initiates mass movement into adjacent soybean fields under drought conditions.
Improves the food quality of soybeans
Diminishes or stops the activity of fungal diseases that attack mites. Disease outbreaks are favored by cool, highly humid conditions that favor spore formation and mite infection. Hot dry weather stops these diseases and allows proliferation of spider mites.
Speeds spider mite reproduction so that predatory insects and mites can't keep up.
Spider mites injure plant leaves by piercing cells and sucking out cell contents. This injury produces a white or yellow spots or "stippling" that is heaviest on the underside of the leaves. Leaves lose their photosynthetic surface as feeding continues. Water lose from damaged leaf surface becomes uncontrolled. As spider mite feeding continues, the plant deteriorates and can eventually die.
Scouting for spider mites involves scouting the fields. Infestations are first observed near field edges or where soybeans are stressed. If lower leaf loss is observed, or yellowed or browning spots are noted at the field edge, it is time for some detective work.
Examine plants at the edge of the field first especially next to roads or drainage ditches or alfalfa.
Pull plants and examine the leaves from the bottom upwards.
Look at the underside of leaves. Note stipling or webbing. Examine for mites with a hand lens or tap infested leaves over a white sheet of paper.
Determine how far mites and symptoms have progressed up the plant
Once mites are observed, move further into the field and continue to look for feeding evidence. Continue to check every 4-5 days as long as drought conditions prevail.