Like weeds, bad bugs are often resilient enough to gain resistance to chemicals that are overused. And menacing mites are among the tiny critters that can become tolerant to a single miticide if it's used too frequently in corn.
Fortunately, there are several miticides that work well in controlling spider mites and other mite types. David Ford, a corn, wheat and cotton producer from Dumas, TX, makes sure he has the right mode of action for his irrigated corn situation.
“In our case, we use Overon when spider mites are about halfway up the plant,” he says. “We hope to get them by spraying just once.”
Mites move from wheat and non-crop vegetation to corn. It takes a week to 10 days to develop from an egg to an adult. Fastest development occurs from 77° to 98° F.
Populations increase rapidly during tasseling and thrive in hot, dry weather. Spider mites have tiny mouthparts modified for piercing individual plant cells and removing the contents, causing tiny yellow or white speckles. Foliage takes on a yellow or bronze color.
Plants see premature leaf death, according to Texas AgriLife Extension entomologists. Mite damage also destroys cells and reduces photosynthesis in the plant, which leads to reduced grain filling.
“You can see lodging and other problems at harvest,” says Ford.
Cost for the 12-oz./acre application is about $20/acre, he says. That's a typical miticide application cost, adds Pat Porter, Texas AgriLife Extension entomologist in Lubbock, adding that Overon is among the miticide arsenal available for control of spider mites, Banks grass mites and others than can plague corn.
“All corn should be scouted for mites,” says Porter. “Bt corn has no immunity to spider mites.”
IF A FIELD NEEDS treatment at tassel, he recommends either Oberon or Hero. “Oberon is a dedicated miticide and does a good job,” says Porter. “It has activity in all mite life stages and has performed well in Texas AgriLife field tests.
“For Bt corn, think about Oberon because you don't need to control southwestern corn borer. However, for non-Bt corn I suggest Hero if you need to control corn borers and/or corn rootworm beetles in addition to mites that are still below the ear leaf in the beginning stages of colonization,” he says.
Hero is a combination of two pyrethroids, including bifenthrin, formerly Capture. “Hero will give good control of southwestern corn borer, western bean cutworm and adult corn rootworm beetles as well as mites,” says Porter.
“It will provide good control on Bt and non-Bt corn,” he says. “But from a resistance standpoint, it might be prudent to not use a pyrethroid mixture if you don't need to control southwestern corn borer or rootworm beetles in addition to spider mites.”
ALONG WITH HERO AND OBERON, other miticides are Onager and Comite II. Comite II is better used in early season when plants are 5-6 ft. tall. It's not recommended for use after tasseling, says Porter.
In the Deep South, where corn acres are increasing annually, mite problems are seen on former cotton fields. Georgia entomologists recommend that growers burn down their winter weeds and cover crops, conserve natural enemies and avoid any unneeded sprays.
Porter says the option of having four miticide modes of action gives growers a leg up on mites. “Choose the one that matches your pest profile,” he says. “Use it only once per season to preserve the genes for susceptibility in your mite population.”
Adds Ford, “You just have to be ready to control them when needed. We're glad there are several miticides out there to use.”