Wed Jan 29, 2014 04:10 PM CST
Winter Annuals and Insects 05/23 14:56 Delayed control of winter annuals in the spring can lead to outbreaks of secondary insects in corn and soybeans. By Daniel Davidson DTN Staff Agronomist OMAHA (DTN) -- Seeing stands of winter annuals in late April or early May even after corn has been planted is becoming much more common; however, delaying control of winter annuals could increase insect pressure on newly planted corn. Outbreaks of winter annuals are most common in soybean stubble that is being planted to corn. Alex Martin, a weed scientist at the University of Nebraska said increased adoption of no-till practices along with a decline in use of residual herbicides in favor of glyphosate has contributed to the increase in winter annuals. Keith Jarvi, an integrated pest management specialist with the University of Nebraska, said that since winter annual outbreaks are a fairly recent development, there is not a lot of information on what possible insect problems may occur with delayed control. The first insect that comes to mind, however, is black cutworm, he said. They migrate into the Corn Belt from Southern states and are attracted to any early green vegetation. The eggs hatch and the cutworm larvae feed on the annuals for a while then move on to emerging corn after the winter annuals have been killed. Winter annuals can also serve as an attractive egg-laying site for white grubs, Jarvi said. "The (white grub) adults are May or June beetles that emerge fairly early in the spring. After mating they may choose winter-annual infested fields to lay eggs in, which causes a potential problem in those fields a year or two down the road." White grubs, similar to cutworms, attack corn from emergence to early whorl (VE to V3 leaf stage) in May. True white grubs, which cause injury by chewing off root hairs and small roots, have a three-year life cycle and can be a problem for several years. Kevin Steffey, an extension entomology specialist at the University of Illinois said some of the two-spotted spider mite soybean infestations in 2005 can be traced back to winter annual weeds that were not killed until Roundup was applied later in the spring. The overwintering two-spotted spider mite females shelter in many species of winter annual weeds, and they get off to a good start when these weeds grow in the spring. "By the time Roundup is applied, colonies have established. If drought conditions persist as they did in 2005, mites simply move from the dying weeds (e.g., chickweed, henbit) to the seedling soybeans, giving the mites an early foothold in the crop," he said. "It's also possible that stalk borers, which overwinter in weedy areas, could become established in the fields with winter annuals in the spring, then move to corn when the weeds die," Steffey said. Daniel Davidson can be reached at (SS) Copyright 2007 DTN. All rights reserved.