With the earliest signs of spring farmers can also begin to look for signs of cereal leaf beetles to start showing up in their wheat fields. CLB generally begins to come out of their winter habitations to colonize wheat fields in the first weeks of March. The adults do little to no damage to the wheat, themselves, but they lay eggs and it is the larvae that hatch that are the wheat pests that cause defoliation and wheat loss.
"We usually see peak egg hatch in the third week of April," says N.C. State University crop scientist Randy Weisz, "so if you are planning to make an insecticide application that gives you the best control of CLB, then usually the best time to apply that is sometime in April, to kill those larvae that are coming along."
However, Weisz says, an increasing number of growers are experimenting with tank-mixing a residual insecticide with the nitrogen applications they are currently putting on their wheat.
"That sometimes can work pretty well," he notes, "but the idea there is that you hope all the adults are finished colonizing the field before that insecticide wears out. If you are successful, if that happens, then you do don't have a CLB problem – and sometimes that works very well."
However, there are occasions it doesn't work out perfectly. Weisz notes his colleague, NCSU entomologist Dominic Reisig, has looked extensively at CLB management strategies on about 170 N.C. farms over the last two years, to determine what growers have been doing to control the pest and how efficiently what they did worked out for them.
Reisig reports he found about 7%-8% of the time when growers put their insecticide out with their N at this time of year (around early March) that the strategy failed. Weisz speculates the application was too early and the CLB adults kept coming into the field from over wintering sites.
"When the insecticide gave out there were still adults coming in that would colonize the field and growers would have some yield loss," Weisz says.
He offers two suggestions for the growers who want to continue to use this strategy. First, he suggests growers who put their insecticide out with their N should go back and take a look at their wheat fields in the second or third week of April and make sure there are no larvae out in the field.
"If there aren't that's great," Weisz says. "That means it worked and it was an inexpensive and easy way to take care of the problem. But if they do see some larvae then they are probably going to want to make another application."
A second consideration for the grower is to make certain he gets good coverage.
"If you are applying an insecticide you want to spray at high pressure, at high gallonage and with a very fine droplet size for good coverage," Weisz says. "That really is just the opposite of what you want when you are applying N. With the case of N, you want the largest droplets with lower coverage, so you don't get a lot of burn. So if you are trying to do both at the same time, you need to think about whether your wheat is strong enough and vigorous enough that it can take some burn, so you can tank mix your N and an insecticide and apply with a good coverage. But if your wheat is struggling a bit so you want to give it every bit of advantage that you can and minimize your burn, you may want to separate those two applications."