Worms of wrath
by James D. Ritchie

 Hungry grubs can can wreak spring damage

You might call them the stealth pests of crop fields and pastures. White grubs burrow into the soil for one or more years, out of sight and usually out of mind, feeding on plant roots and often causing economic injury that isn’t noticed until the damage is done.

“White grubs are the larvae of several different species of insects, from June bugs to Japanese beetles,” said Wayne Bailey, University of Missouri entomologist. “They may be annual grubs, with a life cycle of one year, or true [perennial] grubs if they have a life cycle of two to four years.”

However long it takes, the life cycle of both annual and true grubs is similar. Adult beetles emerge in late spring, and many species congregate in trees bordering fields or pastures—especially willow, cottonwood and sycamore—to feed and mate. In late summer, they move into fields (or pastures) and lay eggs in the soil for the next generation. The eggs hatch into grubs (larvae), which burrow into the soil and feed on plant roots. The larvae go through several stages called instars, then pupate and emerge as adult beetles.

In the larvae stage, the two types of grubs are similar in appearance. But grubs can be identified as annual or perennial types by closely inspecting the raster, or hair, pattern along the lower side of the tail end of the larvae. On annual grubs, the hairs are randomly arranged. If the hairs are paired in distinct rows in a sort of “zipper” pattern, the grubs are from the perennial group.

“The perennial grubs cause 65 to 70 percent of field crop problems; I’m not sure which group of grubs causes the most damage in pastures,” noted Bailey. “However, Japanese beetles, an annual grub, caused a lot of problems in corn fields north of St. Louis in 2008.

“There’s a lot of speculation as to why we’re seeing an increase in white grubs and the damage they cause,” he added. “It’s probably a combination of factors. For example, in no-till and reduced tillage, weeds are allowed to grow longer in the spring prior to using burn-down herbicides. This provides better protection of grubs in fields and more places for female beetles to lay eggs. Also, insecticide use has been reduced with the advent of Bt crops and insecticidal seed treatments, which have limited effect on those species of grubs that exhibit much of their growth in late summer and fall and are large in size by the following spring—more insecticide is needed to kill larger grubs.”

Another possibility for the apparent spike in grub numbers: The now-banned organochlorine soil insecticides that were discontinued 15 or 20 years ago may have had some long-playing effect on white grubs until recently—some of these compounds have half-lives in the soil of up to 30 years.

In field crops or forages, if you wait to do anything until damage appears, it’s probably too late.

“Scouting for grubs before planting is a good practice, especially if you have experienced grub problems in the past,” said Bailey. “Also, be suspicious of fields that are high in organic matter, are bordered by willows or other river-bottom type trees or if you have recently fed hay or pastured cattle in the field.”

It’s a good idea to check pastures, too, said Lyndon Brush, MFA staff agronomist.

“White grub populations are becoming established all over Missouri—building up to damaging numbers in many areas,” said Brush. “Grub damage may be harder to spot in pastures, and it may be worse in orchardgrass or brome than in tall fescue. Look for areas or circles where the grass is yellowing or turning brown, which may also be caused or aggravated by drought. Root injury by grubs reduces the plant’s ability to take up moisture and nutrients. Heavily infested sod may feel soft and spongy and the grass is easily pulled up because the root system is damaged.”

To scout, take a shovel and dig up an area six inches to either side of a foot of row and 12 inches deep. This equates to a cubic foot of soil. Repeat the sampling in several areas of a field or pasture. If you find an average of one or more grubs per sample, that’s an economic threshold for most field and forage crops, especially grasses.

“Control is not easy,” admitted Wayne Bailey. “The difficulty with grub control is finding an insecticide label that fits the crop and needs at the time the grubs are found. Control measures will vary from field to field, depending on the crop and cropping system.

“Often, severe grub problems may cause the need to rotate or replant to a crop on which labeled insecticides may be applied prior to planting or at planting time,” Bailey continued. “Whether the insecticide should be applied as broadcast, T-band or in-furrow treatments of liquid or granular formulations depends on the insecticide chosen.”

“The white grub problem is a complex issue,” agreed Brush. “In some cases, forage production is being reduced as white grubs build up to damaging numbers.”

“Lorsban has been the traditional insecticide to provide good grub control, but the Lorsban label doesn’t cover all situations,” he added. “The use of planting-time liquid Capture LFR1.5 or Regent 4SC or granular products such as Counter 20CR or Force 3G may be helpful. Read product labels for rates and placement of the insecticide. The truth is, management of white grub populations in established crops is difficult at best.

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