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Posted 7 September 2005. PMN Crop News.

K-State Scientist Comments on Late-Season Insects in Soybeans

Kansas State University.

Manhattan, Kansas (September 2, 2005) With soybean harvest drawing nearer, some producers may believe that the time for scouting fields for insects is past, but that's not so, according to a Kansas State University entomologist.


"The calls and e-mails I have received over the last few days show that the need for crop scouting and insect management decision-making for soybean pests has not ended," said Randy Higgins, field crop entomologist with K-State Research and Extension. "Significant defoliation caused by insects is making soybean foliage more ragged than usual in many areas. Plus, other insects have been feeding directly on pods and damaging developing beans to a greater degree than normal in some fields. It is clear that this is a widespread and unusual event."

The calls prompted Higgins to send an alert to K-State Research and Extension agriculture and natural resources agents to help them disseminate information to producers.

Harvesting soybean stem borer-infested fields (SBSB) as soon as possible helps minimize losses caused when plants lodge or break free from their bases after tunneling larvae girdle the stems internally at or above the soil line. The girdling damage is an additional form of plant injury that develops following extensive stem tunneling. Girdling behavior occurs late in the season as SBSB larvae prepare to overwinter within the stem bases of soybean plants.

But soybean stem borers are not the only pests being reported, Higgins said. The culprits are varied, but most callers mentioned one or more species of caterpillars. Most reports tied recent leaf tissue losses with above average to high numbers of green cloverworm (GCW) larvae that were or still are present from the southeast through southwest Kansas.

GCW larvae are light green with three pairs of white stripes running the length of the body. In addition to three pairs of legs near the head, three pairs of fleshy prolegs can be found near the middle of the body, and one more pair is found at the rear of the insect. In some years, GCW larvae can be found feeding on soybeans throughout September.

"Right now, more than one larva per leaf may be present in some areas and scouts are reporting large numbers of very tiny larvae," Higgins said. "Obviously, those sites may experience more significant defoliation unless something halts their growth and survival."

Green cloverworm larvae are susceptible to fungal and viral infections that can destroy an outbreak population in just a few days, he explained. In fact, Wednesday evening (Aug. 31), Higgins detected the initial signs that some GCW larvae in the Manhattan area were beginning to die from a fungal infection that causes their larval bodies to become dried/hardened white remnants of their former selves in death. These mummified remains may be present on the ground or can remain up in the foliage if wind or rain has not knocked them off the plants. Large numbers of spores will have left the bodies and be infecting other nearby larvae, frequently leading to a total population crash in a short period of time. Many GCW also serve as food for a number of predatory insects. In parts of the deep South, green cloverworms are ignored or are sometimes considered beneficial because low populations are thought to serve as a food source for predators that later help hold more serious defoliators in check.

"Our production system is simpler in that Kansas does not generally have follow-up pests, so GCW are important for the damage they themselves inflict," Higgins said. "Unfortunately, when high populations persist in Kansas, natural control through disease or predation may be realized after economic damage has been sustained." This may have been one of those years for some locations, based on the numbers of reports where insecticide sprays seem to have been or are being justified in a number of fields, he said.

In addition to GCW defoliation, some locations have experienced or are experiencing heavier than normal populations of corn earworm (CEW) larvae on top of what has been a high second generation of bean leaf beetle (BLB) adults. Unlike GCW (which is a foliage feeder), earworms and bean leaf beetles can and do feed directly on the pods, chewing into developing seeds.

"It takes just three to four beans per plant at normal plant populations to pay for an insecticide treatment, so pay particular attention to these pests," Higgins said.

Unlike GCWs, corn earworms can be a more difficult pest to control. They sometimes feed deep within the protective canopy and are not as susceptible as GCWs to many insecticides. "Higher insecticide rates may be called for and even then, large larvae frequently are difficult to kill," Higgins said. "That is why the threshold for CEW on soybeans typically calls for treatment when an average of one small earworm larva per foot of row is detected.

Corn earworms can be identified by the presence of many tiny microspines arising from their skin surface, particularly on the back and sides of their bodies. Confirm that earworms are still present and that you did not miss the spray window or you may senselessly apply a spray to plants that suffered damage sometime in the past.

Insecticide sprays do not replace lost yield, but if used when justified they can protect yield potential that is at risk. In some locations, earworms may have cycled through and have pupated, whereas other reports indicate these larvae are still relatively common and are causing damage."

There is also some evidence that growers should be watching newly-seeded alfalfa for spotted alfalfa aphid infestations, he said. Young plants that are just getting established have no reserves and are particularly susceptible to stress and loss. Fall armyworms also sometimes cause serious problems when growers are trying to establish new stands of alfalfa.

"We are also receiving a few reports that indicate some milo fields are becoming infested with fall armyworms and corn earworms," Higgins said. "Thus far, reports of head damage on sorghum have not been widely observed.

"The only way to tell if local fields are infested is to spend time looking. It is not too late for serious insect damage to occur in some fields if high numbers of some of these insects develop. Crop maturity and insect density can be important in determining the benefits or costs associated with applying an insecticide. Remember to consider pre-harvest waiting intervals when selecting among alternative insecticide treatments," the entomologist said.

For more information, interested persons can go to any county or district K-State Research and Extension office or check online: Soybean Insect Management 2005 or the Web site of the Department of Entomology.

Sampling and Treating For Green Cloverworm Larvae

A late flurry of insect activity reported in Kansas soybean fields has spurred numerous calls and emails to K- State Research and Extension agents and scientists. In response, Extension entomologist Randy Higgins issued an Entomology Alert to encourage growers to inspect their fields and take action, if appropriate. His recommendations:

  In planter-sown fields, sample for green cloverworm larvae with a shake cloth placed on the ground between two adjacent soybean rows. Bend about 1 foot of plants from each row over the cloth and shake them vigorously. Repeat the process at 10 or more locations in the field and average the results.

  Treatment thresholds from full pod set through beginning seed development stages revolve around 11 larvae per foot of row if beans can be sold for $8 per bushel, whereas it takes 13 larvae per row foot to trigger treatment if the price is $6.50 per bushel, and as many as 15 larvae per foot of row if the crop value is only $5.00 per bushel. Resample the field in seven to 10 days if five, six or seven larvae or fewer, respectively, are found when sampling. Populations between these levels should be resampled more frequently, possibly every three to four days, to determine if the damage is increasing or diminishing. When pods are forming and beginning to fill, a loss of more than 20 percent of the foliage may decrease yields. Rarely is a yield reduction associated with defoliation beginning after the beans have nearly filled.

  If control costs are higher than $6.50 per acre, adjust thresholds upwards. Factors such as prevalence of natural control agents, previous defoliation, moisture availability, larval age distribution, and yield potential influence this relationship to some degree. Note that a fungal pathogen that kills this insect is showing up in some fields. Experience indicates that threatening populations of GCW can collapse in just a few days without the use of insecticides if the environment is favorable for the spread of the fungus. Dense cloverworm populations and moist conditions help encourage the development of an epizootic that can eliminate most living larvae.

Fortunately, green cloverworm larvae are not difficult to kill with most registered insecticides if that step becomes necessary for some growers" Higgins said.

  Be sure to also read the text about corn earworms because earworms feed directly on beans and pods. That pattern of feeding damage is harder for the plant to recover from than moderate levels of defoliation since yield components (beans that could be harvested and sold) are actually consumed.


Mary Lou Peter-Blecha
K-State Research and Extension

Randy Higgins