Cutworm Just One Pest to Watch For This Year

By Jane Fyksen, Crops Editor
Thursday, May 22, 2008 12:05 PM CDT

Fourth-instar cutworms

Wisconsin farmers hurrying to get their crops in aren’t the only ones busy in their fields. As the season advances, so is the potential for certain pests, in particular black cutworms this year. Delayed weed control encourages migrating moths to lay their eggs in cornfields. This insect is an occasional pest of seedling corn that sometimes causes significant damage in some fields; this might be the year.

According to Wisconsin Department of Agriculture, Trade and Consumer Protection entomologist Krista Hamilton, pheromone traps in Lafayette and Monroe counties have detected a sharp increase in black cutworm activity. High captures of 11 to 12 moths were made near Gratiot, Shullsburg and Sparta on the evenings of May 8-11, presumably due to southerly air currents.

Two separate migrations have been registered this season, the first from April 18-24 and the second from May 8-11. Hamilton says the larval offspring produced by moths that appeared in April are now large enough to sever corn plants, and corn emerging this week should be watched closely for evidence of feeding injury. Larvae from the second, more recent flight of moths will pose a risk to corn emerging in early to mid-June. Pheromone traps placed in southern and westcentral counties averaged four moths per trap from May 8-15.

Iowa State University entomologists Jon Tollefson and Marlin Rice, say pheromone traps are valuable tools, but they “only tell you that the insects are in your area.” “They do not report in which fields the insects have laid their eggs,” they note, mentioning where moths lay their eggs is influenced by weed cover, tillage, when the crop is planted and previous crop residue. “The bottom line is that once adult black cutworms are forecast to be in your area, you must scout your fields to determine if there are larvae present and if they are ‘cutting’ your corn plants,” they state.

To scout for black cutworm injury, walk along rows of corn at several locations in a field looking for feeding on leaves and missing plants. Small larvae will feed on the edges of leaves before they get large enough to cut corn plants.

Cutting may not happen for several days after the first leaf feeding is observed. If leaf feeding is observed, begin looking for missing (i.e. cut off) plants. Sort through soil near the surface in the area of missing plants while looking for cutworm larvae. If larvae can be found, determine whether they are black cutworms or dingy cutworms. Dingy cutworms rarely, if ever, cut corn plants. Record the number of missing plants and determine the percentage of the plants cut.

There are some new variables in the control of black cutworms, say Tollefson and Rice of genetically engineered corn, higher seed prices and increased market prices of corn.

First, with genetically engineered corn, remember that YieldGard is not effective against the black cutworm; only Herculex hybrids give some protection against black cutworm.

Second, with the high cost of seed and expected higher returns from corn, the economic threshold could be lowered. They say that larvae 3/4 inch long are in the fourth stage and will cut several more plants before they finish feeding. If the worms are longer than 1 inch, they are nearly finished feeding, and treatments don’t need to be applied until 5 percent of the plants have been cut. A lower threshold would be one percent stand loss, which would be within normal stand variability and very hard to detect. The previous economic threshold was: If cutworms were less than 3/4 inch, apply an insecticide when 2 to 3 percent of the plants are cut.

“The higher threshold of 5 percent for larger worms could be lowered,” they contend. Assume 32,000 plants per acre with the expectation of producing 200 bushels, then 160 plants would produce one bushel. If corn is $6 per bushel then each plant produces 3.75 cents worth of corn (32,000 plants per acre divided by 200 bushels per acre or 0.00625 bushels of corn per plant, multiplied by $6 per bushel for 3.75 cents per plant.

If control costs are $15 an acre then 400 plants lost would equal the cost of control ($15 divided by $0.0375 or 400 plants). Four-hundred plants out of 32,000 per acre is 1.25 percent of an acre. The result is that losing 1 to 2 percent of the plants at $5 to $6 corn would cover the costs of control

“The higher threshold of 5 percent for larger worms could be lowered to 1 to 2 percent plant loss,” these Iowa State University entomologists contend, noting that actual economic returns will depend on how much cutting larvae continue to do and the percentage of the plants that fail to regrow after being cut.

Scouting seedling corn is the only reliable method to determine whether a problem exists. Then, insecticides can be applied if needed. However, corn hybrids with Herculex I or Herculex Xtra should, as noted, provide significant protection from black cutworm, but fields should still be monitored because some minor cutworm damage can still occur.

Stop scouting when the field is sprayed or when plants have five fully developed leaves (stage V5). Cutworms have difficulty cutting plants in the V5 stage because of the larger stalk diameter, although occasionally they chew into the side of the stalk and kill a larger plant.

Fields with winter annual weeds are more likely to have cutworms than clean fields, and soybean stubble is more attractive to the moths than corn stubble.

If you find leaf feeding and only black cutworms, mark off 100 plants in a row with stakes or flags, and scout these same plants for cutting over a period of several days at several locations across the field. Then you can monitor the cutworm activity and determine whether they are cutting plants and the percent cut plants.

Several insecticides are labeled for black cutworms in corn. Several years ago, research showed that after application, rotary hoeing in dry soils increases the effectiveness of Lorsban, but that the pyrethroids (such as Ambush, Pounce or Warrior) should not be incorporated into the soil as this decreases performance.

According to the Iowa State University entomologists, insecticides for black cutworms in corn are: Ambush, Warrior, Asana XL, Baythroid 2, Discipline 2EC, Capture 2EC, Hero, Mustang Max, Lorsban 4E, Nufos 4E, Penncap-M, Pounce 3.2EC, Sevin XLR Plus.

Watch for alfalfa weevils

Alfalfa weevil development continues to be delayed with very low numbers of first and second instar larvae being found in the most advanced fields in Dane, Dodge, Fond du Lac, Iowa, Juneau, La Crosse, Monroe and Walworth counties late last week. The highest number of larvae which could be obtained by sweeping was four per 50 sweeps. Tip feeding damage was visible but insignificant in southern Wisconsin, and wasn’t yet apparent in the westcentral counties where egg hatch began just days prior. Adults are becoming more active and some stands yielded 16 per 50 sweeps. Larvae and tip feeding injury are expected to increase in first-crop alfalfa after spring-laid eggs begin to hatch (around May 17), reports Hamilton.

Will Wisconsin have significant alfalfa weevil damage in 2008? Bryan Jensen, with UW-Madison’s IPM Program, isn’t sure. “Last year we saw a noticeable increase in weevil incidence and severity compared to previous years. That by itself doesn’t mean weevil damage will be higher this year. But it does give us a reminder that we should continue to scout alfalfa fields in a timely manner. The accumulation of 300 weevil degree days (base 48 degrees) is normally considered the time to start weevil scouting and that event will start very soon in the southern part of the state,” he notes.

There are several sources of degree day information including the Wisconsin Pest Bulletin (http://pestbulletin.wi.gov/) and the Wisconsin/Minnesota Extension weather website (http://www.soils.wisc.edu/wimnext/).

If time permits, spot-checking sandy knolls and/or south facing slopes (areas which warm up quicker) prior to 300 weevil degree days can give you a “heads-up” on weevil activity in your area. Look closely at the terminal leaves for small pin-hole feeding and for larvae. Early instar larvae are commonly found in folded leaflets at the terminal portion of the stem and may not have the typical lime green coloration. Instead they may be a pale yellow color but will have a black head and a white stripe on their backs, says Jensen.

Secondary insect injury possible

Jensen says the cool, wet weather Wisconsin has been experiencing has the potential to increase the amount of secondary insect injury to corn seed and/or emerging seedlings. The longer it takes corn to develop the longer it will be susceptible. “The end results may be reduced stands and poor seedling vigor which are often mistakenly attributed to planter problems or poor seed quality,” he notes.

“Furthermore, rescue treatments are not reliable or suggested,” says Jensen, noting that building accurate field histories is an important IPM practice so we can accurately choose preventive practices in future years.

Corn isn’t as susceptible to seedcorn maggot injury as soybeans. However, cool, wet growing conditions can increase injury potential to both crops as can livestock or green manure. Seedcorn maggots are often overlooked as a potential cause of poor stands. When emergence is uneven, dig up skips in the row to make sure the planter delivered a seed, Jensen recommends. Suspect seedcorn maggots if the seed has been partially fed on or if the white, legless larvae are present. Larvae have a very short life cycle and may not be present when you troubleshoot the field. Also, similar appearing saprophytic maggots may be found feeding on seed which is decomposing from other causes.

Symptoms of seedcorn maggots may also show up as small holes in the first emerged leaf (cotyledon) and is a result of below ground maggot feeding. Leaf injury is not considered economic because the plant has recovered, but it may help confirm below ground symptoms of maggot feeding. Maggot injury is also more uniform across the field compared to wireworms.

Cool/wet conditions also increases the potential for wireworm damage that may also be more common in corn after pastures or other crops which had grassy weed problems the previous year, Jensen reports. Like seedcorn maggots, wireworms can feed on ungerminated corn seed leaving skips in the row that result in similar misdiagnosis as seedcorn maggot (poor planter or seed health issues). Injured seed will appear hollowed out but the majority of the seed coat will be present. Larvae may be found nearby which are copper-colored and have three sets of jointed legs.

Don’t confuse with millipedes (a non-pest) which are also abundant in cool/wet weather. Millipedes are also hard-shelled but are dark gray and have a fringe of hair-like legs the entire length of the body.

Wireworms will also tunnel into the below ground portion of the shoot causing holes in the newest emerging leaves if feeding is concentrated above the growing point. If feeding is at the growing point, symptoms called “dead heart” or “wilted whorl” are evident, Jensen notes.

True White Grubs don’t injure seeds but feed on corn roots and underground shoots. Like wireworms, damage is more common in corn after pastures or other crops with a history of grassy weeds. Unlike wireworms or seedcorn maggots, white grubs will feed on corn roots. The aboveground symptoms are stunted, discolored plants from lack of nutrient uptake.

“White grub injury also mimics wireworms when they feed on below ground shoots that result in wilting of newly emerged leaves or the entire plant. White grubs are relatively easy to find in the soil around damaged plants,” Jensen states.

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