Pest & Crop

Pest & Crop Newsletter

Purdue Cooperative Extension Service
April 7, 2000 - Issue 3
In This Issue

Insects, Mites, and Nematodes

Agronomy Tips
Insects, Mites, and Nematodes

Last Minute Insecticide Decisions for First-Year Corn– (John Obermeyer, Rich Edwards, and Larry Bledsoe)

  • Rootworm damage in first-year corn is most likely in NW counties.
  • Many factors contribute to unsatisfactory rootworm control.
  • Environmental extremes and soil compaction are more harmful to corn than to rootworms.

We are still receiving questions from producers on whether or not they should use a soil insecticide on their first-year corn this season. Obviously, one does not want to spend the 15 to 17 dollars per acre for the insecticide if it won’t pay. In hindsight, many producers wished they had scouted their soybeans last August for the presence/absence of western corn rootworm beetles. This effort certainly would have increased one’s knowledge of fields at risk for larval damage to this year’s corn. Perhaps the following will help in last minute insecticide decisions.

Area of state. As outlined in past issues of the Pest&Crop, the northwestern counties of Indiana have the greatest risk to first-year corn damage. There we saw consistently high concentrations of western corn rootworm beetles in soybean. However, random soybean sweeps last August revealed dramatic variability in beetle numbers, not only from county to county but also field to field throughout northern Indiana. Once again, the best way to assess risk for your farm, area, etc. is to sample individual fields when the western corn rootworm beetles are active.

Insecticide performance. A variety of factors affect insecticide efficacy when applied to the soil. The most obvious, yet overlooked, problem is improper calibration of granular applicators on the planter. Simply put, all boxes should be calibrated every year. April corn planting stretches insecticide efficacy when rootworms are hatching in early June. Rootworm insecticides do not kill eggs, only hatched larvae! Soil moisture, temperature, microbes, and sunlight etc., begin the natural breakdown of insecticides immediately after application.

Wet soils. Tillage and planting in wet soils causes compaction. Compacted soils restrict corn root growth, which may inhibit moisture and nutrient uptake throughout the growing season. Compaction typically concentrates roots into narrow zones, if rootworms hatch within these areas, the larval damage can be devastating to the plant. Damage is further compounded should it get hot and dry (see the following paragraph). Excessive rain after planting may leach insecticides out of the root zone. Though insecticides differ in their solubility, none are immune to failure due to high moisture levels.

Dry soils. Is it going to be a drought year? Some prognosticators think so. Rootworm feeding that may not be economic during a “normal” growing season could be serious in a drought year. Rootworm feeding during drought, combined with compacted soils, just may be the last nail in the plant’s coffin. Insecticides need some moisture to difuse the active ingredient from granular carriers through the soil around the root zone where rootworms feed.

So when is a rootworm insecticide most likely to pay? The correct answer is when rootworm observations in last year’s soybean suggest the risk is high. Other possible, but less accurate, answers are: 1) if you farm in northern Indiana and/or 2) if you compact wet soils and the weather later becomes dry. The wrong answer is if you plant and apply insecticide extremely early, especially if it is a wet spring.

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As Usual, Black Cutworm Moths Are Here– (John Obermeyer, Rich Edwards, and Larry Bledsoe)

  • Black cutworm moths are beginning their annual arrival.
  • Moths arriving mid to late April pose the greatest threat to corn.
  • Scouting fields and treating when necessary makes more sense than the planting-time application of insecticides.

Black cutworm moths have started to arrive in the state, see “Black Cutworm Adult Pheromone Trap Report.” These data will continue to be collected by our cooperators and will be made available on a weekly basis during the early growing season. Black cutworm do not overwinter here, instead they are carried on wind currents sweeping into the Midwest from south Texas and northern Mexico.

Arrival of moths in March and early April is expected and means nothing to corn still in the bag. Moth flights of mid to late April are the ones to watch. Significant captures during this period, along with the use of heat units to predict the beginning of larval activity, gives us an indication of the potential severity of the problem and locations of concern.   Thus, we are able to predict with some degree of accuracy when and where crop damage is likely to occur. Watch for future Pest&Crop articles concerning this pest.

Should one treat for black cutworm before or at planting? The tried, true, and economic approach to black cutworm management is to scout fields, determine infestation and damage levels, and use a rescue treatment if needed. Scouting, treatment thresholds, and control information will be provided in future Pest&Crop newsletters. (See page 4 for Black Cutworm Adult Pheromone Trap Locations.)

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Black Cutworm Adult Pheromone Trap Report

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Alfalfa Weevil Damage in Southern Indiana– (John Obermeyer, Rich Edwards, and Larry Bledsoe)

  • Alfalfa weevil damage observed in southern Indiana.
  • Scouting techniques are given.
  • Use damage and heat unit accumulation information when making management decisions.

Surveys of southern Indiana alfalfa fields this past week (see “Alfalfa Weevil Larval Survey”) reveal that alfalfa weevil feeding has begun with some significant damage. The number of plants with tip feeding reached as high as 60% (range 28 to 60%) with up to 2.7 weevil larvae per stem (range 0.7 to 2.7). Alfalfa in these fields ranged from 9.2 to 10.8 inches in height.

Field scouting for alfalfa weevil damage should begin when approximately 250 heat units, base 48°F, have accumulated from January 1 (see “Weather Update”). Sampling a field to determine the extent of alfalfa weevil damage is best accomplished by walking through the field in an “M-shaped pattern.” Five alfalfa stems should be examined in each of 5 areas of the field, for a total of 25 stems from the entire field. Each stem should be examined for 1) tip feeding by alfalfa weevil larvae, 2) presence of healthy larvae, and 3) maturity of the stem, i.e., pre-bud, budding and/or flowering. The average size (length) of weevil larvae should also be considered. Large alfalfa weevil larvae are relatively easy to find. Small larvae are difficult to see. Thus, very close examination of leaves may be required to detect “pin-hole” feeding and small larvae.

By utilizing heat unit accumulation data to determine when sampling should begin and when a management action should be taken, producers can obtain the greatest economic return. If the application of an insecticide is required early in the weevil season, producers have the option of using a material that has good residual activity. Later in the season, short residual insecticides should be used and producers should pay close attention to harvest restrictions.

The management guidelines listed below have proven to be very effective in determining when alfalfa weevils should be controlled in southern Indiana. The times for sampling and the need for and timing of controls are based on accumulated heat units (HU) at a base temperature of 48°F and percentage tip feeding. Watch for HU information in each week’s Pest&Crop “Weather Update.” This HU information will help one determine when management steps should be taken.

Alfalfa Weevil Management Guidelines, 2000
Southern Indiana
Heat Units
% Tip Feeding
200-250   Begin sampling
300 25 Re-evaluate in 7-10 days using the appropriate HU or treat immediately with a residual insecticide if 3 or more larvae are noted per stem and % tip feeding is above 50%
400 50

Treat immediately with a residual insecticide

500 75 Treat immediately
600 75+ If cutting delayed more than 5 days, treat immediately
750   If harvested or harvesting shortly, return to the field in 4-5 days after cutting and spray if 1) there is no regrowth and weevil larvae are present OR 2) feeding damage is apparent on 50% of the stubble and weevil larvae are present
*As the season progresses, watch for diseased larvae (color progresses from yellow, to brown, to black).

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Alfalfa Weevil Larval Survey - 3/29/00
(Ron Blackwell)
Fields Sampled
Stem Ht. (in.)
Predominant Larval Instar
Total # Larvae*
% Tip Feeding
Dubois 1 9.3 2nd 50 56%
Dubois 2 10.5 3rd 36 36%
Dubois 3 9.3 3rd 17 28%
Dubois 4 9.2 4th 23 36%
Dubois 5 10.8 3rd 68 60%
Dubois 6 10.4 3rd 441 36%
*Number per 25 stems, extracted by Berlese funnel.

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2000 Black Cutworm Pheromone Trap Locations (#/County)

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Agronomy Tips

Corn Planting Fever!– (Bob Nielsen)

  • Early April plantings of corn are susceptible to many problems.
  • Use common sense.

Corn Planting Fever! No, it’s not the name of my favorite 70’s disco song. Rather, it simply reflects a fact of life that warm, dry, sunny days in early spring bring on daffodils, crabapple blossoms, dandelions and corn planters.  While I do not begrudge those who want to begin planting corn in early April, I feel compelled nonetheless to at least remind them of some of the risks of early corn planting.

Figure 1
Figure 2
figure 3

An important fact of corn life to remember is that germination and emergence will not occur rapidly or uniformly when soil temperatures are hovering at or below the 50°F mark.  In Indiana, soils typically do not warm to temperatures consistently above 50°F until mid-April (south) to early May (north). In fact, bare soil temperatures as I write this article are averaging less than 50°F in central and northern Indiana (see figures below). Consequently, it is not uncommon for early April planted corn to emerge three to four weeks after planting.

Early April planted corn will typically yield 4 to 6 percent less than that planted in late April or very early May (see table below). Slowly developing seedlings are more susceptible to damage by soil diseases and insects. Slow, uneven germination and emergence results in uneven stand establishment. If uneven emergence or damage by insects or diseases occur, yield losses can easily increase to as high as 20 percent.

Stand establishment may be so poor as to warrant replanting, which results in greater expense to the producer and possibly less yield than if the field were planted in late April or early May to begin with. By the way, if you have planted prior to April 6 and subsequently need to replant a failed field, don’t count on your crop insurance helping with the replant expenses (personal communication w/ George Patrick, Purdue Ag. Econ.). Check with your insurance agent for details.

The point of this discussion is to simply caution corn growers that there is plenty of calendar time within which to plant the 2000 crop before the proverbial end of the prime planting window in early May. If you are bound and determined to plant corn at the first opportunity, then consider  the following tips:

  • Begin planting with the hybrids in your lineup that are rated best for seedling vigor and/or whose cold germination ratings are the greatest.
  • Use one of several available pesticide planter-box seed treatments to hedge against a lengthy period of exposure to soil diseases and insects.
  • Avoid early planting in no-till fields with heavy levels of surface trash, because temperatures will be even cooler in those seed beds than in more bare fields.
  • Do not plant extra shallow hoping to capitalize on warmer temperatures nearer the surface. A seeding depth of 1 to 1?  inches remains a viable target even with early plantings. Shallower depths are more susceptible to the effects of soil drying, bird and rodent feeding, and uneven planter depth control.
  • Apply starter fertilizer in a 2-by-2 placement at rates of nitrogen no less than 20 lbs of actual N per acre.

Don’t forget, this and other timely information about corn can be viewed at the Chat ‘n Chew Café on the World Wide Web at  For other information about corn, take a look at the Corn Growers’ Guidebook on the World Wide Web at

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Weather Update