Department of Entomology
Planted corn seed and corn seedlings are attacked by a variety of insect pests. Many pests are beetles (adults and/or larvae), such as billbugs, wireworms, corn rootworms, seed-corn beetle, white grubs, and flea beetles. Others include various caterpillars (cutworms, armyworm, sodworm), fly larva (seed corn maggot), aphids (greenbug and corn root aphid), and thrips. If abundant, these pests will injure or kill the sprouting seed or small, sensitive corn seedlings resulting in under-productive fields and lower dollar return.
Since these pests cannot be adequately measured, within the soil prior to planting, granular soil insecticides are often applied at-planting for preventative control. Granular insecticides are burdensome to handle and apply, expensive, potentially hazardous, are often not adequately effective, and can be environmentally unfriendly. New protectants for planted corn seed and seedlings are need.
Neonicotinoids may have fewer problems than granular insecticides. These compounds are
modeled after the basic “nicotine” molecule but have been engineered to improve
insecticide activity. Benefits of the neonicotinoids include:
1. To evaluate the rate responses of clothianidin (Poncho®), imidacloprid (Gaucho® /
and thiamethoxam (Cruiser®) corn seed coatings against wild populations of southern corn
billbug and wireworms on commercial farms.
NEW SEED TREATMENT RATES
Traditionally seed treatment rates have been expressed as "pounds or ounces per 100 pounds of seed" or "grams insecticide per 100 kilogram of seed". Recently the terminology for corn seed treatments has changed to express the amount applied to each kernel. This designation is "millgrams of active ingredient per kernel" (mg a.i./kernel). For instance Gaucho® corn seed treatment is expressed as 0.16 mg a.i./kernel. Table 1 shows four equivalent versions of the new expression terminology. For example Gaucho® corn seed treatment is equal to "0.16 mg a.i./kernel" or "16 grams a. i. per 100000 kernels", or 4 grams a.i. per 25000 kernels" or 0.0088 pounds a.i. per acre (assuming 25000 kernels planted per acre). Table 1 shows these conversions for five neonocotinoid rates. It also shows the equivalent amounts of a.i. per acre if terbufos (Counter 20 CR) is used at 5 and 10 pounds per acre. This allows the reader to recognize the comparatively small amounts of neonicotinoid insecticide seed treatment used per acre.
I. Southern Corn Billbug
Four tests were conducted in 2002 (Hyde, Pamlico, and Pasquotank counties). The Pamlico test provided excellent data and results presented herein are from Pamlico,Co.. All the seed treatments performed relatively well at the higher use rates. Lower rates were not sufficient. Billbug damage accumulated very quickly after seedling emergence. At 11 days after planting the untreated plots averaged about 60% damage but most seed treatments and Counter 15G held damage below 10% (Figure 1). At 25 days after planting clothianidin (Poncho®), at rates from 0.75 mg to 1.25 mg a.i. / kernel, adequately protected seedlings from billbug damage (Figure 2). Thiamethoxam (Cruiser®) results were similar at the 0.57 mg and 114 mg a.i. / kernel rates. Imidacloprid at the highest rate (1.34 mg a.i. /kernel -- Prescribe®) was statistically poorer than the clothianidin but similar to the Cruiser®. The Counter® 15 G standard showed results similar to the better seed treatments at this early date (25 DAP). Other early season data, not shown here, were in general agreement with the data presented in Figure 2. Yields are presented in Figure 3 and illustrate the overall, season long, results from the experimental treatments . Yields were affected by dry weather and billbugs, and were low. Clothianidin (Poncho®) @ 1.25 mg a.i./kernel provided the best protection with clothianidin @ 100 mg and 75 mg a.i./kernel and thiamethoxam @ 1.14 mg a.i./kernel showing similar but somewhat lower yields. Imidacloprid @ 1.34 mg a.i./kernel (Prescribe®) did not perform as well as the previously mentioned treatments. See Photo 1, 2, 3, 4, 5, for pre-tassel pictures of the various treatments.
Wireworm tests in the 2002 season were planted in Camden, Hyde, and Pasquotank counties on mineral and organic soils. All tests provided useful data but the Camden Co. test was outstanding and data shown here is from that test. Corn is attacked by several wireworm species but the corn wireworm (Melanotus sp.) is the primary wireworm in NC corn plantings. It's life cycle may extend for more than two seasons . Eggs are layed in the mid-season in many differing habitats, often deposited in soil within the canopy of no-tillage soybeans. First year larvae can be found feeding on sprouting corn seed and seedlings the following season and are relatively small (ca. 1/2 to 3/4 inch) (Photo 6). The wireworms that growers most often see are the larger, second/third season larvae (Photo 7). Large larvae are much more damaging to corn than the small larvae. However, in the Camden Co. test, small larvae were very numerous and seriously damaged the untreated corn. There were few large larvae.
Data from the Camden Co. test are presented in Figures 4,5,and 6. At 30 days after planting 60% of the seedlings in the untreated plots (UTC) were damaged (Figure 4). All seed treatments at the higher rates (above .57 mg a.i. / kernel) averaged below 20% damaged plants and greater damage was observed in the lower rates, particularly in the imidacloprid 0.16 mg a.i./kernel rate (Gaucho®). By harvest time the plant population in the UTC had declined from the ca. 27000 kernels acre planted to about 7000 plants per acre (Figure 5). All three seed treatment insecticides showed a numerical rate response with the highest rates giving about 25000 plant/acre. However, there were few statistical differences and only imidacloprid @ .16 mg a.i./kernel (Gaucho®) was statistically separated from the other treatments. The yield data (Figure 6) closely paralleled at-harvest plant stand counts, indicating that plant population was the primary yield component and that if plants survived the early-season wireworm attack they were not seriously damaged or regained their health. High yields were obtained with most of the seed treatments. Both clothianidin and thiamethoxam showed the least rate response but imidacloprid showed a steep rate response, giving both the lowest (Gaucho®) and one of the highest (Prescribe®) yielding treatments. Yields from most treatements were statistically similar.
This test indicates that both clothianidin and thiamethoxam may be successfully used for wireworm control at about 0.25-0.30 mg a.i./kernel. Imidacloprid was not as active and needed ca. 2X the amount of active ingredient to give similar results. Low wireworm populations may be adequately controlled with lower rates.
III. Black Cutworm
Black cutworm is often a seedling pest in no-tillage planted corn or corn following weedy
pasture conditions. Less often cutworms occur in conventional tiliage corn fields.
This insect is active in cooler weather and the larvae feeding on
corn developed from eggs laid in the late fall, winter, or early spring. They are medium
to large size when corn seedlings are vulnerable to attack
Data on black cutworm were obtained from a billbug test in Hyde county, a greenhouse test,
and a laboratory test. In the field study
counts of cutworm cut plants
indicated that clothianidin was active against this pest whereas other seed treatments had
less effect, however, cutworm populations were low. Field grown corn seedlings at the
three-leaf stage were placed into petri dishes in the laboratory. Each dish was infested
with a cutworm larvae and scored at 24 hours for feeding and cutworm mortality.
Clothianidin @ 1.25 mg a.i./kernel allowed only minor feeding and killed all the larvae
(Photo 9 )
whereas larvae destroyed the plants in the untreated control
Similar results were seen in the greenhouse test with both clothianidin
@ 1.25 mg a.i./kernel and thiamethoxam @ 1.14 mg a.i./kernel. The UTC is shown in
and thiamethoxam @ 1.14 mg/kernel in
Although data on black cutworm is
currently preliminary it strongly indicates that both clothianidin and thiamethoxam are
active against cutworm if used at an adequate rate.
IV. Neonicotinoid Insecticide Interaction With Nicosulfuron Containing Herbicides
Corn growers commonly use nicosulfuron containing herbicides (e.g. Accent, Steadfast) and must avoid certain phosphate insecticides (e.g. Counter, Lorsban, and Thimet), due to a potential phytotoxic interaction between the two classes of chemistry. Farmers are interested to see if the neonicotinoid insecticide seed treatments show a similar reaction.
A field study was conducted on the Tidewater Research Station, Plymouth. Treatments included Counter 15 G @ 13 lbs/acre applied in-furrow, two seed treatments (clothianidin@ 1.25 mg a.i./kernel and imidacloprid @ 1.34 mg a.i./kernel) and an untreated control. Insecticide plots received three herbicide treatments --- zero Steadfast, 0.75 oz/acre Steadfast, and 1.5 oz/acre Steadfast (applied at the three leaf stage). Data are shown in Figure 8. Counter showed significant levels of damged plants when Steadfest was applied at 0.75 and 1.5 oz/acre but few damaged plants where Steadfast was not applied. The seed treatments did not show an interaction with Steadfast.
NCSU Insect Notes
Corn Production Guide
Vernon James Center
Cotton Insect Corner
Small Grain Production Guide
|Current research and updates are performed by Dr. Dominic Reisig (Extension Entomologist) and Steven Roberson (Research Specialist). Based on original material of Dr. John Van Duyn (Extension Entomologist, Emeritus).|