While sweep net sampling for western corn rootworm beetles in soybean fields this week in northwestern Indiana, we saw plenty of caterpillars munching away on soybean foliage and picked up lots of them in our sweep net samples. While driving between fields we were surprised by the number of painted lady butterflies fluttering about. Of course these are the adults of the thistle caterpillar that can also feed on soybean leaves.
Green cloverworm is a sporadic foliage-feeding pest of soybean and is potentially destructive through September. The larva has a narrow white stripe on each side of its body and four pairs of prolegs. Early larval instars skeletonize the underside of soybean leaves, leaving the upper leaf surfaces intact. More mature larvae consume all leaf tissue, leaving only the major veins and giving the plants a ragged, hail-damaged appearance. Green cloverworm larval populations usually do not reach damaging levels due to their susceptibility to attack by parasites and pathogens. High larval populations now may result in large numbers of moths flying later this summer. These black moths are attracted to lights at night and can become quite a nuisance around homes and buildings.
The thistle caterpillar, Vanessa cardui Linnaeus, is a migratory species flying to and from Mexico and the Southwestern United States in the fall and spring. Adults arrive in Indiana in the spring and lay their eggs on several species of weeds. Although they feed on many weed species, their preferred host is Canada thistle. Several generations occur in Indiana before they begin their migration southwestward in the fall. The larvae are generally dark in color, with conspicuous yellowish markings on their bodies. The larvae are covered with many branching spines (they do not sting, see last week’s Pest&Crop). Full-grown larvae reach a length of about 1-1/4 inches. The larvae feed within webbed enclosures on the upper leaves and may defoliate entire patches of weeds and feed in a similar manner on soybean.
For management guidelines for these lepidopteran pests and other defoliators, consult the following table taking into consideration: 1) whether the number of defoliators present is increasing, decreasing, or appears to be constant, 2) the average defoliation level for the field, and 3) whether the number of diseased and/or parasitized larvae is increasing or decreasing in comparison to previous field visits.
Ron Blackwell, IPM Surveyor, has been busy sampling soybean fields in the state for western corn rootworm beetles. This sampling is to determine the presence and relative number of beetles in randomly chosen fields throughout much of Indiana. Preliminary estimates of beetle captures show very high numbers in several areas of the state. This information will be shared with you in a future Pest&Crop newsletter.
My point is that the only way to know if treating next year’s corn with an insecticide is justified following this year’s soybean is to sample for western corn rootworm beetles. Rootworm beetle numbers will soon be declining, so if you haven’t done so already, sample now. Though it is probably too late for sampling with yellow sticky traps, utilizing sweep nets or even visual observations is better than no information. Very soon the risk to next year’s corn lies within the soil out of view in the form of eggs that will overwinter and hatch next spring and early summer.
The current USDA corn per acre yield estimate for Indiana puts the Hoosier State squarely in first place among the major corn producing states and is identical to the final yield estimate of 147 bu/ac for the 2000 crop. What triggers this fearmonger alert is the memory of last year’s near epidemic of stalk rots that accompanied the high yields. While I’ve not heard of any significant discoveries of stalk rot yet in this year’s crop, I nonetheless believe folks should spend some time walking fields during the next several weeks to confirm that the problem is not developing.
Conceptually, the potential for stalk rots to develop when yield potential is high is due to the fact that high yield potential requires a lot of output from the photosynthetic factory that we otherwise call a corn crop. If stress of any kind during the grain fill period severely limits the photosynthetic output, the corn plant is more likely to remobilize stored carbohydrates from the lower stalk reserves to the developing ears. When the carbohydrate concentration declines in the lower stalk and root tissues during grain filling, the plant becomes more susceptible to the development of root and stalk rots.
Fortunately, this year’s crop is generally healthier than last year’s crop. Soil moisture conditions, on average, are also more favorable at this moment. Indeed, few if any reports of stalk rot have come to my attention.
Nonetheless, if you know that certain fields or areas within fields have been under considerable stress during the past few weeks AND the yield potential of the field was heretofore pretty darn good, it would behoove you to spend some time walking those fields and inspecting the condition of the lower stalks. The inspection technique is simple: Squeeze the lower stalk. If it collapses easily, you’ve got trouble. If you cannot squeeze it easily, you’re probably okay.
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 <http://www.kingcorn.org/cafe >. For other information about corn, take a look at the Corn Growers’ Guidebook on the World Wide Web at <http://www.kingcorn.org/>.
Severe drought stress in parts of Ohio, especially in the northeast, has raised questions concerning the potential for toxic levels of nitrates in corn harvested for silage. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. Under unfavorable growing conditions, especially drought, this conversion process is retarded, causing nitrate to accumulate in the stalks, stems and other conductive tissue. The highest concentration of nitrates is in the lower part of the stalk or stem. For example, the bulk of the nitrate in drought-stricken corn plants can be found in the bottom third of the stalk. If moisture conditions improve, the conversion process accelerates and within a few days nitrate levels in the plant returns to normal.
The highest levels of nitrate accumulate when drought occurs during a period of heavy nitrate uptake by the corn plant. A drought during or immediately after pollination is often associated with the highest accumulations of nitrates. Extended drought prior to pollination is not necessarily a prelude to high accumulations of nitrate. The resumption of normal plant growth from a heavy rainfall will reduce nitrate accumulation in corn plants, and harvest should be delayed for at least 1 to 2 weeks after the rainfall. Not all drought conditions cause high nitrate levels in plant. If the supply of soil nitrates is in the dry soil surface, plant roots will not absorb nitrates. Some soil moisture is necessary for absorption and accumulation of the nitrates.
If growers want to salvage part of their drought damaged corn crop as silage, it’s best to delay harvesting to maximize grain filling. Even though leaves may be dying the stalk and ear have enough extra water for good keep. Kernels will continue to fill and the increases in dry matter will more than compensate for leaf loss unless plants are actually dying or dead. Moreover if nitrate levels are high or questionable, they will decrease as plant get older and nitrates are converted to proteins in the ear. The kernel milk line can be used as a guide in determining the best time to cut corn for silage. When the kernel milk line has moved 1/4 to the distance from the top (or crown) of the kernel to the base, the whole plant generally contains about 60-70% moisture — usually the recommended moisture range for making corn silage.
If the corn is to be harvested for silage it is important to test the nitrate levels at the cutting height of the forage chopper to determine the highest levels of plant nitrate.
If the plant is marginal to high in nitrates then a lab analysis is recommended before feeding. To get a quantitative nitrate determination, send or deliver a sample of 6 to 10 plants (cut at the stalk height of the forage chopper) to a plant analysis laboratory. Lists are available from your county extension office. Results of the test can generally be obtained over the phone, fax, or email when tests are completed.
The following feed laboratories will test corn samples for nitrate:
Nitrate Field Test Kits are available from:
For more information on feeding corn with varying nitrate-nitrogen levels, consult: Nitrates in Dairy Rations, Ohio State University Fact Sheet AS-0003-99 available on-line at <http://ohioline.ag.ohio-state.edu/as-fact/0003.html>
There are a number of good extension publications on this topic - the following is one I recently came across from Kansas which is available on-line.
Nitrate and Prussic Acid Toxicity in Forage - Causes, Prevention, and Feeding Management. Kansas State University Extension MF-108 <http://www.oznet.ksu.edu/library/crpsl2/MF1018.PDF>