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Caterpillars and Corn: Sweet Corn Insect Pest and their Control

The Natural Farmer, Spring 1991
by Ruth Hazzard, IPM Specialist University of Massachusetts, Amherst, Mass.

CONTENT

INTRODUCTION
THE CATERPILLARS THAT LOVE CORN
CATERPILLAR CONTROLS
THE MINOR DINERS: Other pests
INFORMATION AND SUPPLIES

INTRODUCTION

Sweet corn is the queen of vegetables in New England. Whether from the garden or the farmstand, it is best steamed and buttered as soon as possible after harvest. Locally grown com is the best, and many believe that eating freshly-picked, locally-grown organic sweet corn is a culinary experience just this side of Nirvana Unfortunately, we're not the only animals that relish sweet can. Far more abundant than the bold raccoon, woodchuck blackbird are the coin-feeding insects. ID this article well familiarize you with the insects that relish sweet com, and some low-impact techniques to deal with them. But first, a little more about the different markets for sweet corn.

Economic returns. Corn prices vary through the season, with early crops bringing the highest price, and mid-to late August harvests selling the cheapest Direct marketing is the way to go for a good return on sweet corn, though of course there are costs that go along with retailing. With a typical wholesale price of $7 to $10 per bag (one bag = one bushel = 5 dozen ears), and a good yield of 200 bags per acre, you would gross $1400 to $2000 per acre. Contrast that with a wail price of $2.50 to $3.50 per dozen at the farmstand, for which you would gross $2500 to $3600 per acre. Of course, prices can sag lower in some markets. But an organic corn marketer may be able to keep prices higher than average throughout the season.

Organic: is there a premium price? In wholesale markets that want organic produce, the price differential can be quite substantial-- $3 to $6 per bag. For instance, in 1990 Northeast Coops paid $12-$15 per bag for northeastern organic corn, versus $6-$8 for conventionally grown corn. At a farmstand, the premium depends on whether your customers value organic enough to pay more for it. This depends on your region, your market and competition from both conventional and organic farms outside the region. But a loyal farmstand following can see you through a drop in the regional market paces- as well as a period of wormy corn. Customers at a farmstand will tolerate higher levels of worm-infested ears than wholesale buyers. Northeast Coops expects no more than 4% infested ears in organic com - not much different from the expectation for wholesale conventionally-grown corn, which is <2%. That's a demanding job, especially during corn earworm season!

Regional pest differences. How easy is it to grow corn that is free of "worms? That depends a lot on where you live in New England. The three major insect pests of sweet corn are all caterpillars that feed in the ear. All of New England is affected by the resident moth, European corn borer, and throughout the interior, northern, and western regions this is the primary pest throughout the season. Coastal and southern areas are in the path of migratory moths that move up the coast on summer stone fronts. These are the corn earworm and fall armyworm. Migratory moths are unpredictable in their arrival and, especially with corn earworm, are difficult to control in an "Organic farming system.

Crop growth stages are important for understanding when your com is susceptible to each of these pests. Corn first emerges as a spike from the soil then the leaves unfurl into the whorl stage in which only leaves are visible. At about 1840 inches, the tip of the tassel can be seen by looking down into the whorl, initiating the pre-tassel stage. Green tassel stage lasts from when the tassel is fully emerged from the whorl until pollen is shed. Appearance of green sap usually coincides with release of pollen. Silk grows quickly, about 34 inches in the first day and 2 more inches in the next four days. After pollen lands on the silk and fertilizes the ear, there is a period of dry silk during which kernels develop and swell to the milk stage, at which time it is ready for harvest. The period from first silk to harvest is usually about 21 days.

THE CATERPILLARS THAT LOVE CORN

European corn borer (ECB ) , Ostrinia nubilalis overwinters in New England and typically has two generations per year. It was introduced in the early 1900 s into New England on infested broom corn from Europe, and rapidly spread across the continent. The larvae food on many crop plants -- potato, peppers, beans -- as well as about 200 weed species. The winter is spent in final stage (instar) of larval growth, inside corn and other plant stalls. In spring the caterpillar pupates and the adults emerge in late May in Massachusetts, and in June farther north. After mating at night in grassy areas near cornfields, females move into corn to lay masses of 5-50 eggs on the underside of leaves. A single female can lay 500 to 600 eggs. Eggs hatch in about a wee}, depending on temperature. Larvae are found feeding first in the succulent tissue of leaf axils and newly formed tassels, inside the individual florets. After the tassel expands and dries out, they move to leaf axils or bore into the stem. Some of them move down into the ear, entering from the top or boring through the husk on the side. If you see fresh white yellowish frass (droppings) or a small round entrance hole, you will usually find a light bodied, dark headed caterpillar inside.

Adult flight periods can be monitored with pheromone baited traps set out in weedy areas near cornfields. Pheromone lures mimic the chemical composition of odors released by female moths at night, to attract males of their own species for mating. Since we are not trapping the half of the population that produces eggs, nor are the lures disrupting the mating process, these pheromone traps do not work as a control measure. What they do tell us is when adults are flying and in what quantity.

In most of New England, European corn borer has two distinct races which respond to different pheromone blends. These are called E (New Y - ) and Z (Iowa) strains, and they require two separate traps and Lures. The trap recommended by U. Mass. IPM is the Scentry-tm Heliothis trap, a nylon mesh trap which is hung on a 7-foot fence post with the trap base just above the vegetation. Use the pheromone rubber septum lure (one E and one Z) made by Trece. Not all commercial lures for a Riven insect world squally well, but this one has proven reusable in our comparison studies.

Figure 1 shows ECB trap catch at two sites in Massachusetts over 3 years. These sites are representative of regions in Massachusetts that show distinct patterns of insect pressure inland (Hadley) and coastal (Seekonk).

You can see that the onset of flight varies somewhat - it is later after a cold, wet spring -- but always occurs around late May or the first week of June. Local variations in terrain, warmth, nearby corn crops and other hosts, and management practices will change the size of the ECB population on your farm.

You may not need to set traps out on your own farm if your state IPM program is monitoring ECB and putting out flight information by codaphone or newsletter. When you know a flight has begun, it's time to get out into the field to check corn that is in the pretassel stage. The plant can tolerate larval feeding in the whorl without any loss of yield. However, borers that feed in the tassel may move down into the ears as the plant develops. If you find more than 15% of your pretassel plants with larvae, control is needed to be sure that ear infestation will be below 5%.

The first flight ends in mid-July, creating a "window" that is consistent across years and regions (Figure 1). This period of 2-3 weeks without adult activity lets some blocks of corn pass through tassel and silk stages unscathed. It should be noted, though, that in the Berkshires of Massachusetts and parts of New York State, a one-generation strain of ECB occurs which flies between the two generations of the more common races. This one is caught by Z-baited traps. Second-generation flight begins in early August, and may continue into September. (If vou are growing~ peppers, it is this second flight that results in borer in your peppers.)

Corn earworm, Hedicoverpa or Heliothis zea (also know as "tipworm" and tomato fruitworm) reaches us through long migratory flights from the southeast. High level winds of storm fronts carry them north. It is generally a late-season pest, but you can't predict exactly when and where the adults will arrive. See Figure 2, which shows earworm (CEW) captures in Hadley and Seekonk. In 1990, massive flights came virtually overnight on an early August storm -- earlier than usual, and reaching even into southern Vermont and New Hampshire. For earworm, monitoring with pheromone traps is especially critical. Because flights come in so quickly, having one trap on your own farm can be useful.

The same Scentry-tm Heliothis trap works well for CEW, baited with a Hercon-tm luretape and placed in a block with fresh silk The males caught in the trap are light buff-colored with a single dark spot in the middle of the front wing and a dark band along the margin of the hind-wing. You may find some non-pest moths in the trap, which have a plain wings or are rust-colored. Females, which may lay 1000 eggs each, are attracted to the silk odor. Eggs are laid on the sill and hatch in 2-6 days, depending on temperature (6+ days below 65 F, 2 days at 90 F). Larvae move directly down the silk channels after hatching, usually remaining near the tip of the ear and fouling their feeding area with frass. Fully-grown larvae leave the ear and pupate in the soil. Because damage is restricted to the tip, growers are often able to market earworm-infested ears successfully (retail only!). Cut the tips off, flick the worm out of the tip, let the customers pick ova the ears, husk the ears, trim and bag them, offer a 13-ear dozen, put up a sign that explains that it's wormy (but unsprayed!): you name it, it's been done

Our IPM action thresholds for earworm are based on trap captures and temperature. In Massachusetts they are based simply on pheromone trap captures. A capture of only 2 males per week will mean that infestations may exceed 20%. Non-organic growers may apply up to 5 sprays through the silking period, if captures are high, to keep infestation below 5% of ears.

Fall armyworm (FAW), Spodoptera frugioerda, is another strong flier that does not overwinter here but migrates north from the deep South. Figure 3 shows captures over three years at Seekonk and Hadley. Hadley is not typical of all inland areas, which tend to have lower flights. There is a lot of variation in size, timing, and duration of the flight, depending on location and weather patterns. In general, FAW is more sporadic and causes less serious ear damage than corn earworm. However, it can do serious damage to whorl-stage corn because it is a voracious feeder in the heart of the whorl.

Females lay scale-covered clusters of eggs on a variety of plants including sweet corn, where they prefer whorlstage plants. Eggs hatch in 2-10 days, and larvae feed deep in the whorl. Ragged holes and piles of brownish frass tell you that, if you dig inside the rolled leaves, you will find a fall armyworm. Or, you may find that it has already left the plant to pupate in the soil. Larvae are brownish to greenish with thin light stripes down the back and a dark brown head capsule that bears a distinctive inverted Y. The whorl feeding damage delays maturity and can reduce overall yield.

Fall armyworm will also feed in the ear, if flights occur when the corn is silking. They enter through the silk channel or bore through the husk.

A rigid plastic trap known as a Multipher-tm type 1, with a Raylo rubber septum lure, works well for FAW. It should be hung on a slanted fence stake, about 3 l/2 feet high, in whorl-stage corn. Mark it well with bright surveyor's tape or you may not find it again! A strip of pesticide-impregnated vaportape is usually placed in the trap to kill moths before they escape or beat their wings up, which makes them very difficult to identify. If you use these, handle them with gloves. Unfortunately, these traps do not work without a vaporous insecticide, and we are not aware of any botanical alternative. These traps also capture non-target moths, some similar to FAW, but generally these have plain wings. Fall armyworm males are light to dark tan, with a wingspan of about 1 1/2 inches, gray mottled light and dark patches on the wings.

CATERPILLAR CONTROLS

Tillage. Before your corn is even planted, fall or early spring tillage of corn stubble helps control European corn borer. field corn harbors ECB just as well as sweet corn, and so should be tilled as well. Some of the worst infestations of ECB in sweet corn occur in areas where it is surrounded by field corn which has not been tilled to suppress the borer. In the years following the ECB's introduction into the Northeast, laws were passed requiring plow-down of corn stubble. These laws are still on the books in Massachusetts. Illegal or not, you shouldn't leave the stubble standing!

Weed control. Corn borer females prefer a moist microclimate, and they may be attracted to weedy fields by the higher humidity or specific plant odors. The young larvae, before boring into plants, survive much better under humid conditions. It is also interesting that female ECB are repelled by the smell of injured corn plants and the smell of larval frass. As to the borer's culinary tastes, the world is its salad. Some of their favorite weeds are pigweed (the weedy redroot pigweed and cultivated amaranth as well), lambsquarters, cocklebur (Xanthium), curly and Mexican dock (Rumex), and stout-stemmed grasses like fall panicum. Research at U. Mass. has found that ECB infestation tends to increase within small weedy patches in a larger cornfield. So good weed control will tend to discourage ECB, and if you have spotty weed control, pick the weedy patches last!

Cultivar. Unfortunately the whims of the weather are most important in determining earworm and fall armyworm abundance in our area, and tillage has no impact on them. If you are in coastal or southern areas and don't plan to move to the hills to escape earworm, your choice of varieties can be important Those with longer silk channels, in which the husk wraps tightly around the ear's tip, prolong the time it takes for the earworm to reach the tip, reducing the eventual damage. Many of the supersweet varieties are notoriously lacking in husk protection at the tip; growers in Massachussets call these "grinning ears" because they show their tasty kernels, encouraging birds as well as earworm damage.

Bacillus thuringensis can be an effective control for fall armyworm in the whorl stage, and for ECB prior to its boring into the plant. A granular formulation can be applied by hand rather rapidly to small plots of corn, and will lodge right in the center of the whorl where the worm is feeding. Liquid sprays will also flow into the center of the whorl. Treating for ECB with B.t. must take place as the tassels are emerging. This is when the larvae typically re-distribute from the tassel to the lower parts of the plant, including the developing ear. Surfaces must be mated with B.t. at this time because the borer must ingest the B.t. before it begins internal feeding. This may require more than one application. ECB may also continue to infest ears to a lesser extent during the silking stage.

Note that not all B.t. materials are registered for sweet corn! Follow the label restrictions for both target pest and crop. Refer to chart included here, but be aware that registrations differ from state to state and from year to year.

Corn earworm, while it is susceptible in the lab to B.t., cannot in our experience be controlled by B.t. in corn. This is because the female earworm moth, unlike the ECB and fall armyworm, lays its eggs directly on the silk, and the newly-hatched Larva eats very little in its direct route through the silk channel to the tip of the ear. Since B.t. is a stomach poison, few larvae are affected by spraying the silk. In contrast, there has been some success in suppressing earworm in research trials with Neoplectana nematodes, applied in a liquid spray directly to the silks. Following ingestion of very few nematodes by the young caterpillar, they multiply rapidly and kill it. Finally with earworm, if you get a really bad infestation, you can take some comfort in rarely finding more than one per ear. It's the ultimate biocontrol: cannibalism.

Whorl stage controls. One of the important ways the Massachusetts IPM program has cut back on conventional sprays is by never recommending whorl-stage sprays for ECB. A healthy corn crop can outgrow this indirect damage, and there is no concern at this time for ear infestation. Also, especially in the cool moist days of the early season when first generation borer eggs have just hatched, a large proportion of larvae are often killed by a protozoan pathogen of ECB Nosema pyraustae research at Uconn. has shown.

In our IPM program, treatment for fall armyworm is recommended in the whorl stage only when 15* or more of plants are infested with live larvae. This threshold may be a bit on the overly-protective side, since given a fertile soil and good growing conditions, the corn will outgrow much damage. One grower we worked with near New Bedford had exactly 15% of plans infested in one late block. according to our scouting of 200 plans in the 3-acre field: To protect the crop, we recommended one whorl spray. One week later, the crop had less than 5% of whorls infested with live larvae. We commented on how the treatment had worked, and the grower expressed surprise, since he had never gotten a chance to spray the field! Some of the larvae had probably already left the plants and pupated, and some had probably succumbed to disease. In any case, the yield was excellent.

Release of Trichogramma wasps. Some growers have used releases of insectary-reared Trichogramma against ECB. These tiny parasitic wasps lay their own eggs in ECB egg masses. In theory this should work if sufficient numbers of vigorous, fertile Trichogramma wasps search efficiently in com for ECB eggs. However, they have not always proved reliable. There are many variables which may result in variable effects of these parasite releases rearing and shipment methods, wind, temperature and rain at the release site, proper timing with ECB oviposition, and adaptation of the particular strain and species of Trichogramma to the com habitat and its prospoctive host, the corn borer. There are many species and strains, not all of which parasitize ECB. Work is underway to import a species of Trichogramma which is adapted to the Asian corn borer, a very close relative of ECB, and is capable of overwintering in our climate. The use of Trichogramma has great potential, has received a lot of research attention both in this country and abroad, and will, we nope, become an effective and reliable strategy in the near future.

Generalist predators. Already out there on the farm are several insects that feed on insect eggs and small larvae, including ladybeetles (notably the 12-spotted ladybeetle, which feeds on eggs), predatory stinkbugs, lacewing larvae, minute pirate bugs (Orius) and ground beetles. The twelve-spot (Coleomegilla maculate) also feeds on corn pollen and aphids. Ladybeetles also help to reduce buildup of corn leaf aphids, which invade tassels in late June or July.

Floating row covers. If you are pushing for an early corn crop by planting into clear plastic or using floating row covers, you may find that this com reaches pretassel stage just as ECB oviposition begins. Research at U New Hampshire has shown that wide row covers left on into silk stage provides complete protection against ECB -- if it is anchored against winds, sealed at all sides, and enough slack to let the com grow tall! It pays for itself if you can re-use the row cover on subsequent crops and you will ga a premium price on early coml

THE MINOR DINERS: Other pests

If you've watched your corn plants closely, you know we haven't dealt with all the pests of sweet corn by describing the "key three. " In addition to these caterpillars, there are two other moth larvae which commonly infest whorl corn: the true armyworm (Pseudaletia unipuncta) and the stalk borer. In terms of thresholds, these are considered along with fall armyworm in a decision as to treatment of whorl corn crops. Both may be present throughout the corn season.

Late in the season, blue-green corn leaf aphids may be especially abundant in the tassel, and sometimes on the ear and flag leaves, producing sticky honeydew which supports black sooty mold. This is primarily cosmetic damage, and the aphid populations support the build-up of beneficial populations of lady beetles, lacewings, and minute pirate bugs.

Finally, several different types of beetles may pose occasional problems. Japanese beetles are fond of corn silk, and their feeding may result in spotty pollination if large numbers descend on your small plot. Sap beetles (also catted picnic beetles), typically yellow and black and about 1/4" in length, are attracted to fragrant vegetables and rotting plant matter of all sorts. Especially with ECB damage, they may be found in the silk or even in the ear, where they may eat the ECB larva as well as corn kernels. Supersweets with poor tip coverage may be more attractive. Finally, corn rootworms -- a real problem elsewhere on the continent are rarely a problem in New England sweet corn. You may see an occasional adult beetle on the silk-a But the southern corn rootworm is much more of problem on cucurbits, where it is known as the spotted cucumber beetle.

INFORMATION AND SUPPLIES

Traps and pheromones

Pest Management Supplies Co, P.O.Box 938, Amherst, MA 01004. (413) 253-3747 Great T akes IPM, 10220 Church Rd. NE, Vestburg, Ml 48891. (517)268-5693.

Pest Biology, Traps, Threshold and IPM Methods: If you are serious about using traps, the color photos, scouting information and threshold guides in these pamphlets will be especially useful.

Managing Sweet Corn Pest in Massachusetts, by David Ferro and Weber. Available from the coop. Extension Bulletin Center, Cottage A, UMass, Amherst MA 01003, for $1.50.

IPM for Connecticut Sweet Corn. by Roger Adams and Jude Boucher. Coop Extension, College of Ag & Natural Resources, UConn, Storrs CT 06268. Monitoring for Sweet Corn Pests, by James Dill, David Handley and on Mairs. Univ. of Maine Coop Extension, Highmoor Fann, P.O. Box 178, Monmouth ME 04259.

For a comprehensive list of scientific literature on biological control, write to Rodale Research Center, Box 323, RD1, Kutzeown, PA 19530 for their Sweet Corn IPM Bibliography.

Code-a-phones and newsletters:

Massachusetts: Weekly pest message from Vegetable IPM, Entomology Dept., Fernald Hall, UMass Amherst MA 01003, at a yearly subscription of $25. Codaphone number, 413-545-1724 Other states: contact your Vegetable Extension agents!

Insectaries for Trichogramma, other natural enemies

Producers of Beneficial Organisms, available from Bio-Integral Resource Center, P.O. Box 7414, Berkeley, CA 94707, for $3 plus $1 shipping. This will list sources. Best to order directly from an insectary that rears wasps, and to ask about quality control

 


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