Circular 646 Borer Control in SWEET CORN UNIVERSITY OF ILLINOIS COLLEGE OF AGRICULTURE EXTENSION SERVICE IN AGRICULTURE AND HOME ECONOMICS In cooperation with ILLINOIS STATE NATURAL HISTORY SURVEY.
3 DEFENSES AGAI NST CORN BORER DAMAGE IN SWEET CORN Before May 1 plow under all cornstalks and other plant refuse. Adiust planting dates, so for as practicable, to escape heavy infestations. Use insecticides as recommended on all corn likely to be heavily infested. THESE 3 MEASURES, properly applied will do the iob. Urbana, Illinois June, 1949 Cooperative Extension Work in Agriculture and Home Economics: University of Illinois, College of Agriculture, and the United States Department of Agriculture cooperating. H. P. RUSK, Director. Acts approved by Congress May 8 and June 3, 1914.
CORN BORER CONTROL IN SWEET CORN By G. C. DECKER and J. W. APPLE 1 CORN BORER LOSSES in Illinois have increased each year since this pest2 first appeared here in 1939. In 1948 damage to Illinois sweet corn alone was estimated at about $9,. In areas where there is a moderate to heavy infestation of borers, growers of sweet corn will need to apply all available control measures. On early and late sweet corn the use of insecticides is practically a must. This circular tells how to use both planting practices and insecticides to the best advantage. Growers cannot yet look to resistant strains of sweet corn for protection, as no commercially grown strain has shown any special advantage in this respect. Search for such strains has gone on for some years and will continue. MARKET AND CANNERY GROWERS' PROBLEMS DIFFER The types of sweet corn grown for the fresh market (roasting ears) and for canning are so different that control measures for them have to be considered separately. Market grower's chief problem is first-generation borers. The young worms of this generation put in their appearance the very last of June and during July. (For further details about the time the insect appears in its different stages, see pages 15-16.) Since the first sweet corn to reach the market brings the greatest dollar return, most of it comes from early-maturing varieties (66- to 74-day) planted early. Moderate to heavy infestations of first-generation borers are to be expected on it. The 1 G. c. D ECKER, Entomologist, Illinois Natural History Survey and Illinois Agricultural Experiment Station; J. W. ApPLE, formerly Entomologist, Illinois Natural History Survey. 2 Pyrausta nubilalis (Hbn.). 3
4 CIRCULAR NO. 646 grower who wants to hit the early, high market will have to depend on insecticides for control on this early corn. He will.have to kill at least 95 percent of the borers. Applications from the air usually fall so far short of giving this control that they are not recommended for early market corn. Ground spraying or dusting must be used. The small percentage of the market crop that is planted late may suffer damage from second-generation borers. These borers are in the worm stage from the latter part of August through September. Control on late corn may be harder to get than control on the early corn since many of the eggs will be laid on or near the ears. Such corn is often not treated because of its lower cash value. Cannery grower's chief problem is usually second-generation borers. This is true for two reasons: canning corn takes longer to mature, and it is not planted until growing conditions are at their best (May 1-15). Plantings are then continued at regular intervals until possibly as late as July 1. Thus canning corn is planted too late to receive many eggs from first-generation moths. But any corn planted after June 1 is likely to be infested moderately or heavily with secondgeneration borers, and a large part of the acreage for many canners is planted after that date. The severity of a second-generation infestation in any field or community depends, of course, on the number of first-generation borers in the community. SOME GROWERS CAN ESCAPE DAMAGE BY REGULATING PLANTING Growers who are not interested in getting their corn on the early market can avoid serious first-generation damage in two ways: ( 1) by delaying planting a few days, or (2 ) by choosing varieties for early plantings that mature in 8 to 85 days instead of 66 to 74 days. "Safe period" planting. Varieties such as Golden Cross, that mature in 8 to 85 days, planted during the middle of the planting season will have the fewest borers from each genera
CORN BORER CONTROL I N SWEET CORN 5 Table 1.- CORN BORERS PER PLANT AS RELATED TO PLANTING DATE AND VARIETY Planting date Spancross (66-day) Harvest date First Second genera- generation tion borers borers Carmelcross (73-day) Harvest date First Second genera- generation tion borers borers April 28.............. May 11...... May 19............ May 31............. June 16............... June 28............. July 7............ July 14.............. July 14 July 2 July 27 Aug. 7 Aug. 15 Aug. 29 Sept. 7 Sept. 19 11. 3.2 1. 8.4.4 4.3 4. 8 4.3 2.2 July 19 July 27 Aug. 2 Aug. 15 Aug. 28 Sept. 7 Sept. 19 Oct. 5 6. 3.4 1..5 1.7 5.4 4.5.2 tion. In northern Illinois the safe period falls about June 1 in a normal year (Table 1). Early or late seasons will shift the period a few days to a week. The farther downstate corn is grown, the earlier the safe planting period will be. Suitable adjustment should be made for early- or late-maturing varieties. Earliest and latest plantings have most borers. This is true of both market corn and canning corn. Late June plantings show increased infestation as a result of eggs laid by second-generation borers. It is these late plantings that give the canner his biggest control problem. Difference in varieties. The late-maturing Carmelcross, a 73 day market corn, attracts fewer borers than Spancross, a 66-day variety (Table 1). An 8-day variety such as Golden Cross, when planted at the same time as Carmelcross, has even fewer firstgeneration borers than Carmelcross. WHEN AND WHERE TO TREAT Watch for Egg Masses Only corn that is in danger of developing a damaging infestation will justify treatment with an insecticide. When in doubt about. whether to treat, get a count of the egg masses. To count egg masses, examine thoroughly all parts of the plant, especially both sides of all leaves (Fig. 1) and all flag
6 CIRCULAR NO. 646 Note the position of this recently laid corn borer egg mass along the midrib on the underside of a corn leaf. Moth near egg mass is a female, the other a male. Photograph enlarged two and one-fourth times. (Fig. 1) 1 9 8 FIRST GENERATION SECOND GENERATION (!) 7 ~ GOr ~ 5 I- z 4 UJ :!j 3 a.. 2 1 Down State or ~ Early Season 5 1 15 2 25 3 5 1 15 2 25 3 4 9 14 19 24 29 3 8 13 ----JUNE---- ----JULy AUG.-- - SEPT. Graph lines show average hatching dates of corn borer eggs in the northern half of the state. The first-generation curve will shift to the left for early seasons and for downstate areas and to the right for late seasons and for upstate, as indicated. A variation of 1 days either way will include most Illinois conditions. The time that second-generation eggs hatch varies more from location than from any other cause. (Fig. 2)
CORN BORER CONTROL IN SWEET CORN 7 leaves (leaves at the tips of the ears). Distinguish between hatched and unhatched egg masses. Examine fields being considered for treatment every few days to find out when the first hatch occurs. The date when hatching starts and the date when 7 percent of the eggs have- hatched are important in timing insecticide applications to control first-generation borers on canning corn (Figs. 2 and 3). A grower unable to recognize corn borer egg masses will need to ask the help of an experienced observer. Treat Market Corn at First Sign of Egg Hatching Make applications every 5 days until four have been made. After a rain of more than half an inch, lay on another treatment as soon as possible. Reasons for starting treatment at this time are explained on pages 8 and 1. Treat any sweet corn that comes into tassel, or shows the tassel down in the whorl during egg hatching, if it has an average of 4 or more egg masses to 1 plants. Treat such corn even though it already has had four treatments. Corn in this stage is very likely to be damaged. Treat Canning Corn According to Egg Masses For first-generation control, treat only corn that has at least 1 egg masses to 1 plants and only corn that will tassel in about 1 to 14 days. Count as an egg mass each plant showing damage even though the eggs may have disappeared. For instance, consider 25 damaged plants and 75 egg masses equal to 1 egg masses. Apply insecticide when about 7 percent of the eggs have hatched (1 to 12 days after the eggs start to hatch). One treatment at this time can be expected to kill 5 to 65 percent of the borers (Figs. 2 and 3). Give the crop a second treatment 5 days after the first one if there are more than 2 egg masses per 1 plants at the time of the 7-percent hatch. (Count egg masses as directed above.) A third or fourth treatment is seldom necessary for first-generation c.ontrol on canning corn.
8 CIRCULAR NO. 646 1 CORN-BORER CONTROL SINGLE TREATMENT 9 OF DDT DUST Broken line shows percent of borers hatching on given dates. Solid line shows percent of borers killed when crop was dusted once with DDT (different test plots were dusted on the different dates indicated). The best control came when the dust was applied 1 to 14 days after the first hatch. (Fig. 3) To get second-generation control, start treatments at the first sign of egg hatching if the corn is more than 15 days from harvest and has 5 egg masses or more to 1 plants. For importance of early treatments, see Table 2. Examine again, on alternate days, any fields that do not have the above number of egg masses when hatching starts. If within 1 days after the first count was made, the egg Inasses have increased to more than 5 to 1 plants, and the corn is still more than 15 days from harvest, apply treatment. Repeat treatments every 5 to 7 days until the crop is about 1 days from harvest, or until the count of unhatched egg masses drops below 2 to 1 plants. Cover Feeding Areas Thoroughly The feeding habits of the borer determine where insecticides should be applied. A grower who understands those habits is likely to get the best results. Lay insecticide on leaves and first feeding points at time eggs are hatching. Treatment at this time will kill many of the newly hatched worms before they reach the stalk since they crawl along the leaves to the stalk before they start to feed.
CORN BORER CONTROL IN SWEET CORN 9 Corn borers feed on the leaves while the leaves are in the whorl. Small, irregular holes in the leaves of this young corn plant show that borers have been feeding on them. Many borers can be killed by applying insecticide at this stage. Applications to market corn should be made before and to canning corn shortly after the borers start work here. (Fig. 4)
1 CIRCULAR NO.646 Apply insecticide to whorl (curl) and kill many borers at that point. Put it on market corn before and on canning corn shortly after the borers start work there. The worms that reach the main stalk may seek shelter between a leaf sheath and the stalk. On young corn they are more likely to go into the whorl. A few days after they go into it, small, irregular holes in the new leaves will show that they are at work there (Fig. 4). Cover emerging tassel. As soon as the tassel develops in the whorl, the worms stop feeding in the whorl itself and move into the pollen sacs of the tassel to feed. A thorough covering of insecticide on the emerging tassel is therefore very important. Apply another dose to worms that leave the tassel. When the tassel reaches its full length and the silks appear, the borers leave the tassel, move down to the ear shoots and leaf sheaths, and begin their first burrowing into the stalk. These worms will be small to half-grown. An insecticide will kill most of them. Coat shoots and small ears if worms hatch after these parts are formed. Also keep the whorl, leafaxils (places where leaves join stalk), and ear zone well covered. Worms that hatch after the ear shoots or small ears are formed go directly to these shoots and ears. Protecting these parts is imperative. Table 2.-EFFECTIVENESS OF SINGLE APPLICATIONS OF INSECTICIDE ON SECOND-GENERATION BORERS Time insecticide was applied By DDT oil spray Percentage of borers killed in 1945 1946 1947 1948 By 5- percent DDT dust By 5- percent DDT dust By DDT oil spray By 4 percent Ryania dust First hatch................. 49 5 37 37 49 Five days later...... 33 29 34 13 53 Ten days later..... 19 11 8 35 Fifteen days later...... 5 27
CORN BORER CONTROL IN SWEET CORN 11 TREATING WITH GROUND MACHINERY Ground treatments do much better iob than treatments from the air. They can be directed to the exact area where needed, and the machinery can be adjusted to put the dust or spray on the part of the plant where it is most needed. For canning corn, ground treatments also have the advantage of giving greater protection to the ear. This is very important since half or more than half of the eggs from second-generation borers are sometimes laid on the flag leaves of the ears or on the main leaves adjoining the ears. Ground machinery can be used for either sprays or dusts. A ground sprayer or duster must, however, be mounted on a machine with high clearance in order to operate in tall canning corn in August and September. A detasseling machine makes an excellen t carrier. Standard row-crop sprayers give best control. These machines must be equipped with a boom, have three nozzles to the row, and provide pressure of 1 to 4 pounds to the square inch (Fig. 5). The boom must be adjustable. Here an operator is getting excellent coverage with an orchard sprayer adapted for use in the cornfield. Note that the spray is directed straight down into the tops of the plants. (Fig. 5)
12 CIRCULAR NO. 646 Nozzles that deliver a solid cone of spray give the best coverage. To convert hollow-cone nozzles to solid-cone nozzles, drill a 3/64-inch hole in the center of the whirl plate.. Use 5 to 1 gallons of dilute spray to the acre (see recommended insecticides and required amounts given below). On young corn, direct most of the spray into the whorl and the upper leaf sheaths. On corn that is developing ears, lower the two side nozzles and direct the spray toward the ear zone. Dosages of Insecticides for Dilute Spraying In 5 to 1 gallons of water use per acre Use wetting agent DDT, 25-percent emulsifiable concentrate.. 3 quarts None DDT, 5-percent wettable powder.... 3 pounds lis pound Ryania, IOO-percent powder.... 6 pounds lis pound Parathion, 15-percent wettable powder ( experimental).... 2 pounds lis pound A wetting agent is necessary with powders for adequate coverage. Use 1 pound of wetting agent to a 3-gallon tank. Some emulsifiable concentrates and wettable powders contain more DDT than listed above, so when using such products include an amount that will provide 1Y2 pounds of DDT to the acre. Low-gallonage weed-control sprayers can be modified for borer control. This is a relatively new method of spraying which has proved about as effective as ground dusting. The standard nozzle arrangement that comes on the sprayer must be changed. For best results use the arrangement described above for standard row-crop sprayers. Use 5 to 25 gallons of water to the acre to carry the insecticide. The nozzle openings of these sprayers are usually too small to permit the use of wettable powders. To use the wettable form of DDT, get special nozzles from the manufacturer. If emulsifiable DDT is to be applied, put on 3 quarts to the acre in whatever amount of water is used. Emulsion concentrates having a solvent (xylene type) with a low boiling point are not as injurious to corn as those having solvents with a high boiling point.
CORN BORER CONTROL IN SWEET CORN 13 Ground dusting is not so effective as high-gallonage spraying. But the method is reasonably effective and is recommended where ground spraying is not practical. When applying dust with ground machinery direct at least two dust nozzles at each row. Point the dust into the whorls on young corn and where it will cover the ear zone on older corn. A 4-percent Ryania dust applied at the rate of 4 pounds to the acre gives somewhat better control than 5-percent DDT applied at 3 to 4 pounds to the acre. Parathion dust (2-percent) applied at the rate of 3 pounds to the acre has given excellent control, but its use on sweet corn is still in the experimental stage. TREATING BY PLANE Spraying or dusting by plane is generally less effective than ground treatment. Either is a broadcast method. Nevertheless, the plane has three advantages: ( 1) it is the fastest way to cover large acreages; (2 ) it can be used when soil conditions will not permit ground operation; (3 ) it can be called in on short notice to meet emergencies or can be used when ground equipment is not available. Spraying. On a plane with a fixed wing or on a helicopter, use a spray boom to get the most even distribution. Provide each boom with multiple nozzles. How many nozzles to use on the boom will depend on how much spray per acre is to be put on and on the capacity of the nozzles. It is best to use at least 2 to 3 gallons to the acre. A few high-capacity nozzles will apply this quantity, but there are two objections to their use: ( 1) the spray will be deposited in large droplets and give poor coverage; and (2 ) large droplets containing DDT will, in some solvents, cause extensive injury to the corn plants. A number of low-capacity nozzles having Y1 6-inch openings will give better coverage than the highcapacity nozzles. The size of the droplets will also be nearer right (about 1 microns in diameter). The width of the swath should be limited to about 2 feet on
14 CIRCULAR NO.646 cub-type planes and 3 feet on Stearman biplanes. Provide flag men to guide the pilots. Use 3 quarts of 25-percent emulsifiable DDT to the acre in whatever volume of water is applied. Wettable DDT powders also have been used in airplane spraying where there is enough agitation in the spray tank to keep them in suspension. Use a wettable powder at a rate that will put on I1h pounds to the acre. Dusting. Dusting from the air is the poorest way to apply insecticides. The tendency of the dust cloud to drift means that much of the insecticide is not evenly deposited on the field where it is needed. Also there is too much dust in the middle of the swath and not enough for control on the edges. A 4-percent Ryania dust applied at the rate of 4 pounds to the acre gives better control than DDT dust. But either a 5-percent DDT dust applied at 3 to 4 pounds to the acre or a 1 percent DDT dust at 15 to 2 pounds to the acre will give reasonably satisfactory borer control. SOME CAUTIONS ON THE USE OF DDT Don't feed fodder or silage treated with DDT to dairy animals or cattle being finished for slaughter. Especially don't feed silage or roughage treated for second-generation borers to such animals. Where such crop remnants are to be fed, Ryania may be substituted for DDT. Even though residues are small, DDT may be stored in fat or thrown off in the milk. The extent to which DDT taken into the system may prove injurious to people is not yet known. Normal DDT residues on fodder or silage should not, however, be injurious to the animals. There is little or no danger that DDT used as recommended for corn borer control will poison roasting ears or canning corn.
CORN BORER CONTROL IN SWEET CORN 15 CORN BORER CALENDAR Growers who are not yet familiar with the life cycle of this pest will be interested in this brief statement of where and in what forms it lives through the year. An understanding of its different forms and its habits will show why clean plowing and time of planting are so important in its control and why insecticides, to be effective, must be used at certain times. Borer goes through four stages of development: egg, worm, cocoon, and moth. The moths of one strain produce only one generation a year. Moths of the other produce two generations a year, and in southern Illinois a few try to produce three. Usually about half of all the early-season worms change into moths that lay eggs for the second generation of borers. Strain that produces more than one generation a year is the more important. It may produce a very large number of borers late in the season even when early conditions are not favorable for it. These late-season borers not only damage the crop of the current year but live through the winter to threaten the next year's crop. It is impossible to tell the worms of the two strains apart by looking at them. Beginning with the winter and early spring months, this is the way the borer goes through the year: November - April. The corn borer passes the winter as a mature worm (larva) within old cornstalks or in large-stalked weeds. May - June. These worms become cocoons (pupae) while still in their winter quarters. About two weeks after the cocoons are formed, moths begin coming out of them (Fig. 6). The moths normally lay their first eggs on corn plants during the last two weeks of June. The eggs, in masses of about 2 each, are most often laid on the underside of the leaves along the midrib. The eggs hatch in 5 to 7 days. The young worms do not stay on the exposed parts of the plant but travel to the leaf sheaths, curl (whorl), tassel, or ears. In other words, they go to the tender, sheltered part of the plant at the time they hatch. The worms are usually half grown before they burrow into the stalk itself.
16 CIRCULAR NO. 646 July. About half the worms that mature in cornfields during J uly stay on as full-grown worms throughout the summer and the following winter; t hese belong to the single-generation strain. The other half of the borers remain worms about a month and then become cocoons; they belong t o the multiple-generation strain. August - October. Moths come out of the cocoons formed in J uly and lay their eggs. The moths of this two-generation strain are plentiful during August and continue in fewer and fewer numbers through September. The worms that hatch from their eggs are present during the latter part of August and in September. The ones that become full grown by cold weather spend the winter in stalks, as do those of the single-generation strain.. Many things affect borer numbers. Initial population, winter mortality, effectiveness of clean-up practices, earliness of spring planting, varieties planted, and climatic conditions such as wind, rain, and drouth - all combine to determine the intensity of each year's infestation. ~ ~. Q~ -.~ _~ e~~ A c o Four stages of the corn borer: (A) cocoon, or pupa; (B) moth; (C) an egg mass and a borer just hatched; (D) a partly grown and a full-grown borer. From June to October in infested corn the borer can be found in all these stages at the same time, though one stage will be most common at one time, one at another. (Fig. 6) - ~ ~. ~ '" 1M-5-49--41487