Alfalfa webworm (Loxostege commixtalis Walker), The

Circular 58

THE ALFALFA WEBWORM

OFFICE OF &rATE E~TO:\IOLOGIST

COLORADO AGRICULTURAL COLLEGE GEO. Y. LIST, State Entomologist

FORT COLLI~S. COLORADO

THE ALFALFA WEBWORM

(Loxostege commixtalis WALKER)

By ,J. L. HOER~ER, Deputy State Entomologist

The alfalfa webworm caused considerable injury to crops in Colorado in the years 1914 and 1920. In 1932, heavier losses occurred than in either of the previous outbreaks. In Northern Colorado over 10 thousand acres of sugar beets \vere abandoned and many thousands of acres of alfalfa seedings were lost in 1932. In this same year in-jury to crops occurred in practically every county east of the conti-nental divide. Injury to truck crops varied from 10 to 100 percent.

\Vhile the reasons for the sporadic occurrence of this pest are not fully understood, a study of the weather records shows that each outbreak has been preceeded by a dry year. The precipitation In those years was below normal, especially during :May and June.

Life History

Adult.-The moths appear very early in the spring, having been observed in the field as early as the last of ·March. This time varies, depending upon the climatic conditions. The first-brood moths continue to appear until the latter part of June, being most numer-ous during the first part of JY:Iay. They are seldom seen during the day except \vhen disturbed, or as they feed on flowers that are in blossom. On warm nights they often collect in large numbers around e1ectric lights.

This moth is rather small, having a wing expanse of 1 to 1 and one-fourth inches and is of a buff color, with gradations of light and dark shades of gray. The old moth is dull gray in color, due to the absence of wing scales.

This insect is closely allied to the sugar-beet \Vebworm and re-sembles it in general appearance, but can be distinguished by th~

markings on the underside of the hind ·wings. The alfalfa webworm moth has a row of spots on the underside of the hind wing, while the sugar-beet webworm has a broad continuous dark line in this area.

Eggs.-A few days after the adult moths appear, they mate, and the females soon begin to lay eggs, continuing to do so for about 2 weeks.

The eggs are about half the size of a small pinhead, oval-shaped, somewhat flattened on the under surface, and strongly convex on the upper surface. When first laid, they are pearly white in color, turning yellow as the worm develops within. A black spot appears a day or two before hatching. This is the head of the small worm

4 STATE ENTOMOLOGIST Circular 58

vvhich is developing inside. Very few of the eggs are laid singly. 'l'hey are almost always deposited in groups of 2 to 20, over-lapping each other, and usually placed on the under surface of a leaf, seldom on the upper surface. The average number per female is about 200. When fully developed, which takes from 4 to 6 days in warm weather, the small larva eats a ragged hole in the egg shell and crawls out, leaving the semi-transparent shell.

It is difficult, if possible at all, to distinguish the single eggs from those of the beet webworms. In groups, however, a distinction can usually be made, as the latter place their eggs in single rows, over-lapping, end to end. The former deposit theirs in over-lapping masses, and not in single rows.

Larvae or Worms.-When first hatched, the worms are about one-sixteenth of an inch in length, pale yellow or greenish-yellow in color, becoming somewhat darker as they increase in size. The first feeding is on the underside of the leaf to which the young worms attach themselves with a flimsy web. This web, on small plants, usually leads in tubular form to a place of concealment, sometimes several inches from the base of the plant and under a clod of earth. When disturbed, they retreat rapidly down this tubular web, or, if on the ground, they rapidly crawl back to the plant, traveling as well backward as forward.

Small worms skeletonize the leaves. As they increase in size, they eat the entire plant down to the midribs and stems. On large plants the web is generally attached so that it draws the leaves to-gether and within this area the worms feed until all the green tissue is devoured; then they web a new area and proceed as before. The amount of food they consume especially the last 4 or 5 days before they enter the ground to change to moths, is surprisingly large.

It takes 4 or 3 ·weeks for the worms to become full grown, at which time they are about 1 to 1 and one-fourth inches long and a little thicker than a common match. They vary in color from

green-i~h-yellow to almost black on the dorsal half, with a broad light stripe extending down the back, while the under side is almost free from stripes or markings. Laterally on each segment are 3 dark spots, each containing 1 to 3 bristle-like hairs.

The sugar-beet webworm has a narrow black stripe extending down the back. This marking makes it possible to distinguish be-tween these worms.

Pupa.-When ready to pupate, the worms enter the ground one-half to 1 and one-one-half inches and construct earthen cells, which are lined with silk. In a few days they shed their skins and change

April, 1933 A.-Alfalfa webworm, top to bottom : :Moths Eggs Worms Pupae and Pupae cases

THE ALFALFA WEBWORM

B

from worms to pupae, resembling nothing of their former worm-like appearance, or the adult moths that they turn into later. They are light yellow to dark bro·wn in color, depending upon their ages, about one-half inch long and one-eighth inch in diameter. The anterior end is rounded and the posterior comes to a point, bearing eight small spoon-shaped appendages. This character makes it easy to dis-tjnguish the alfalfa webworm pupae from the sugar-beet webworm pupae, which have eight bristle-like appendages on the posterior end. 'l'he pupa stage takes from 2 to 3 weeks, and then the adult moth emerges, and soon after copulation, lays eggs for the next genera-tion.

In 1920, three broods were reared in the insectary of the Colo-rado Experiment Station. This is the usual number for Northern

6 STATE ExTO:M:OLOGIST Circular 58

Colorado. The broods over-lap and different-sized worms and adult moths can be found in the field at the same time. This insect passes the winter as a larva in its silk-lined cell beneath the surface of the ground.

The second brood of moths occurs about the middle of July, and the third brood of moths, the first part of September.

In 1932, only a few of the first-brood worms that entered the ground and constructed a cocoon pupated at once. Some pupated at longer and longer intervals and emerged as moths thru the summer. About a third of these worms remained in their silk-lined cells and waited until the following spring to emerge as moths.

Food Plants

The larvae or worms of this insect feed on a large number of }Jlants. If short of food, the larvae will migrate like the army worm, going in large numbers in search of food.

In several instances the worms have migrated from alfalfa and '"·eeds on waste land to adjacent fields and have been noted feeding on cabbage, beans, corr1, young cherry trees, etc. The larvae are gen~

eral feeders and will freely eat almost any succulent growth, ex-cept small grains and grasses. Even these may be eaten if nothing else is available.

The following is a list of the food-plants on which worms were found feeding:

Cultivated Food-Plants

1. Sugar beets 7. Beans 12. Cherry trees

2. Alfalfa 8. Corn 13. Peas

3. Cabbage 9. Sunflower 14. Strawberries 4. Cherry 10. Spinach 15. Raspberries 5. Carrot 11. Garden beets 16. Honey dew melons 6. Parsnip

Uncultivated Food-Plants

1. Sunflower 7. Erect door weed 14. Salt bush 2. Lamb's quarter 8. Wild parsnip 15. Ragged robin 3. R.ussian thistle 9. Lady's thumb 16. Wild carrot 4. Pigweed 10. lVIalva (cheeses) 17. Thistle 5. Wild morning glory 11. Old witch grass 18. Red root 6. Resin weed 12. Ragweed 19. Sweet clover

April, 1933 _{THE ALFALFA WEBWORM 7}

Control on Sugar Beets

This pest has be.en successfully controlled on sugar beets by spraying with paris green, 4 pounds to 50 gallons of water, and the sprayer set so that 50 gallons cover 1 acre. The sprayer commonly used is the traction ''Iron Age'' machine with one nozzle for each row,

Sugar beets injured by migrating webworms.

with a total of eight nozzles. These machines have a good mechanical agitation and maintain a pressure of about 75 to 150 pounds. The height of the nozzle above the beets should be about 1 foot and on small beets, should be directly over the row.

Home-made sprayers for small patches can be made by mounting a hand spray pump on a barrel, and this placed on wheels. When this is done, some means of mechanical agitation should be provided, as a pipe transferring some of the pressure to the bottom of the tank cannot be depended upon to keep the material stirred, and soon clogs up. Paris green is a heavy powder and settles to the bottom of the tank if not constantly stirred.

The spray machine used should maintain sufficient pressure to apply the spray in a fine mist or fog, as coarse spray collects in drops and runs off.

Sugar beets are not easily burned by arsenical sprays, conse-quently these materials may be used without danger to the foliage. Paris green is a quick-acting poison but does not stick well to foliage. Arsenate of lead can be used, and is recommended by the Great

8 STATE ENTOMOLOGIST Circular 58

pounds of the powdered form to 50 gallons of water. Apply in the same manner as described above.

As high as 6 to 8 pounds of paris green to 50 gallons of water have been used per acre on sugar beets, with good results and very little burning of the leaves, but 4 pounds to 50 gallons for the same area will give just as good results when properly applied, and the expense for material is much less.

The best time for applying the spray is when the worms are small. At this stage they are hard to see and easily overlooked. The first indication that this pest is in the field will be the moths that fly up when walking thru the field. After this, the undersides of the leaves should be examined daily for eggs and young worms. Picking leaves at random in the field and examining them is the best way. When the eggs are observed in considerable numbers, all the material for spraying should be on hand and the field sprayed when the eggs are found to be hatching. If spraying is delayed until the worms have the plants webbed over, results will not be so rapid as when sprayed sooner, due to the web tending, to a certain extent, to keep the poison from the leaves, and one must not expect results until the worms web a new area.

Co,ntrol on Truck Crops

When the worms are small, good control can be obtained by thor-oly spraying the infested plants with lead arsenate or calcium arsenate at the rate of 3 pounds to 100 gallons of water. After the worms are over one-half inch in length this spray should be increased to 5 pounds to 100 gallons of water. Arsenate of lead at these strengths has successfully poisoned the worms on carrots, parsnips, celery, corn, garden beets and small cherry trees. Injury to truck crops during the month of May was for the most part in weedy fields, where the weeds had attracted the moths for egg-laying. When the weeds were eaten or the fields cleaned from weeds, the worms moved to the cultivated crops. Clean culture following a heavy flight of moths, will help keep the moths from depositing eggs in the field.

On crops such as spinach, lettuce and strawberries, spraying with lead arsenate or calcium arsenate, should be at least 2 weeks or more before they are marketed. Usually this period is sufficient so that the crop will not contain the arsenical residue in injurious amounts when sent to market. Powdered hellebore, at the rate of 1 ounce to 1 gallon, may be used as an emergency spray a few days before marketing these crops, without danger of poisonous residues. However, hellebore is expensive and it is important to obtain fresh material as it looses its strength rapidly on exposure to the air, and on standing from one year to another.

April, 1983 _{THE ALFALFA WEBWORM 9}

It is very difficult to make a liquid spray stick to such plants as cabbage and peas. Here lead arsenate or calcium arsenate, 1 part by weight, mixed with 5 to 8 parts of hydrated lime (plaster lime) used as a dust, gives satisfactory control. Where the infestation of worms is very heavy a second application of spray or dust may he necessary.

Control on Alfalfa

Alfalfa that is heavily infested with worms, should be cut and harvested early as it will decrease in quality on standing. After the hay is cut a weighted corrugated roller, run over the field, will mash many of the worms. If some of the worms have entered the ground to pupate, thoro harrowing will break many of the silken cocoons and drag others to the surface where exposure to the sun will kill many of the worms or pupae.

If it is necessary to spray alfalfa or forage crops, calcium arsen-ate at the rarsen-ate of 1 pound to 50 gallons of warsen-ater should be used. Calcium arsenate is not an accumulative poison and is eliminated by livestock when taken in small quantities.

In young alfalfa fields, when irrigating water is available, the best procedure for combating this pest is to apply water in order to force the growth of the plants to exceed the injury of the worms. Spraying with calcium arsenate, 1 pound to 50 gallons of water, may be advisable in some cases before irrigation.

It is questionable whether early seeding of alfalfa should be attempted in areas where injury occurred the previous year, unless planted without a nurse crop and the equipment for spraying is avail-able. Late seeding of alfalfa will usually escape injury.

Stopping Migrating Worms

If the worms are short of food they will migrate in search of new food, feeding on almost any succulent growth. Heaviest migra-tion has occurred from abandoned fields where the worms stripped the weeds and were seeking a new food supply. Heavy migration has also occurred from alfalfa and cut alfalfa fields.

Trap Furrows.-Trap furrows are used to control the migrating worms. The furrows are of two types: (1) The dusty-sided and (2) the vertical sided.

The dusty-sided furrow is adapted to dry, loose soil. It is more efficient and less expensive. Whenever possible this type of fur-row should be made.

It is constructed by turning a deep furrow, throwing the earth toward the crop to be protected. Running the plow thru the furrow

10

Oireclion

or

miqrafion

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1.8"

.Prolecfed

lle/d

Diagrammatic cross-section of dusty-sided furrow.

a second time will give a depth of 16 to 18 inches from trough to crest. A smooth log or keg, weighted down and following the plow, will mash the clods and reduce the sides and bottom to a dust. If a dust does not result the first time thru, repeat as often as necessary. In moist, firm ground, the vertical-sided furrow can be con-structed. The vertical side of the trench is made by means of a disc cutter attached to the plow. It may be necessary to trim it with a sharp spade to make it sufficiently smooth to prevent the worms crawling up it. The vertical side is next to the crop to be protected.

It may be impossible to construct the dusty-sided furrow during rainy weather.

.Direction or miqration

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Profecfed

lie!

a'

April, 1933 _{THE ALFALFA WEBWORM 11}

Neither of these furrows will entirely stop migration. Worms can get by them. Poisoned bran mash, as used for grasshoppers, or poisoned dried beet-pulp scattered in the furrow, will poison many of the worms. The heaviest migration of worms usually takes place from 8 to 11 a. m. The poisoned bait applied during this time, at the rate of 50 to 100 pounds per mile, will poison the worms. Dragging a weighted log or keg thru the dusty-sided furrow will mash many of the worms.

Spraying a strip of crops on the side of the field to be protected will poison the worms that succeed in passing the furrow.

Mashing the Worms with a RoUer.-A smooth, weighted roller may be used to mash the migrating worms. The roller can be run over small plants without injuring them to any great extent. On large plants this would not be advisable. After rolling, cultivate row crops as the worms can crawl very rapidly, about 3 feet per minute, over the rolled ground. Repeat the rolling and cultivating as often

a~ necessary to stop the worms. The use of a corrugated roller is advisable on cut alfalfa, but is too severe on other crops.

Use of Water as a Barrier.--A large stream of water will stop the migrating worms. The worms are able to cross a small stream if not flowing swiftly and are not injured by the wetting. One irri-gation lateral carried the worms down stream about three-fourths

12 STATE ENTOMOLOGIST Circular 58

mile and delivered them, apparently uninjured, onto a beet field. Still or slow-flowing water, with a little kerosene or oil on the sur-face, makes an effective barrier. A container of oil with a slow drip will supply the oil to the surface of the water.

Oil Barriers.-Oil barriers can be used to stop the marching worms. They are constructed by making a shallow furrow along the field to be protected. An automobile driven back and forth a few times makes a good furrow in soft ground, and if run thru a fur-row, mashes any clods. Next, ordinary building cement is mixed with water, about a pound to 1 gallon, and applied with a sprinkling can, to the bottom of the furrow. Sufficient cement must be used to make a light crust of cement in the furrow. After this dries slightly, a narrow strip of crude oil is poured in the furrow. Oil poured directly on the ground soon sinks into the soil so that the worms can cross it. A sack of cement and 50 gallons of crude oil will maintain a quarter mile of barrier for 3 or 4 days. It may be necessary to renew the oil each morning, to prevent the worms cross-ing. Worms getting only a slight amount of oil on them will be killed even tho they crawl back from the barrier.

Use of T·urkeys.-Invading worms have been stopped by mobiliz-ing a large flock of young turkeys and lettmobiliz-ing them range over the infested fields. The turkeys fed freely on the worms and stopped the injury that would have occurred.

Natural Enemies

A few natural enemies of this insect are known. One of the ground beetles was found feeding on the worms. Four tachnia-flies,

Zenillia vulgaris Fall, Z. caesar Ald., Z. trisetosa Cog. and Achaet-oneura archippivora Will, and 3 brachonids, Agathis media Ores.,

J.lleteorus lomostegei Vier., and Bracon vulgaris Ores., have been reared from the worms. Not much aid can be expected from these enemies, as only about 3 percent of the worms have been found parasitized.