VJTATE PLAiVi BOAR^ E-519 December 1940 CONTROL OF THE PALE WESTERN CUTWORM IN THE SOUTHERN GREAT PLAINS REGION By H. H. Walkden, Division of Cereal and Forage Insect Investigations INTRODUCTION During recent years the pale western cutworm (Agrotis orthogonia Morr.) has become a pest of considerable importance in the small-grain areas of the southern Great Plains region. Prior to 1911 this insect was not known to be of economic importance in this region. In 1911, accord- ing to Arthur Gibson, it caused extensive injury to grain crops in southern Alberta, Canada. The first known outbreak in the southern Great Plains area occurred in 1923. Since that time outbreaks have occurred at intervals in various localities in the western parts of Kansas and Oklahoma, the Texas panhandle, northeastern New Mexico, and eastern Colorado. In the spring of 193S a severe outbreak developed in western Kansas and was still in progress in 1940. In 1939 and 1940 this cutworm caused extensive damage to winter wheat in northeastern New Mexico. NATURE AND TIME OF APPEARANCE OF THE INJURY The first sign of damage to winter wheat is usually noticed about April 10 to 15. The greening wheat fields begin to "fade out" in spots, taking on the appearance of wheat killed by drought. These spots enlarge rapidly, and frequently the entire field soon is destroyed, with the ex- ception of a few spots where harder soil prevented penetration by the cut- worms. The greatest damage usually occurs during the last part of April and the first week in May. In this period the worms consume the largest amount of food. The rapidity with which a large field of wheat or barley is de- stroyed is one of the astonishing characteristics of an outbreak of this cutworm. Plants attacked by the pale western cutworm usually are severed below the surface of the ground. Working along the drill rows, the worms pass from one plant to another, cutting them off at or just above the crown. Feeding occurs almost entirely underground, and at a depth depending on the looseness of the soil and the position of the moisture line. The worms are unable to penetrate hard or crusted soil. This fact accounts for the small areas which escape injury in most infested fields. -2- HOW TO RECOGNIZE THE PALE WESTERN CUTWORM The pale western cutworm usually may be recognized by its smooth and shining slate-gray color and the absence of body markings or stripes. The head is light brown, with a vertical darker brown band in the center. When viewed from the front, this band has the appearance of the letter "H." The absence of any markings or "freckles" on the sides of the head distin- guishes this species from all other common cutworms. SEASONAL HISTORY The pale western cutworm has but one generation annually. The eggs are deposited in the soil by the parent moth, or "miller," late in September or early in October. These eggs are very small, about the size of a small pinhead, and, since they are placed in the soil, it is diffi- cult to find them. The eggs remain in the soil during the ensuing winter and ordinarily hatch during the first warm days of the following spring, although some hatching may possibly occur during warm spells late in the fall or during the winter. The newly hatched larvae, or worms, feed at first on the tender leaves of wheat or other available food plants. This early feeding by the small worms gives the leaves a ragged or skeletonized appearance. Soon, however, the worms enter the soil and thereafter feed almost exclusively beneath the surface of the soil unless excess moisture brings them to the surface. Feeding continues throughout April, and by the last week in Kay the majority of the worms have reached their full growth and are then about 1-1/2 inches in length. In northwestern Kansas a small proportion of the worms have been observed to feed well into June in some seasons. When fully matured, the worms construct small elliptical cells from 2 to 4 inches below the surface of the soil, depending on the depth of the plow sole or "hardpan." After constructing their cells, the worms become inactive within them and reu-ain so until early in August, v/hen they transform into the pupa or chrysalis stage. This is the transition period from the worm to the adult. or "miller." When development is complete, the moths begin emerging from the soil, usually late in September or early in October. After mating, the females begin depositing eggs in the soil, thus completing the life cycle. The eggs are laid during the night, and if automobile headlights are turned on infested fields just after dark the moths can be seen in flight, alighting here and there to deposit their eggs. The moths are short-lived and feed only on the nectar of flowers. The seasonal history of the pale western cutworm is shewn graphically in figure 1 . FACTORS WHICH INFLUENCE ABUNDANCE The worms prefer a dry, loose soil and avoid moisture whenever pos- sible. A dry spring is favorable for their survival, whereas excess moisture during the active feeding period of the worms is conducive to the spread of a disease which kills them in large numbers. Excessive moisture in the soil also drives the worms to the surface, where they are subject to attack by their parasitic and predatory enemies. -3- Soveral successive dry springs are likely to result in an increase in the pale western cutworm population to outbreak proportions, v/hereas a wet spring as a rule tends to reduce their numbers to a point where no ap- preciable damage occurs the following year. CROPS ATTACKED Observations made during recent years indicate that winter wheat and spring barley are the principal crops attacked in the southern Great Plains region. In rare instances newly seeded alfalfa has suffered severe damage. Row crops planted before June 10, on ground where the worms destroyed wheat or barley, are also subject to attack if active worms are still present at seeding time. CONTROL MEASURES Cultural Practices Extensive field observations have indicated that certain land-manage- ment practices are of definite value in preventing injury by the pale western cutworm. First, wheat sown in the fall on land which has been cleanly fallowed during the preceding suff-mer, and which had been planted during the preceding year to a row crop such as sorghum, almost without exception has escaped serious injury, notwithstanding some marginal damage where such fields bordered infested wheat-stubble land. Second, alternate wheat culture and summer fallow appears to be an effective means of reducing the chances of severe injury. In applying this method, the stubble after 1 year's crop is left undisturbed until the follow- ing spring, when the ground is cultivated and kept clean throughout the sum- mer until wheat-seeding time. The spring cultivation should start as early as possible without incurring danger of spring soil blowing, preferably be- fore April 15. With but few exceptions, wheat grown in this manner has es- caped severe injury. It should be noted that wheat on sum.mer-f allow ground in which insufficient moisture has been stored during the fallow period, or which supported any weed growth during Ihe egg-laying period of the moths, was subject to attack during periods of heavy general infestation. It should also be noted that the practice of clean summer fallowing in alternate years is recommended for the purpose of minimizing the effect of outbreaks of the pale western cutworm. From the standpoint of soil conservation, subsurface cultivation may be more desirable than clean fallow, but its effectiveness as a substitute for clean fallow in controlling the pale western cutworm has not been determined. A control measure developed by H. L. Seamans of the Canadian Depart- ment of Agriculture, and found effective in southern Alberta", is based on the starvation of the newly hatched worms. This is accomplished by thorough cultivation of the stubble fields to destroy all green vegetation early in the spring as soon as the weeds and volunteer grain show 1 to 2 inches of growth above ground, followed by a delay of 10 days before seeding to a spring grain crop. In the southern Great Plains, however, where fall-sown wheat is the chief crop, the starvation method of control would not be applicable un- less the seeding of a spring grain crop, such as barley, on wheat-stubble -4- land could be delayed until the starvation of the young cutworms is ac- complished. The method is believed not to be generally applicable in this area for the reasons that (1) the hatching period of the pale western cut- worm overlaps the seeding date of spring barley and (2) the starvation kill would have to approach perfection because a cutworm population which would not materially injure well-established winter wheat plants would be highly destructive to young, 2-leaved barley plants. However, this method of con- trol has not been tried in the southern Great Plains, and is mentioned here because circumstances may arise which might warrant giving it a trial. Utilization of Land on Which the Crop Has Been Destroyed Once a destructive population of the cutworms has become established in a field, it appears to maintain itself until starved out or until cli- matic conditions occur which are unfavorable to it. Consequently, land on v;hich wheat or some other small-grain crop has been destroyed by the cut- worms in the spring should not be returned to wheat the following fall. Many instances have been observed during the present outbreak where the continu- ous planting of wheat has resulted in the loss of three consecutive wheat crops. Wheat or barley seeded into wheat-stubble land is almost certain to be destroyed under outbreak conditions. However, land on which a crop was destroyed in the spring may be planted to row crops with safety, provided the seeding of the row crop is delayed until the feeding period of the cutv/orms is past. Ordinarily, row crops can be planted safely under the above conditions by delaying seeding until June 10. In certain years, however, cutworm development is delayed, and in northwestern Kansas actively feeding worms have been found as late as June 25. The safe date should be determined by a careful examination of the infested field, and if all worms found are in their cells or have passed into the paler, somewhat shrunken summer stage, no further damage will occur. Use of Furrow Barrier as a Means of Protecting Crops Where the worms are invading the margin of a noninfested field from an adjacent infested area, furrow barriers are of some value in stopping the advance of the worms. Considerable success was obtained in western Kansas in protecting wheat or barley on fallow ground from invasion by list- ing or plowing several deep furrows between the crop to be protected and contiguous heavily infested fields. CONTROL MEASURES THAT ARE OF DOUBTFUL OR LITTLE VALUE Once the pale western cutworms are generally distributed in a field of small grain there is little that can be done to prevent destruction of the crop. Poisoned Bran Bait Repeated attempts to c'estroy the worms with poisoned bran bait spread on the soil, or placed in the ground with a wheat drill, have generally resulted in complete failure, or in such a small percentage of kill that the reduction in numbers of worms would not warrant the expense of baiting. Use of Lights to Attract the Moths At night the moths are rather strongly attracted to lights, and this habit suggested the possibility of trapping the moths as a means of reduc- ing the number of egg-depositing females. However, during four seasons of work with light traps in the area it was found that only 18 percent of the adults captured were females, and the majority of these had deposited nearly their full complement of eggs before capture. Furthermore, the expense of oper'ating a sufficient number of light traps would be prohibitive. IMPORTANCE OF KNOWING THE IDENTITY OF THE SPECIES INVOLVED IN CUTWORM OUTBREAKS There are many different species of cutworms in the Great Plains region. When cutworms are found in crop or pasture lands, it is very iirportant to know the identity of the species involved. Most of the cut- worms can be successfully combated with poisoned bran, but the pale western cutworm is the notable exception in the Great Plains region, owing to its habit of subterranean feeding. Specimens of cutworms should be submitted to the local county agent, who will have them identified by some qualified person. Control measures cannot be recommended until the species is known. STATE PU^ BOARD Digitized by the Internet Archive in 2013 http://archive.org/details/controlofpalewesOOunit I • o o • 1 1 1 1 1 1 1 ■ 1 1 1 ■ 1 1 u 4 t 1 1 4 1 1 1 i 1 1 1 • ■ 1 ^ ^ ! I JNIVERSITY OF FLORIDA .iililillillliilll 3 1262 09224 7484