UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY 4, CALIFORNIA CONTROL OF THE ALFALFA BUTTERFLY RAY F. SMITH,' A. E. MICHELBACHER/ and G. L. SMITH' The alfalfa butterfly, Colias eurytheme Bois- duval, is one of the most important insects which attack alfalfa. Although it is found everywhere in California where alfalfa is grown, it has proved to be a serious pest only in the warm in- terior valleys. Damage has been observed in the Imperial, Antelope, Sacramento, and San Joaquin valleys. Experiments conducted at the Califor- nia Agricultural Experiment Station by Smith and MacLeod 4 have shown that dusting sulfur is effective in killing the caterpillars. Further Description and Life History of the Insect The adult alfalfa butterfly is the common yellow butterfly with black markings seen about alfalfa fields (fig. 1). The males are distin- guished from the females by their slightly smaller size and by the solid black border on the upper, outer surface of the wings. In the female the border is broken by spots of the base color of the wings. The female also occurs in a white color phase. r -ยป | Fig. 1. --Adults of the alfalfa butterfly. Left, females; right, males; upper row, dorsal view; lower row, ventral view. (Natural size.) investigations seem necessary, however, before definite recommendations can be made concerning the use of sulfur. Since sulfur should be used with a great deal of caution, this publication has been prepared in order to point out the ad- vantages and disadvantages arising from its use in controlling alfalfa butterfly larvae on alfalfa. Principal Laboratory Technician, Division of Entomology and Parasitology. 2 Assistant Entomologist in the Experiment Station. 3 Associate in the Experiment Station. 4 Smith, Ray F., and G. F. MacLeod. Alfalfa butterfly control with sulfur dusts. Journal of Economic Entomology 26 ( 5) : 665-71 . 1943. The ribbed, somewhat cigar-shaped eggs (fig. 2, A) are laid singly, usually on the upper sur- face of the leaves of the alfalfa plants. When first laid they are pearly white, but as incuba- tion continues, they turn faintly pink; before hatching, the black head of the larva, which is apical in position, shows through the delicate eggshell. The hatching larva chews its way out, fre- quently eating the entire shell before starting to feed on the leaves. The black head of the newly hatched caterpillar distinguishes this first stage from the other stages in which the head is green. Before reaching maturity the larva casts its skin four times. The full-grown larva (fig. 2,B) is about 1-1/4 inches in length. It is velvety green in color, bearing along. [1] UNIVERSITY OF CALII I IBRARY ALLEGE. OF A ,-UCULTURE DAVIS either side a white line with red markings. Down the back there may be two fine white lines. At first the larva grows slowly and its feeding is not noticeable. However, by the time it reaches 1/2 to 3/4 inch in length a great quantity of food is consumed and, during the final stages of growth, the caterpillar has a voracious appe- tite. Upon reaching maturity it changes into a light-green pupa (fig. 2,C) found attached to the stubble or stems of the alfalfa. At this period, transformation to the adult takes place and as the butterfly develops, its form can be W?T r Fig. 2. --Life stages of the alfalfa but- terfly. A, Egg of the alfalfa butterfly; B, mature larvae as seen on alfalfa plants; and C, pupa. (A, x 10 ; B and C_, actual size.) seen within the pupal case . Soon after this stage of development is reached the adult emer- ges, mates, and gives rise to a new generation. The alfalfa butterfly passes through five or six generations in a year in the warm interior valleys and can be found in alfalfa fields dur- ing any month of the year. However, it is found in abundance only in the summer and early fall. In the high temperatures that prevail during this period, the pest completes a life cycle in 30 days or less. This length of time is suffi- cient for it to complete a generation on each cutting of alfalfa during the hottest part of the year. Destructiveness Over most of the state the caterpillars are found in maximum numbers from about the middle of July through the first part of September. It is during this period that alfalfa is seriously injured. In general, damage is confined to the fourth and fifth cuttings; with severe infesta- tions the alfalfa may suffer complete defolia- tion. The amount of damage varies from year to year, from district to district, and from field to field. These fluctuations unquestionably are controlled by climate, parasites, and predators. Natural Control The natural control of the caterpillar by environmental factors is very important. It is so effective that, in many areas, destructive caterpillar populations fail to develop. These natural factors can be divided into both cli- matic and biological factors. Hot or cold spells at critical periods may affect the pest favorably or unfavorably; and, unquestionably, the cool climate of the coastal valleys is the principal reason why the alfalfa butterfly is not a pest in those areas. Many biological factors affect this butter- fly. Among the more important are insect para- sites and a wilt disease. Although the cater- pillar stage is attacked by a large number of parasites, the one most thoroughly investigated, and that appears to be the most important, is Apanteles flaviconchae Riley (fig. 3). It para- sitizes the first three larval instars, and de- Fig. 3. --Adult Apanteles flaviconchae . (x 19-) stroys the alfalfa caterpillars before they have had opportunity to do much damage. Under field conditions nearly 100 per cent of the small but- terfly larvae (fig. 4) may be parasitized. This parasite every year effectively protects a great part of the alfalfa grown in the hot interior valleys from serious damage by the alfalfa but- terfly. As the parasite develops, it distends the posterior end of the caterpillar (fig. 5) ; parasitized caterpillars in this condition can [2] Fig. 4. --Stages in the development of Apan- teles f laviconchae . Left, host caterpillar from which the parasite is about ready to emerge. The upper row shows at the left a mature parasite larva, and at the right two cocoons of the para- site . The lower row shows three cocoons from which the Darasite has emerged, (x 5.) The wilt disease (fig. 6), which is so import- ant in killing the caterpillars of the alfalfa butterfly, usually does not become effective un- til after the caterpillar population has reached destructive proportions. Where the host popula- tion is very large, the disease may kill off nearly all the caterpillars in a very short time. Fig. 5.--A, Normal alfalfa butterfly larva in early instar stage. B, Parasitized larva, show- ing swollen posterior end. (Both x 5.) be detected at a glance. However, if there is any doubt as to whether a caterpillar is parasi- tized, this can be determined by holding it be- tween the thumb and forefinger while the head is pulled off with forceps. Then enough pres- sure should be exerted on the larva to expel its intestines. If parasitized with a large parasite larva, this will spurt out; if the para- site larvae is small, however, it usually ad- heres to the intestinal tract where it can be seen if carefully examined. In determining parasitism, caterpillars that measure from about 1/4 to 1/2 inch should be examined. Apanteles flaviconchae effectively controls the alfalfa butterfly for the following reasons: (l) it has a shorter life cycle than its host; (2) it ap- pears to find its host with ease; and (3) it destroys its host before the host has had an op- portunity to do much, if any, damage. Fig. 6.--A cabbage butterfly cater- pillar which has been killed by a wilt disease. The alfalfa butterfly is af- fected in the same way: the dead cater- pillars are commonly found attached to the plant as illustrated. (After Slingerland . ) Under some conditions the disease reaches an ef- fective level before the host population has had an opportunity to do much, if any, harm. Cases have been observed where the disease and Apanteles flaviconchae have worked together to limit damage. Where the wilt disease is present in epidemic proportions, it has been ob- served to be harmful to Apanteles flaviconchae . Moist, warm, humid conditions favor the rapid development of the disease; frequently, also, a timely irrigation will create conditions that favor it. Chemical Control with Sulfur Research work conducted at this station has shown that dusting sulfur is effective in kill- ing the alfalfa butterfly caterpillar, '//here it was applied under favorable conditions, ex- ceptionally good control was obtained. However, the work has not advanced to the point where un- qualified recommendations can be made. Informa- tion available at the present time strongly in- dicates that dusting sulfur is extremely injuri- ous to the very useful parasite, Apanteles flavi- conchae. Even the drift of sulfur across an alfalfa field may be lethal to the adult para- site. Therefore, until definite effects of sul- fur on the parasite are known, the dusting of alfalfa fields, as insurance against attack by the alfalfa butterfly, is hazardous and is not recommended. Apanteles flaviconchae is one of the alfalfa grower's best friends and should be given every possible protection. '[3] Sulfur applied late in the growth of a cut- ting is very likely to cause irritation to the eyes of 'men who bale the hay. This is certain to arouse complaint, and, under extreme condi- tions, might make it impossible to get the hay baled. Further study also is needed to deter- mine whether or not hay treated with sulfur can be milled or otherwise processed without increas- ing the fire hazard. At least research should be conducted to determine the limits of safety under which sulfur can be used. Information at the present time indicates that sulfur should be evenly applied to the al- falfa at the rate of 75 pounds to the acre . It should never be applied after the alfalfa has reached a height of 8 or more inches. It is best to apply the sulfur when the alfalfa is 4 to 6 inches tall. At this stage of growth the alfalfa butter- flies have laid most of their eggs, and an ex- perienced person can determine fairly accurately whether or not there is much danger of a sizable population developing. Control should be con- ducted under supervision and, before sulfur is applied, the supervisor should be certain that Apanteles flaviconchae and the wilt disease will not take care of the situation. The young, de- veloping caterpillars should be examined for parasites; if parasitism is found to be 40 per cent or more when the alfalfa is 4 to 6 inches in height, dusting should be avoided. Even ,vith low parasitism, dusting should not be done unless an average of 10 or more larvae can be collected to a sweep of an insect net. When the alfalfa is 8 inches tall, and damage is imminent, the recommended procedure is to cut. There are caterpillars other than the alfalfa butterfly that seriously injure alfalfa. The most important of these is the yellow-striped armyworm (fig. 7). This insect is not killed by sulfur; therefore, sulfur should not be applied until the cause of the damage is definitely known to be the alfalfa butterfly. As a final word of caution, it should again be stated that investigations now underway on the use of sulfur for the control of the alfalfa butterfly are far from completed. Sulfur dust- ing for the control of the alfalfa butterfly is not recommended unless done under adequate su- pervision and when it is apparent that the field will not be protected by the parasite. The recommendations given here are only tentative. Fig. 7 ---The yellow-striped army- worm, Prodenia praefica Grote , shown actual size. (From Ext. Cir. 61.) [4] 6m-May, -45(4569)