UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE VEGETABLE WEEVIL LISTRODERES OBLIQUUS OLIVER H. LOVELL BULLETIN 546 DECEMBER, 1932 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA Adult vegetable weevil. (From: Essig, E. O. A history of entomology. By permission of The Macmillan Company, publishers.) The Vegetable Weevil, Listroderes Obliquus 1,2 OLIVEE H. L0VELL3 DISTRIBUTION AND HISTORY The vegetable weevil, Listroderes ohliquus Gyll.,^ was first described by L. Gyllenhal in 1834^ from specimens collected in southern Brazil. Nothing more was written concerning this weevil until 1908, when it was reported as damaging truck crops in Australia and found to have a wide range on the south and east coast of that continent. Since then it has been reported in South Africa and the United States. The first report of the vegetable weevil in the United States was from Mississippi in 1922,^ and it has now spread throughout the Gulf region. The vegetable weevil has been known in California, about the San Fran- cisco Bay region, since 1926, where it is known to occur in eleven coun- ties, namely, San Francisco, San Mateo, Santa Clara, Santa Cruz, Alameda, Contra Costa, Solano, Napa, Marin, and Monterey, and also in Humboldt. The vegetable weevil was without doubt introduced into California a few years prior to 1926, but the exact time and manner of its intro- duction are not known. "^ Early in 1926, L. R. Cody, Agricultural Com- missioner of Santa Clara County, called the attention of the State Department of Agriculture to a larval form of insect that was doing damage to truck crops about San Jose. At that time it was supposed that the insect was one of the clover or alfalfa weevils. Later, however, when adults were found, they were sent to H. S. Barber, of the United States National Museum, who identified them as the Australian tomato weevil, Listroderes ohliquus Gyll. 1 Eeceived for publication August 26, 1932. 2 The work on which this paper is based was made possible by a cooperative arrangement between the Division of Truck Crop Insect Investigations of the Bureau of Entomology, United States Department of Agriculture, and the Division of Entomology and Parasitology, of the College of Agriculture, University of California. 3 Eesearch Assistant in the Division of Entomology and Parasitology. 4 Order Coleoptera, family Curculionidae. 5 Gyllenhal, L. Original description of vegetable weevil. In: Schoenherr, C. J. Genera et Species Curculionidum, etc. vol. 2, part 1, p. 277. 1834. 6 Chittenden, F. H. The Australian tomato weevil introduced in the South. United States Dept. Agr. Dept. Cir. 282:1-8. 1923. Essig, E. O. A history of entomology, p. 203-216. The Macmillan Co., New York. 1931. 7 It may have been introduced into California from either Brazil or Australia. 4 University of California — Experiment Station The common name, tomato weevil, was found to be too limiting, be- cause, while the tomato is used by the adult weevil as a host and is considerably damaged, many other plants, such as carrots, turnips, potatoes, and spinach are also attacked. It can also maintain itself upon some of the uncultivated plants which grow adjacent to cultivated fields. Therefore, the common name of the weevil was changed by the Economic Entomologists Committee on Nomenclature to the broader title, vegetable weevil. LIFE HISTORY AND DESCRIPTION Estivation. — During June, July, August, and part of September the vegetable weevil is in the adult stage and is inactive, remaining hidden in grass, weeds, and rubbish accumulated about the borders of truck- crop plantings. In September the adults emerge from estivation, or oversummering, and enter fields planted to carrots, turnips, or spinach and lay their eggs on these plants. Eggs. — On carrots and turnips the weevil lays its eggs usually in the crown of the plants, but eggs may be found on the petioles of the leaves and in the soil near the bases of the plants. The egg-laying period is rather long, extending from the middle of September to the middle of March. During normal fall and winter seasons in California there is usually a peak of egg laying in October and November. During the coldest months of the winter — December and January — egg laying will almost cease. In February, egg laying is resumed but gradually dwindles away and completely stops in March. In the laboratory one female may lay from 1 to more than 30 eggs a day and from 300 to more than 1,500 eggs a season. The females prefer laying their eggs in moist places. Weevils kept in petri dishes with carrot leaves and no other moisture except that in the foliage itself will lay their eggs almost anywhere on the foliage and also on the glass floor of the dish. Sometimes a weevil may be seen with as many as 2 eggs hanging from the posterior end of the abdomen to be scraped off as the weevil passes over some obstruction, or forced off by other eggs emerging from the body. However, when a piece of wet blotting paper is placed in the dish, the weevil will come to the paper and lay all her eggs there ; only rarely will they be found elsewhere. The weevil, when laying eggs on moist blotting paper, has a very definite method of depositing them. She first selects a place on the paper to lay the eggs, then extends the ovipositor, and with a slight swaying motion separates the fibers leaving a small opening in the paper ; she forces an egg into the hole and as it descends to the bottom, the ovipositor is BuL. 546] The Vegetable Weevil, Listroderes Obliquus TABLE 1 Incubation Period Under Laboratory Conditions, Percentage of Eggs Hatched, and Average Time of Hatching (Day temperatures varied from 18° to 20° C; night temperatures varied from 8° to 14° C approximately. Hatching dates, 1930 Total eggs laid Incubation period for Per December cent Date eggs laid 1930 November First egg hatched Last hatched of eggs 12 13 14 15 17 18 19 20 21 22 24 25 26 28 29 1 hatched days days October 28 1 2 15 50.00 October 29 2 6 14 33.33 October 30 3 4 3 13 14 17 76.92 October 31 1 5 16 2 28 13 20 89.28 November 1 1 1 6 15 17 33.33 November 2 4 5 9 14 15 100.00 November 4 3 20 23 13 1 1 63 13 18 96 82 November 5 8 1 9 16 18 100.00 November 6 1 4 10 1 4 1 1 25 13 20 88.00 November 7 1 5 36 2 1 47 12 21 95.74 November 8 1 16 10 8 1 44 14 19 81.81 November 10 - - - - - - - - - - 6 48 28 5 1 103 14 19 85.43 Average 13 91 18.4 77.56 Hatching dates, 1930-31 Total eggs laid Incubation period* for Per December January cent Date eggs laid, 1930 First egg hatched Last hatched of eggs 23 27 29 30 31 2 3 5 6 7 8 9 10 12 hatched days days December 10 1 1 23 11 4 2 58 13 18 72.41 December 11 1 24 15 6 1 4 65 18 23 78.46 December 12 1 1 16 36 7 2 71 11 18 88.73 December 13 43 22 4 2 80 18 21 88.74 December 15 3 48 31 3 2 90 16 20 93.33 December 16 2 18 3 6 84 18 21 34.52 December 17 1 13 40 2 1 1 64 17 23 90.62 December 18 4 7 6 26 11 3 1 121 16 23 93.38 December 19 1 3 20 53 6 2 2 92 18 24 91 30 December 20 - - - - - - - - - 4 39 9 2 1 55 18 22 94.54 Average 16.3 21.3 82.60 * The average temperature was somewhat lower for the second group so that the average incubation period of the first eggs hatched was longer, but the range of time between hatching of first egg and last egg is about the same. 6 University of California — Experiment Station raised until withdrawal and the fibers are then packed closely about the egg by the weevil. After the egg has been deposited, the weevil moves to a new position and repeats the process until oviposition is completed. Fig. 1. — Larvae of tlie vegetable w^eevil, about four times natural size. (Courtesy of E. O. Essig.) TABLE 2 Measurements of Head Capsules of Weevil Larvae in Microns (1/1000 Millimeter) at Different Stages of Grom^th Size at hatching Second instar Third instar Fourth instar Mature larvae (February 11) (February 18) (February 25) (March 3) (March 10) M M M M M 200 288 480 680 760 224 288 440 680 760 216 288 400 720 760 240 288 440 720 720 200 296 440 744 744 208 320 456 680 760 216 320 400 680 * 208 312 * * * Average 214 300 436.6 700 750 * Died. Hatching takes place from 11 to 24 days after the eggs are laid (see table 1). The egg, when first deposited, is white, spherical, and about a millimeter in diameter. As it grows older, the color becomes dark yellow, and when about to hatch, is almost black. Just before the egg hatches, the larva may be seen through the shell, curled up with the head and tail touching. The head is fully chitinized, or covered by a horny capsule, and sometimes the movement of the mandibles may even be BuL. 546] The Vegetable Weevil, Listroderes Obliquus 7 seen through the shell. The larva ruptures the shell by extending itself, and when the shell breaks, it crawls away and seeks food. Larvae. — The larvae (fig. 1) are of the usual curculionid form, slender, strong^ convex, and of a cream- white color when first hatched Fig, 2. — Photograph showing turnips injured by the larvae of the vegetable weevil. (From: Essig, E. O. A history of entomology. By permission of The Mac- millan Company, publishers.) and about 1.66 mm in length. As they grow, the body color changes from cream-white to green owing to the presence of food which they ingest. The head of the larva is dark-brown and heavily chitinized. Just back of the head is a chitinized plate called the prothoracic shield. It is narrow and divided in the middle, and of slightly paler color than the head. 8 University of California — Experiment Station The larvae have no jointed legs but have ventral and lateral lobes which ma}^ be projected and retracted for locomotive purposes. The larvae increase in size with each molt of which there are four (see table 2). The head, just after molting, is white but soon becomes dark brown. After the last molt, the larvae leave the host plants and enter the soil. The entire active life of the larvae, from the time they hatch until they enter the soil for pupation, is spent above ground on the foliage of their host plants. Larval Injury. — As soon as the larva leaves the egg it begins feeding. Carrots and turnips are the preferred host plants. By the first part of November the population of young larvae has increased so that there may be from 5 to 15 feeding in the crown of one host plant and all the infested plants in the field begin to show some injury. The principal damage is due to the stunting of plants by the destruction of the tender, young, growing crown leaves which are eaten away. The large leaves of carrots lose their plumy appearance and only the main stems and the heavier veins are left. In severe cases the foliage is eaten com- pletely, leaving only the roots in the ground. Turnips are injured in much the same way, except that the turnip roots at the level of the ground may be gouged by the larvae while carrot roots are rarely attacked (fig. 2). Besides denuding the plants of leaves, which deprives them of their supply of food, the loss of the leaves is detrimental to the sale of such vegetables. All carrots and turnips are bunched, and in order to accom- plish this the plants must have leaves. At times the housewife may also find larvae while preparing vegetables for the table, with the result that further purchases may be curtailed. The peddling of infested vege- tables from door to door or from farm to farm is probably one of the best means of disseminating the weevil. In most cases the vegetable tops are simply cut off and thrown out of doors, which permits the larvae, in many instances, to complete their life cycle. While carrots and turnips are the preferred hosts of the vegetable weevil, spinach may be attacked when grown near the former. Spinach, however, is a secondary host and is never so badly infested as carrots and turnips. Other host plants of the larvae are Swiss chard, radish leaves, young cabbage plants, parsley, parsnips, Malva sp., ragweed, milk thistle, Rumex sp., wild asters, and lettuce. Pupae. — After the mature larvae enter the ground, they form cells of earth where they undergo a prepupal, or resting, stage. This is fol- lowed by a final molt after which they enter the true pupal stage (fig. 3 ) . The pupae are about 7.5 mm in length and a little over half as wide. BuL. 546] The Vegetable Weevil, Listroderes Obliquus 9 Tlie lieads of the pupae are white, the legs and wing pads yellow, and the abdominal segments green. The rostrum, or snout, is broad and folded along the body as far as the middle of the first pair of legs (fig. 4) . The pupal stage is, perhaps, the most critical period in the life history of the insect, the pupa being so extremely delicate that only the slightest pressure is required to rupture the body wall and cause death. Fig. 3. — The pupae of the vegetable weevil, as they appear in the cells which are 1 or 2 inches below the surface of the soil. (From: Essig, E. O. A history of entomology. By permission of The Macmillan Company, publishers.) Fig. 4. — Pupae of the vegetable weevil, about four times natural size. (Courtesy of E. O. Essig.) The length of the pupal stage varies greatly with climatic conditions. Usually the larvae begin leaving the plants in December and January, and enter the ground to a depth of from 1 to 2 inches. The adults do not emerge until April or May. In the laboratory the insects complete the pupal period in from 14 to 16 days. 10 University of California — Experiment Station Because the pupae are buried in the soil during this transition stage and take no nourishment, they do no damage to crops. Adults. — The complete transformation from fully grown larva to weevil takes place in the pupal cell. After the transformation the weevil remains in the cell for a few days in order that the body covering may become hardened, after which it emerges from the soil and seeks food. Fig. 5. — The adult vegetable weevil, about four times natural size. (Courtesy of E. O. Essig.) The adult is about 8 mm in length and about half as wide. The snout, or beak, is short and stout, bearing geniculate (elbowed) and clubbed antennae. The prothorax is widened anteriorly. The color is a dull grayish-brown that blends very well with that of the soil and conceals the adults when they are resting on the surface of the ground. The body is covered with scales and hairs. On the wing covers there is a well defined V-shaped figure of lighter scales, while back of this figure and BuL. 546] The Vegetable Weevil, Listroderes Obliquus 11 at each side there is a pair of small tubercles which project posteriorly (fig. 5). The weevils are gregarious and collect in large numbers about the stems of young potato and tomato plants and also under boards or rubbish about infested areas. The vegetable weevil, in California at least, has never been observed in flight, though there are observers in Australia and in the southern states who have reported its ability to fly.^ The weevil has well formed wings, but they do not function. It uses its legs well, however, and may cover a considerable distance at night, when it is most active. It feeds at night, both in the larval and adult stages, but hides by day about the bases of the host plants or under clods and rubbish. The vegetable weevil is parthenogenetic ; hence, every individual that reaches maturity is a female capable of producing fertile eggs which in turn produce other parthenogenetic females. A single weevil is therefore capable of establishing the species in any new territory where climatic conditions are favorable for existence. No males of this species have been found to date in California. Adult Injury. — In early April, when the adult weevils begin to emerge from the soil, instead of using carrots and turnips as their prin- cipal food supply, as do the larvae, they prefer young tomato and potato plants. They attack these plants in a manner like the cutworm ; that is, they cut the stems at the surface of the ground. At other times, when these young plants have reached a more mature stage, they ascend the stems and eat the leaves or cut the stems just below the bud. In the case of tomatoes this injury usually means that replanting must be done. As many as 25 to 40 weevils have been found congregating in the soil about the base of a single tomato or potato plant. Besides tomatoes and potatoes, the weevils may feed on the following plants : pear foliage, turnips, carrots, spinach, wild parsnips, milk thistle, cabbage, collard, mustard, chick weed, pig weed, dock, radish, chard, and Malva. During late May and early June the weevils gradually go into estiva- tion and remain inactive until September. No weevil damage to crops has ever been noticed during this period of estivation. CONTROL For the control of the vegetable weevil the following methods are recommended: Cultivation, rotation of crops, poison baits, field sani- tation, and dusting with poison dusts. 8 Mr. M. M. High of the United States Department of Agriculture, stationed at Gulfport, Mississippi, reports that the weevil will fly when the temperatures are high. 12 University of California — Experiment Station Cultivation. — During the months of January, February, March, and April a large percentage of the immature insects are in the ground, usually only an inch or two below the surface, in either the prepupal or true pupal stage. In this stage the insect is practically motionless and very delicate ; the slightest pressure brought to bear on them will rupture the body wall and cause death. Therefore, thorough and re- peated cultivation of infested areas during this season with a disk or harrow, followed by a clod breaker, will break up many pupal cells and crush insects. This tends to lessen greatly the adult population in the spring. Crop Rotation. — In the fall and early winter the larvae of the vege- table weevil are most active, and all the damage sustained by truck crops because of them is done at this time. Preferred host plants such as turnips and carrots, grown in succession, will insure an increase of weevil population. However, if known infested areas could be planted to crops which are immune to weevil attack, it would tend to force the insect to either leave the field or to starve for want of proper food. The larvae, being legless, cannot travel far, and since the adults do not fly they cannot seek food at long distances. On the other hand, fields of tomatoes and potatoes, if planted in the spring immediately adjacent to land that was heavily infested during the previous fall and winter, can be expected to suffer attack by adult weevils. Poison Baits. — The very noticeable habit of congregating about the bases of potato and tomato plants, especially in the late spring, provides a favorable condition for the use of poison baits in controlling the adult weevils. A poison bait, broadcast in a field of tomatoes and potatoes in May or June, or as soon as the first damage is noted, will kill a great number of the weevils and thus lessen the population of insects which estivate, or over summer, in rubbish about cultivated areas, and conse- quently will reduce the number of weevils which begin their egg laying in September. One of the best baits for the purpose is dried apple pulp coated with sodium fluosilicate, a product which is patented under the trade name ' ' Go West. ' ' This is broadcast in the field at the rate of 75 pounds per acre. Other poison baits flavored with dried apricots and lemon juice have been used experimentally with rather good success.® 9 These baits used experimentally by H. C. Lewis, formerly of the California State Department of Agriculture, were made after the following formulas: (1) 2 lbs. ground dried apricots; 1% oz. calcium arsenate, dampened with a small amount of lemon juice. (2) Juice of 6 lemons; 2 qts. of light cooking molasses; 20 lbs. bran; 1 lb. calcium arsenate, and sufficient water to make a crumbly mash. (Lewis, H. C. The vegetable weevil in California. California State Dept. Agr. Mo. Bui. 16:378-392. 1927.) BuL. 546] The Vegetable Weevil, Listroderes Obliquus 13 Fruit juices and pulps seem to be far more efficient than poison bran baits with molasses. Field Sanitation. — In May and early June the adult vegetable weevils go into estivation and remain inactive until September. Estivation, or oversummering, takes place in almost any kind of shelter that the insects can find, such as rubbish and grassy fence rows or weed patches adja- cent to their feeding places. In the fall, the weevils begin to lay their eggs upon the first suitable host plants for the larvae. In the majority of cases the first larvae and the severest injurj^ to truck crops will be found immediately adjacent to places that have sheltered the oversummering adults. When the time comes for the larvae to leave the plants, there may be seen a sharp demarcation between severely injured plants and plants less severely injured, and the greatest damage is always found next to the estivating shelters. In view of these facts, a great deal of the danger of insect injury would be removed if old fence rows were kept clean, and weed patches and rubbish were completely eliminated from all truck-crop areas. The mere cutting of weeds and grass is not sufficient to stop the vegetable weevil. Such places must be cleared to the bare ground and kept so to insure absolute freedom from reinfestation in the fall. To secure the best results shelters should be cleared up before the weevils go into estivation, that is, during early May. Cage tests to determine the survival of oversummering adults were made at San Jose, California, as follows : Three screen cages were used ; the first one was placed over Malva plants, the second over a few boards on bare ground, and the third over bare ground. Into each cage 100 adults were introduced in May. In early October, 29 live weevils were recovered in the first cage, 4 live weevils in the second cage, and no live weevils were found in the third cage. Dusting with Poison Insecticides. — The surest method for controlling this weevil is the application of insecticides in dust form. Until recent years the common dusts used for foliage-eating insects contained arseni- cals. While arsenicals are highly toxic to insects there is danger to the human consumer when such truck crops as carrots, turnips, and spinach are treated in this manner. Eecently fluorine compounds have been developed which are toxic to insects and much less toxic to humans. There are now two fluosilicates on the market, namely, sodium fluosili- cate and barium fluosilicate, both of which are highly efficient and inexpensive. Sodium fluosilicate may be used as a dust on carrots, turnips, spinach, and tomato plants at any time of the year, with only slight damage by burning even when the plants are wet. It cannot be used safely on potato plants because of the danger of severe burning of 14 University of California — Experiment Station foliage. Barium fluosilicate, however, may be used with safety on potato plants as well as any other plants on which sodium fluosilicate may be used. Barium fluosilicate is, however, a little slower in action and costs slightly more than sodium fluosilicate. These fluosilicates can be used against either the larvae or adult weevil with excellent results. To protect crops from larval injury the fluosilicates should be applied as soon as the damage is first observed, usually about the first of November. If infested plants are dusted at this time, there is a great likelihood of killing both the larvae which are doing the damage and adults which may be still laying eggs. Fluosilicate dusts are commonly used at 70 per cent or 80 per cent strengths, the remaining 20 per cent or 30 per cent of the dust being diatomaceous earth used simply as a carrier. They are best applied to vegetables which are grown in rows, such as carrots, turnips, or spinach, by a rotary duster which insures even distribution over the tops. When the foliage is dense, from 30 to 40 pounds per acre should be used. For younger vegetables with less foliage, 15 to 25 pounds of dust per acre is sufficient. The nozzle of the duster should be fan shaped to be most effective and the dust should be aj^plied as evenly as possible over the entire growth of foliage. Usually one application of fluosilicate dust will be sufficient to mature a croj) without injury. In cases of very young vegetables becoming infested by the weevil, two applications may be necessary because of reinfestation after the first application of dust. Observation of young vegetables by the grower will tell him when a second application is needed. In the spring most of the damage to vegetables is due to the adult weevil, especially to young potatoes and also to young tomato plants when they are being transplanted from coldframes to the fields. These vegetables are planted at some distance apart and can be most efficiently dusted with a knapsack, or bellows, duster, using a puff or two to each plant, and being careful to cover not only the foliage but the ground at the base of the plant as well. Young tomatoes and potatoes require only 10 to 12 pounds of dust per acre to insure adequate crop protection. Young tomatoes should be dusted when set out in the field, especially if planted on ground which was infested the previous winter with larvae. If the land adjacent was infested, it is good crop insurance to dust the young plants even though there are no insects to be seen at the time of planting. Tomatoes will tolerate either sodium or barium fluosilicates in the San Francisco Bay region but as previously stated it should be borne in mind that sodium fluosilicate will injure potato foliage in this region. BuL. 546] The Vegetable Weevil, Listroderes Obliquus 15 Attrahents. — At San Jose, California, about 60 different chemicals were used in experiments to attract the adults of the vegetable weevils. ^^ Paper cups were filled with wet bran and the various chemicals were sprinkled on the top. Some of the cups were sunk into the ground with screen guards about them to hold the weevils and some were placed on the surface in wire cages. Wide-mouthed jars were sunk into the soil with the mouths level with the surface and the several chemicals were placed at the bottoms of the jars on cotton pads; also a combination of the sunken jars and paper cups was used, that is, the jars were sunk into the ground, the paper cups w^ere attached over the mouths of the jars by wires, and the chemicals were put onto the bran in the cups. Some of these chemicals having a cinnamon or anise flavor, or odor, seemed to have slight attractant qualities but were not uniformly so from one season to another. None of the many chemicals used were sufficiently attractive to be of commercial value in the control of the pest. Soil Fiimigants. — Paradichlorobenzene and carbon disulfid emulsion were used against the larvae and pupae in the soil. Paradichlorobenzene was used in infested carrot patches at the rate of 1 ounce per 4 feet of row, and was applied in a shallow furrow made between the double rows of carrots ; the chemical was poured in and covered. After 120 hours, all the larvae and pupae which were recovered were alive. There was no apparent damage to the carrots. 10 The following is a list of chemicals used as attrahents. These materials were left in the fields at least 3 days before changing. Amyl salicylate Bornyl acetate Ethyl acetate Ethyl benzoate Methyl benzoate Methyl ethyl ketone Benzyl alcohol Benzaldehyde Nitrobenzaldehyde Malic acid Asparic acid Hippuric acid Butyl acetate Ethyl salicylate Methyl salicylate Butyl alcohol Carvocral alcohol Salicylaldehyde Propionic acid Iso valeric acid Capric acid Phenyl salicylate Terpenyl acetate Citrol Anthole Terpin hydrate Ethyl acetoacetate Iso propyl acetate Nitrobenzene Geraniol Citronellal Terpineol Menthol Safrol Petroleum ether Cymene Borneol Lactic acid Calcium benzoate Eugenol Hexane Cumene Anthracene Limonene Thymol Formic acid Succinic acid Amyl acetate Cinnamyl propionate Benzyl cinnamate Butyl cinnamate Methyl anthranitate Anisaldehyde Ethyl butyrate Sodium salicylate Butyric acid Oxalic acid Ammonium acetate Calcium acetate Ground cinnamon Cinnamaldehyde Liualool cinnamate Cinnamyl butyrate Cinnamic aldehyde Benzyl benzoate Anethol 3-Nitro 4-acetyl amino anisole Cinnamyl alcohol Cinnamic acid Hydrocinnamic acid Methyl cinnamate Sodium cinnamate Terpineol cinnamate Nitro cinnamic acid Phenyl ethyl cinnamate Anisic acid 5-Nitro 2-amino anisole Anisol 16 University of California — Experiment Station TABLE 3 Efficiency of Carbon Disulfid Emulsion as a Soil Fumigant Against Larvae and Pupae of the Vegetable Weevil Concentration Area treated (1 sq. ft. each) Time examined, hours Number live Larvae Pupae Number dead Larvae Pupae Per cent dead larvae and pupae First trial, February 6, 1929; soil temperature 8° C 15 cc CS2 to 1 gallon H2O 30 cc CS2 to 1 gallon H2O.. 45 cc CS2 to 1 gallon H2O.. 60ccCS2to 1 gallon H2O... Total. First Second Third.. Fourth First Second Third.. Fourth First Second Third.. Fourth First Second Third.. Fourth 3 3 Second trial, March 6, 1929; soil temperature 18° C 15 cc CS2 to 1 gallon H2O.. 30 cc CS2 to 1 gallon H2O.. 45 cc CS2 to 1 gallon H2O.. 60 cc CS2 to 1 gallon H2O.. Total. 24 48 72 120 24 48 72 120 24 48 72 120 24 48 72 120 13 27 10 30 22 16 4 13 11 15 4 6 270 BuL. 546] The Vegetable Weevil, Listroderes Obliquus 17 TABLE 3 (Concluded) Third trial, March 8, 1930; soil temperature 14° C 15 cc CS2 to 1 gallon H2O. 30 cc CS2 to 1 gallon H2O. 45 cc CS2 to 1 gallon H2O. 60 cc CS2 to 1 gallon H2O. Total 48 3 3 72 7 2 96 8 1 120 5 48 1 1 72 1 5 96 7 120 6 5 48 9 6 72 6 2 96 5 9 1 120 8 5 48 11 72 12 2 96 4 6 120 7 3 4 100 44 11 9 67 Carbon disulfid emulsion was used on the soil at different concentra- tions : 15 cc, 30 cc, 45 cc, and 60 cc to 1 gallon of water. This material was a 50 per cent CSg emulsion as formulated by the Japanese Beetle Laboratory in Morristown, New Jersey, and was applied to infested carrot rows at the rate of 1 quart of mixture to 1 square foot of row. Three trials were made as follows : The first one was made on February 6, 1929, temperature of the ground was 8° C ; the second trial was made March 6, 1929, temperature of soil was 18° C ; and a third was made March 8, 1930, temperature of the soil was 14° C. All these experiments were made with the same material and applied in the same way. Each square foot of the 4 square feet treated was examined at a different time beginning at 24 hours after application and continuing up to 120 hours, as shown in table 3. After 120 hours, the highest percentage of killing was only 9.67 per cent. The reason for the failure of this chemical to kill the larvae and pupae may be explained by the fact that when the larvae leave the ground to pupate it is midwinter and the ground is wet and so cold that only a minimum amount of gas is evolved but not enough to displace the air in sufficient quantities to cause suffocation. Or it may be that as the larvae and pupae are buried to a depth of only 1 or 2 inches, the gas, being much heavier than air, is carried below the level of the transforming insects and has no lethal effect on them. Either one of these factors would tend to preclude the use of this fumigant against the larvae and pupae in the soil. 18 University of California — Experiment Station Light Experiments. — Experiments were undertaken to determine whether or not the vegetable weevil would fly, with the idea in mind that if they did, light traps might be used for control. A large gasoline lantern was set up in a heavily infested field of carrots. The lantern was lighted about seven o'clock in the evening and kept lighted until nine o'clock. Surrounding the light were large frames covered with paper smeared with tanglefoot. Besides the white light of the lamp, blue, amber, red, and green screens were used for a number of evenings in May when the young adults were emerging from the ground. At no time were weevils seen in flight, but were readily found walking on the ground and feeding on the foliage of potato plants growing closely adjacent to the infested carrot field. SUMMARY The vegetable weevil, Listroderes ohliquus Gyll., is a native of the Southern Hemisphere and was introduced into the United States prior to 1922. Its first noted appearance in California Avas in 1926. The weevil is active throughout the fall, winter, and spring months, undergoing during this time four stages of development, namely, the egg, larva, pupa, and adult. The larva does considerable damage to truck crops, especially carrots, turnips, and spinach, in the fall and winter, while the adult does most of its damage to tomatoes and pota- toes in the late spring and early summer. After June the weevil goes into estivation and remains inactive until September. The vegetable weevil is best controlled by dusting infested carrots, turnips, and spinach with either sodium fluosilicate or barium fluosili- cate of 70 per cent to 80 per cent strengths at the rate of 30 to 40 pounds of dust per acre when the foliage is dense, and at the rate of 15 to 25 pounds when the foliage is thin. The dust should be evenly distributed. It is best done by a rotary duster with a fan-shaped nozzle. For larger acreages a power duster is most efficient. For the control of the weevil on young tomato plants, a knapsack, or bellows, duster is recommended. A puff or two to each plant, or enough to cover thoroughly the foliage and the ground at the base of the plant, will give the plants protection ; 10 to 12 pounds per acre is sufficient. Field sanitation is next in importance in reducing the damage due to vegetable-weevil attack. It should, however, be emphasized that the mere cutting of weeds and grass about infested areas is not sufficient to stop the invasion of truck-crop areas by oversummering w^eevils. Grass,, weeds, and rubbish of all kinds must be removed to the bare ground, and the cleaning should be done in May before the insects go into estivation. BuL. 546] The Vegetable Weevil, Listroderes Obltquus 19 Crop rotation and thorough cultivation are other means of reducing the weevil population and should be used when possible. Poison baits may also be used to advantage in some instances. Carbon disulfid as a soil fumigant does not appear to have any effect in the control of the vegetable weevil because of the prevalence of low winter temperatures when fumigants should be used, and also because of the heavy nature of the gas evolved, which carries it below the cells of the transforming weevils. Attraction to chemicals or lights has shown only negative results. ACKNOWLEDGMENTS The writer is indebted to a number of entomologists, especially to Professor W. B. Herms, under whose supervision this work was under- taken. Professor E. 0. Essig and Professor E. C. Van Dyke have been very helpful with suggestions and criticisms. Mr. Roy E. Campbell, of the United States Bureau of Entomology, and Mr. Stewart Lockwood and Mr. W. C. Jacobson, of the State Department of Agriculture, have been helpful in many ways, and to them the writer desires to express his obligations for many courtesies. Mr. L. R. Cody, Agricultural Commis- sioner of Santa Clara County, and his staff have fully cooperated in all field work undertaken near San Jose. Miss Helene Diepen of the Uni- versity of California Deciduous Fruit Station at San Jose gave gener- ously and kindl}^ of her time for egg counts and the rearing of larvae as well as other experiments carried on at the Station near San Jose. ll?n-12,'32