Several different insect pests pose serious threats to Maryland’s tobacco crop. These pests must be managed properly to prevent economic damage to the crop while avoiding overuse of pesticides. Integrated pest management (IPM) makes use of natural, cultural, and chemical controls to maintain pest populations below levels that will cause economic damage to the crop. IPM promotes the application of pesticides only when they are needed and decreases production costs, pesticide residue levels, environmental contamination, and human exposure to pesticides. IPM also encourages beneficial predators and parasites that help regulate insect pest populations. IPM reduces the chance that an insect pest will develop resistance to insecticides, and, therefore, these chemicals should be effective for longer periods of time.
IPM recognizes that a certain amount of insect damage will not reduce tobacco yield or quality enough to pay for the cost of treating the crop. This tolerable level of damage is called the "treatment threshold."The treatment threshold is that pest population or injury level that requires treatment with an insecticide to prevent significant economic damage to the crop. Cultural control practices can help reduce insect infestations and feeding injury, make the tobacco plant a less desirable host, and eliminate overwintering sites for several pests. Practices include: plowing in early spring, applying proper nitrogen fertilizer, adjusting transplanting date, destroying plant beds immediately after transplanting is completed, keeping field margins clear of weeds, early topping in the button or prebutton stage, maintaining effective sucker control, cutting stalk and destroying roots soon after harvest, and rotating crops. Promote natural control by delaying insecticide applications until an insect reaches its treatment threshold and by using insecticides with low toxicity for the natural enemies of insect pests, such as Bacillus thuringiensis-based insecticides, pymetrozine (Fulfill), and spinosad (Tracer).
Insects, including flea beetles, cutworms, aphids, and vegetable weevils, can cause serious problems in tobacco plant beds. The tobacco flea beetle, the most common insect pest in plant beds, can riddle young seedlings and reduce their vigor. Cutworms reduce stands by feeding on leaves or by cutting off the entire plant and killing it. Vegetable weevil larvae occasionally cause moderate damage by feeding in the buds of seedlings. Aphids can build up moderate populations and rarely cause damage to the seedlings, but they can be carried to the field on transplants and cause early-season infestations. Although insects may cause serious problems in plant beds, high-quality transplants can be produced without extensive use of insecticides. A preseeding or postemergence application of Di-Syston 15G provides effective control of flea beetles and aphids but not cutworms. Remedial applications of foliar insecticides will also control these and other pests of the tobacco plant bed. Plant beds should be checked once or twice a week for insect problems. If a pest problem develops, apply an insecticide recommended for its control.
Recently, transplant production in greenhouses has increased rapidly. Orthene is the only insecticide labeled for use on tobacco transplants grown in greenhouses. It provides good to excellent control of cutworms, flea beetles, and aphids. Discourage such pests as cutworms, crickets, ants, and grubs by keeping the area around the greenhouse clear of weedy vegetation and excess organic debris. Thrips do not cause economic feeding injury to greenhouse-grown seedlings; more importantly, however, thrips can vector tomato spotted wilt virus, a disease that is a potential problem for Maryland’s tobacco growers.
Wireworms, the larval stage of click beetles, are hard, yellowish-brown grubs that live in the soil and tunnel the roots and piths of young tobacco plants during the first month after transplanting. This injury stunts plant growth, causes irregular stands, and can reduce yields. The life cycle of wireworms takes about a year to complete and ranges from 1 to 3 years. The larvae emerge from eggs in the late summer and fall, feed on the roots of tobacco and other host plants, and overwinter into the following season. Wireworms are most serious in tobacco fields with a past history of wireworms, or in those following sod, weeds, or small grains. In these situations, the use of pretransplant applications of soil insecticides are recommended for wireworm control. Treatments should be broadcast and incorporated by double disking at least 2 weeks before transplanting. Plowing fields in early spring will help reduce wireworm populations. Sturdy, healthy, thick-stemmed transplants should also be used since they are less susceptible to wireworm damage than tender transplants. There is no remedial control for wireworms once damage has occurred. If the stand is seriously reduced, the crop may be plowed under and replanted after treatment with a recommended preplant insecticide. If replanting is not possible, cultivation often helps plants recover from wireworm damage.
Cutworms are active at night, feeding on leaves and cutting off plants. This injury can cause enough damage and stand loss to require replanting. Cutworm problems are difficult to predict. However, cutworm populations can be reduced by plowing fields in the early spring. In fields plowed late, potential cutworm problems may be predicted by placing clumps of green clover at various locations throughout the field. If large numbers of cutworms are observed under the clumps after 3 to 7 days, a pretransplant treatment for cutworm control is recommended. Otherwise, fields should be checked for cutworm damage once or twice a week during the first month after transplanting. This will help warn the grower when a remedial foliar treatment is needed. Foliar treatments for cutworm control should be applied during the late afternoon or early evening.
Adult tobacco flea beetles feed on the leaves and stalks of tobacco in the plant bed and in the field, while the grubs or larvae feed on tobacco roots. Extensive feeding by both beetle stages on newly set transplants may cause stunting of scattered plants in the field, resulting in uneven stands. When checking tobacco fields for flea beetles, look for the characteristic "shot hole" feeding damage, and then count the flea beetles on 20 plants (2 per field-sample location). Begin treatment for tobacco flea beetle control on newly set tobacco when there are four or more beetles per plant. Though larger plants can tolerate very high flea beetle densities, treat with insecticides when densities exceed 60 beetles per plant and the bases of the lower leaves have a ragged, lacy appearance. The most effective cultural practices for reducing flea beetle damage are stalk cutting and root destruction immediately after harvest and harvesting at the normal time.
Pretransplant applications can provide effective control of wireworms, flea beetles, aphids, cutworms, and hornworms on tobacco. However, the timely use of foliar insecticides also provides effective control of tobacco insects feeding on tobacco foliage at a lower cost than with a systemic insecticide applied to the soil.
Several factors should be considered before selecting a soil insecticide. Lorsban, Mocap, and Vydate control both insects and nematodes. If tobacco is to be planted in land that has been in sod, weeds, or small grain the previous year or has a history of wireworm problems, an insecticide should be applied for wireworm control. Several chemicals are labeled at lower rates for wireworms than for nematodes.
Proper scouting of fields and the application of foliar insecticides at the insect’s threshold are effective alternatives to the use of soil insecticides. Orthene and Vydate applied in the transplant water provide good early season (2- to 4-week) control of flea beetles. The Orthene treatment also gives some early season control of aphids, cutworms, and hornworms.
Tobacco fields should be scouted at least once a week throughout the season to determine whether or not insect pests are abundant enough to require treatment. Accurate samples are essential for determining the proper timing of insecticide applications. Samples should consist of observations on insects and insect damage on at least 20 percent of the plants in each field. Make counts on consecutive plants at each of 10 different locations throughout the field. Large fields will require larger samples. Resulting counts should be compared to the threshold levels listed in the following treatment threshold table. If pest populations meet or exceed the treatment thresholds, a labeled insecticide should be applied for their control.
Hornworms, budworms, flea beetles, and aphids are key foliage pest species in Maryland and scouting should be targeted for them. Use the procedure described below to scout fields for insects and their damage on individual plants. First, check the bud region carefully for budworms and the white cocoons of the budworm parasite, Campoletis sonorensis. If there is budworm damage, but no worm, do not count the plant as infested. Examine the upper one-third of the plant for aphids. Check the entire plant for hornworm damage, locate any hornworms, and note their size and whether they are parasitized by Cotesia congregata (white, egglike cocoons on hornworm’s back). The undersides of lower-, mid-, and upper-stalk tobacco leaves should be examined for the presence of aphids, and the upper surfaces of the middle and lower leaves should be checked for honeydew, flea beetles, and flea beetle feeding holes. If an unidentified insect is observed and it appears to be causing serious damage to the crop, the insect and samples of its damage should be collected and taken to a local Extension agent for assistance in identification. This can be a valuable step because beneficial insects are often mistaken for pests. In addition, the misidentification of a pest may result in the selection of the wrong insecticide for its control. Tobacco fields should be treated when one or more insect pests meet or exceed the treatment threshold levels given in the first table.
Tobacco budworm larvae feed in the buds of young tobacco plants, causing many holes in the tiny developing leaves. As these leaves increase in size, the feeding holes increase proportionally, giving the leaves a ragged, distorted appearance. Tobacco plants are sometimes topped by budworms, resulting in early sucker growth that can cause stunting and the need for extra labor to remove the suckers. Tobacco budworm control should be initiated anytime prior to buttoning when there are five or more living budworms per 50 plants. After the button stage, budworms rarely cause economic damage to tobacco. Apply foliar sprays for budworm control with one or three solid-cone or hollow-cone nozzles over each row using 40 to 60 pounds of pressure per square inch (psi) to deliver 10 to 25 gallons of spray mixture per acre. Control with foliar sprays rarely exceeds 80 percent. However, Bacillus thuringiensis (Dipel) baits applied by hand or with a granular applicator usually give better than 90-percent control. When tobacco is checked for budworms, the cocoons of a wasp (Campoletis) that parasitizes budworms are often observed on the leaves near the bud. They are about a quarter of an inch long and white or grayish in color with two black bands. These cocoons are often mistaken for budworm cocoons, which are reddish-brown, three-fourths of an inch long, and formed in the soil beneath the plant. Living budworms are rarely found on plants with Campoletis cocoons in the bud region. Campoletis provides good natural control of budworms in Maryland and should be promoted as much as possible.
Tobacco and tomato hornworms are large caterpillars (up to 4 inches long) that consume considerable amounts of tobacco leaf. Infestations may develop anytime from transplanting until harvest, but the most severe damage occurs during June, August, and September. Control should be initiated when there are 5 hornworms an inch or more in length per 50 plants. Do not count parasitized hornworms. Hornworms with white, egglike cocoons of the parasitic wasp Cotesia congregata on their backs eat much less than healthy horn worms and provide a source of parasites that will help reduce the next generation of hornworms. In some cases when large numbers of small hornworm larvae are found (over one per plant), an insecticide should be applied for their control. For optimum hornworm control, insecticide sprays should be directed to the upper one-third of the plant. Several cultural practices are helpful in managing hornworm populations on tobacco. Early topping, early transplanting, and effective sucker control reduce late-season infestations. Stalk cutting and root destruction as soon as possible after completion of harvest reduce overwintering populations.
The tobacco aphid has been the most severe insect pest of tobacco in Maryland for the past decade. It may infest tobacco seedlings in plant beds, but the most severe damage occurs on field tobacco from early July through August. Aphids can be introduced into the field on infested tobacco transplants, but winged aphids that move into the field and deposit young, wingless aphids on tobacco plants are the most destructive source of infestation. High aphid populations can reduce tobacco yield by 5 to 25 percent. Aphids deposit honeydew on tobacco leaves, and a dark, sooty mold often develops. This interferes with curing and reduces quality. The presence of sooty mold indicates that aphids have been a problem, but these materials often remain on leaves after aphids have been controlled.
A red form of the tobacco aphid has been common on tobacco in Maryland for the last 10 years and has almost completely replaced the green form. The red form reproduces more quickly and at higher temperatures and is more difficult to kill with many labeled insecticides than the green form. Early planted tobacco can suffer less aphid damage than that transplanted near the middle of the planting period. Early topping gives some reduction in aphid populations.
From late June to the end of August, producers should watch for increases in aphid populations. The undersides of leaves from the lower, middle, and upper portions of tobacco plants should be examined at regular intervals to determine the extent of aphid population buildup. Producers should also be on the lookout for honeydew, a sugary substance produced by the aphids, which gives the lower leaves a shiny appearance. Treatments should be initiated for aphid control when 10 of 50 plants are infested with colonies of any size.
The following practices can be used to manage aphids on tobacco:
1. Control aphids in plant bed. This reduces the chances that aphids will become established in the field early in the season. Destroy plant beds after transplanting is completed.
2. Use recommended nitrogen rates. Excessive nitrogen keeps the leaves green for a longer period of time and promotes excessive sucker growth, which favors aphid infestation.
3. Top early. Aphid populations decline rapidly after topping, especially during hot, dry weather. Be alert after topping because aphid populations can build back up to damaging levels that require insecticide treatment.
Apply insecticides properly for maximum insect control. For effective control on small tobacco, use a single solid-cone or hollow-cone nozzle per row directed to the bud. Operate equipment at 40 to 60 pounds of pressure per square inch, do not exceed 4.5 miles per hour, and use at least 6 to 8 gallons of mixed spray per acre. After tobacco is 2 feet tall, use one or three hollow-cone nozzles per row. If three nozzles are used, orient the two side nozzles at a 45-degree angle toward the upper third of the plant. Use 40 to 60 pounds of pressure per square inch (60 to 100 pounds of pressure per square inch for aphids) and 18 to 25 gallons of spray mixture per acre. Set the nozzles 12 to 18 inches above the tobacco. Do not exceed a speed of 4.5 mph. Drop nozzles oriented to the undersides of the leaves and used in combination with one to three nozzles over the row may improve aphid control (Figure 5).
