Scouting Tobacco in North Carolina

Scouting for Insect Pests

Cutworms, Vegetable Weevils, Slugs
Several pests may kill or severely injure tobacco in the first weeks after transplanting. Among the most common are several species of cutworms. These pests are in the soil at the time of transplanting and damage tobacco by cutting off the stem near ground level or by cutting off individual leaves (Figure 7). They feed primarily at night. During the day, cutworms are usually found in the top inch or two of soil near injured plants.



Figure 7. Black cutworm and damaged plant.


If cut plants are seen (check wilted plants for partially cut stems), dig around the base of several damaged plants to verify presence of cutworms. Most cutworms are gray to black and curl up when disturbed.

Adult and immature vegetable weevils can damage the leaves of newly set plants. Immature weevils are pale green, legless grubs with dark heads. Adults are snout beetles with characteristic V-shaped, light markings on their backs (Figure 8). Damaged leaves are ragged and in extreme cases may be stripped (Figure 9).



Figure 8. Adult vegetable weevil.

Figure 9. Vegetable weevil damage.

Slugs, which also damage leaves, are most common in wet weather. The leaf surface may be scarred--slugs rasp small holes in the leaf--or large areas of leaf may be eaten away.

Slugs hide beneath the soil or in trash during the day. Slime trails may be the best clue to their presence.

Scouting procedure. Once a problem is detected, determine during a regular scouting check if the pest is still present. If the pest is still in the field, count the plants damaged within a group of 100 plants. This can be done as you walk between regular scouting stops. Counts of damaged plants should be made at several locations in the field.

Economic threshold. Treat only if the pest is still present. Treatment is justified if 5 percent or more of the plants are killed or severely injured within four weeks of transplanting. In some cases, treatment is justified in border rows but not in the field as a whole.

Wireworms
Wireworms are soil insects that are in the field when tobacco is transplanted. During the first few weeks after transplanting, wireworms feed on the exterior surface of the underground stem and often tunnel into the center of the stem (Figure 10). Young plants may be stunted or killed. The result is an uneven stand and possible yield reduction (Figure 11). Wireworm damage may cause even resistant varieties to become susceptible to soil-borne diseases.



Figure 10. Plant stem injury by wireworm.

Figure 11. An uneven stand may result from
wireworms.

Scouting procedure. You should check for a wireworm infestation if wilted or stunted plants are observed, although these symptoms are also indicative of other problems, such as fertilizer injury. Carefully dig up several affected plants (a garden trowel or small spade will be helpful) and check the stems for feeding and tunneling. Wireworms are hard- thin, yellow-brown worms up to 3/4 inch long. Only occasionally will a wireworm be found in the stem or even in adjacent soil. Sometimes webworms (soft-bodied) or whitefringed beetles (grub-like) cause similar damage.

Economic threshold. No remedial control for wireworms exists, and thresholds have not been developed. It is important, however, to record wireworm problems so that preventive control decisions can be made for future years. Estimate the percentage of plants stunted or killed by wireworms; record this figure. It may also be helpful to record or map areas of a field which are most affected.

Flea Beetles
Flea beetles are small, jumping insects that chew small, rounded "shot holes" in the leaves of tobacco (Figure 12). Occasionally their feeding may also injure the buds of small plants. Adult beetles generally do the most damage during the first three weeks after transplanting and again during the topping and harvest period. Immature or larval flea beetles injure tobacco roots, but neither a scouting method nor thresholds have been developed for this damage.



Figure 12. Flea beetles chew small "shot holes"
in tobacco leaves.


Scouting procedure. If damage is evident, examine the entire plant and count or estimate the number of beetles per plant. Look for flea beetles before checking for other insects, as flea beetles may jump from the plant when disturbed. Look closely on large plants at the base of lower leaves for damage (Figure 13). Divide the total number of beetles counted or estimated by the number of plants examined and determine the average number of beetles per plant.



Figure 13. When scouting, look closely on
large plants at the base of lower leaves for
flea beetles and damage.

Economic threshold. Treat when small plants (first four weeks after transplanting) average four or more beetles per plant. Treat large plants (after buttoning) when an average of 60 or more beetles per plant is estimated or when the lower leaves begin to look ragged or lacy at their bases (near the stalk). Drought conditions may increase the impact of flea beetles late in the season. Overripe leaves are often more heavily damaged. Thus timely priming can reduce damage. A threshold has not been developed for the period of rapid plant growth that occurs from about four to eight weeks after transplanting; however, flea beetles are very rarely a problem during this period.

Budworms
In tobacco, the term "budworm" usually refers both to the tobacco budworm and the corn earworm. The tobacco budworm is by far the more common of the two pests. In any case, these pests are very similar, and species identification is not critical. Before buttoning, budworms spend most of their time feeding on the developing leaves in and around the bud (Figure 14). As damaged leaves grow, feeding damage expands and is seen as large, smoothly rounded holes. Occasionally these pests will top plants (that is, eat the bud out) and tunnel into stalks or midveins. If the plant has not been topped after buttoning, budworms feed primarily on the flowers and seed pods.



Figure 14. Before buttoning, budworms spend
most of their time feeding on developing
leaves in and around bud.

Scouting procedure. Before buttoning, carefully open the bud and look for damage. If damage is found, look closely for the budworm. You may have to separate the small leaves of the bud or look at both sides of leaves outside the bud (a check of the top third of the plant is sufficient) in order to find budworms (Figure 15). Count and record the number of plants with one or more live budworms present. Count only plants on which live budworms are found. Budworm counts are not necessary after topping, but heavy infestations should be noted.



Figure 15. Budworms can be found by carefully
separating the small leaves on the bud.

Economic threshold. Treat when one or more live budworms are found on 10 percent (one of every 10 plants) or more of the plants examined before buttoning.

Aphids (plant lice)
Aphids are small, soft-bodied insects that can build to large colonies on tobacco. Winged aphids fly to plant beds or fields, where they produce a wingless form of the insect. Typically their numbers increase slowly until about six weeks after transplanting, then the rate of colony growth accelerates. Populations usually peak shortly after topping, then decline. Aphids are found in the greatest numbers on the undersides of leaves just below the bud (Figures 16 and 17).

Aphids damage tobacco by sucking plant juices from the leaves. This insect also deposits on tobacco plants a clear, sticky waste product called honeydew. The presence of honeydew encourages the growth of sooty mold. The leaves of plants infested with aphids may be distorted, thin bodied, difficult to cure, and darkened by sooty mold. Quality as well as yield may be reduced.



Figure 16. Red aphids.

Figure 17. Green aphids.

Scouting procedure. To scout for aphids, check the undersides of leaves on the top third of the plant (or the entire plant if it is small). Count the number of plants with 50 or more aphids on any one leaf. (Once you have learned what a concentration of 50 or more aphids looks like, it will not be necessary to count individual insects.) If suckers are a problem, check them also for aphids (colonies will persist on suckers even though declining on marketable leaves).

Economic threshold. Treatment is justified when 10 percent or more of the plants checked have 50 or more aphids on any one leaf. If aphids are present primarily on suckers, they should be counted toward the threshold as well. In such a case, however, removing suckers may be as effective, or more effective, in controlling aphids than using an insecticide.

Hornworms
Hornworms (including the tobacco hornworm and tomato hornworm) are caterpillars that feed on the leaves of tobacco . They may be more than 3 inches long when fully developed and are potentially very destructive. Small hornworms chew small, slightly irregular holes in leaves; large caterpillars are capable of stripping leaves, even whole plants.

Scouting procedure. Examine the entire plant for feeding damage and hornworms; do not look too extensively if damage is not immediately apparent. Count and record the number of worms 1 inch long or longer. Parasitized hornworms do less damage and should be counted as one-fifth of a worm. These worms may have the parasite's white cocoon attached to them or have very short horns . Parasitized worms may also be identified by the presence inside the hornworm of the parasite. If cocoons have not yet appeared, it is necessary to break open the hornworm to find the parasites. Hornworms less than 1 inch long are not counted toward a threshold, but be alert for large numbers of small worms because damage can build rapidly after the threshold is reached.

Economic threshold. Treat when an average of at least one large worm (1 inch long or longer) is found for each 10 plants checked. If large numbers of small worms are found (about one per plant or more) check the field daily (which is best) or treat.

Other Leaf Feeders
Several other insect pests feed on tobacco leaves. Those most commonly causing significant damage include several species of grasshoppers, Japanese beetles, and cabbage loopers.

Scouting procedure. Examine the entire plant for damage and the presence of these pests. Count the number of plants with significant damage. If you do not recognize the pest, collect several specimens and take them to the local Extension agent.

Economic threshold. No exact thresholds exist for these pests. As a general rule, however, treat when anticipated damage is equal to or greater than that caused by a 10 percent budworm infestation.

Scouting for Weeds            Flowers and Suckers

Scouting Tobacco in North Carolina