Cutworms, aphids and hornworms are insect pests that can attack tobacco plants after transplanting. High population density of these insects can cause significant losses. Thus, the need for field scouting to ensure that these insects are kept under damaging levels, or below economic thresholds. This is when the population density at which the costs associated with pest management may be offset by the value of potential yield loss associated with the pest feeding or damage.
Scouting for cutworms, aphids and hornworms requires adequate selection of field sites for a good estimate of each insect’s distribution in the field. Plants selected for observation should be pre-selected at random before starting scouting to avoid biasing counts by selectively stopping at damaged plants. The number of sites selected in a field depends on its size. The more selected and checked sites, the higher the scouting accuracy. For instance, for fields 2-4 ha (5-10 acres) in size, 5-10 randomly selected sites throughout the field are required. For fields larger than 4 ha (10 acres) the number of sites checked is increased by 1 site for every hectare (2.5 acres).
Scouting for Cutworms
Cutworms are post-transplant soil insects that damage tobacco during the first four to five weeks after transplanting. The darksided cutworm is the most commonly seen cutworm on tobacco. It can damage a tobacco plant by creating holes in leaves, feeding extensively on the growing point (heart) or cutting off the stem at the soil surface. Studies indicate that a low to moderate population level of cutworms can result in 20-40% damaged plants and a 14-25% loss of yield.
Damage to a tobacco transplant from cutworm feeding
Scout fields for cutworms weekly, from transplanting through to early July. Cutworms are most active at night; therefore, evenings and early mornings are the best time to scout for these insects. Select and stop at 5-10 sites or more and examine 20 plants per site for signs of cutworm feeding (holes in leaves, missing growing points, or stems severed at soil line). Treat the field when at least 5% of the plants show signs of cutworm feeding, or when 2 or more cutworms are found per 100 plants.
Information about cutworms and their control can be found in the technical bulletin entitled “Tobacco Pests and Disorders in the Field – Insect Pests: Cutworms, Wireworms and Seedcorn Maggots” posted in the Plant Protection area of this website and in OMAFRA publications 842 and 843.
Scouting for Aphids
Aphids, mostly the green peach aphids are often seen on tobacco anywhere from about four weeks after transplanting through to topping. Aphids can be black, orange or green in colour and winged or wingless. Adults and nymphs damage tobacco by piercing the leaves and drawing large quantities of plant sap. Studies indicate that a heavy infestation of aphids reduce the price of cured tobacco leaves 5-50% and also have an effect on chemical constituents of the leaves and yield. Aphids are also vectors (carriers) of important tobacco viruses such as Potato Virus Y (PVY), Cucumber Mosaic Virus (CMV); Tobacco Etch Virus (TEV) and Tobacco Vein Mottling Virus (TVMV).
Wingless aphids – adult and nymphs; and, aphids feeding on upper leaves of tobacco
Scout fields for aphids from June through to topping. Stop at 10 or more sites per field and examine 20 plants per site. Check for aphid adults and nymphs on the underside of the upper tobacco leaves. Early in the season, special attention should be given to areas near the field edges, since most aphids migrating from other crops land in these areas of tobacco fields. Treat the field when 10% of the plants have 50 or more aphids on the upper leaves. An exception would be when aphids are detected in tobacco fields near potato, tomato and pepper fields. These fields should be treated as soon as possible to reduce the possible risk of a virus spreading in the tobacco from the neighbouring crops, particularly PVY.
Information about identification of aphids and their control can be found in the technical bulletin entitled “Tobacco Pests and Disorders in the Field – Insect Pests: Aphids” posted in the Plant Protection area of this website and in OMAFRA publications 842 and 843.
Scouting for Hornworms
Hornworms, mostly the tomato and tobacco hornworms, are caterpillars that attack tobacco plants about five to six weeks after transplanting. The two species of hornworms are biologically similar and are distinguished mainly by the number and shape of the white stripes on each side of their bodies and the colour of the horn on their back end. The tobacco hornworm has seven diagonal white stripes with black margins and its horn is usually red. The tomato hornworm has eight V-shaped stripes with green margins and a bluish black horn. Hornworms are voracious feeders. Young hornworms feeding on tobacco cause small holes in the leaves, while mature hornworms can strip the top and mid leaves of plants leaving only the midribs and main leaf vein. Studies on hornworm damage and crop loss, indicate that 100 hornworms feeding on tobacco leaves can result in 2 kg (72 oz) cured leaf loss, which is an average of 20.4 g (0.72 oz) of cured leaf per hornworm.
Tomato hornworm and leaf damage caused by a hornworm
Scout fields for hornworms from July to August. Because of their green colour and preference for the underside of the leaf, hornworms are difficult to find until the damage has occurred. The presence of distinctively barrel-shaped insect droppings, a sign of hornworm infestation, can help locate the insects in the field. Stop at 5-10 sites per field and check the underside of leaves of 20 plants at each site, and record the number of hornworms. Treat when the population averages 1 or more hornworms per 20 plants. Often, hornworms are parasitized by other insects such as the Cotesia wasp. Parasitized hornworms should not be included in the scouting counts.
Hornworm parasitized by Cotesia wasps
Information about identification of hornworms and their control can be found in the technical bulletin entitled “Tobacco Pests and Disorders in the Field – Insect Pests: Hornworms” posted in the Plant Protection area of this website and in OMAFRA publications 842 and 843.
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Mailing Address: CTRF, P.O. Box 322, Tillsonburg, ON N4G 4H5
Telephone: 519-842-8997
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