Soon after transplanting, tobacco seedlings can be damaged by several insect pests. The population of these insects can vary from year to year and field to field during the growing season. Regular scouting of tobacco fields is the best way to know the pests present and the potential damage level they might be causing to tobacco. For some insect pests there are specific economic thresholds (ET) established. ET is the pest population level at which control should be initiated to keep the pest population from reaching economically damaging numbers. Insecticides should be applied only when the established ET for a given pest is reached. Applying insecticides based on ET is economical because it reduces insecticide use and associated costs, protects workers and the environment, and has the potential to reduce pesticide residues by avoiding unnecessary applications.
Field Scouting – Site Selection
Tobacco fields should be scouted for cutworms, aphids, hornworms and Japanese beetles, since these insect pests can attack tobacco after transplanting. At high population densities these insects can cause significant losses, thus the need for field scouting to ensure that these insects are kept below the ET.
To scout tobacco fields, it is recommended to adequately select the field sites for a good estimate of each insect’s distribution in the field. Plants for observations should be pre-selected at random and before starting scouting to avoid biasing counts by stopping at damaged plants. The number of sites selected in a field depends on its size and the more selected and checked sites, the higher the scouting accuracy. For fields 2-4 ha (5-10 acres) in size, it is recommended to select 5 to 10 randomly selected sites throughout the field, and for fields larger than 4 ha (10 acres) the number of sites checked is increased by 1 site for every hectare (2.5 acres).
Cutworms, mostly the darksided cutworms, are soil insects that damage tobacco during the first four to five weeks after transplanting. Cutworms damage tobacco plants by creating holes in leaves, feeding extensively on the growing point (heart) and/or cutting off the stem at the soil surface. Fields should be scouted for cutworms weekly, from transplanting to early July. Scouting should be done early morning or evenings because cutworms are most active at night. When scouting, 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 point, or stem 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.
Aphids can begin to infest tobacco shortly after transplanting. The green peach aphid is the aphid most commonly found on tobacco, and it can be black with wings and green or orange/red without wings. Aphids damage tobacco by piercing the leaves and drawing large quantities of plant sap and by depositing large amounts of honeydew on the leaves, which makes these leaves non-marketable and/or reduce the price of the cured leaves by 5-50%. Leaves highly infested with aphids can also affect yield and the chemical constituents of the leaves. Aphids also damage tobacco by carrying and transmitting viruses such as Potato Virus Y (PVY), Cucumber Mosaic Virus (CMV); Tobacco Etch Virus (TEV) and Tobacco Vein Mottling Virus (TVMV).
Field scouting for aphids should start in June and continue until topping. To scout for aphids, 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 edges of fields, since most aphids migrating from other crops land in these areas. Treat the field when 10% of the plants have 50 or more aphids on the upper leaves. Tobacco fields near potato, tomato and pepper fields should be treated as soon as possible to reduce the possible risk of viruses 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.
Tomato and tobacco hornworms attack tobacco plants about five to six weeks after transplanting. The tomato hornworm is the hornworm most commonly found feeding on tobacco. Hornworms are voracious feeders. Young hornworms damage tobacco by creating small holes in the leaves and mature hornworms strip the top and mid leaves of plants leaving only the midribs and main leaf vein. A damage estimate for hornworms is 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.
Fields should be scouted for hornworms from July to August. Hornworms are green in colour and prefer the underside of the leaf, which makes them very difficult to detect until damage has occurred. A sign of hornworm presence and feeding that helps locate the insect is the presence of distinctively barrel-shaped insect droppings around the infected leaves/plants in the field.
To scout for hornworms, 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 seen covered with white cases (cocoons) of the Cotesia wasp that parasitizes them. Parasitized hornworms feed less – the feeding of five parasitized hornworms causes damage equivalent to that of one healthy hornworm. Therefore, five parasitized hornworms should be considered as one healthy hornworm in the scouting counts.
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.
Japanese beetles feeding on tobacco has been occurring in some tobacco fields in recent years and infestations appear to be increasing each growing season, however, damage so far has typically not been economically significant. The Japanese beetle was first detected in Canada in 1939 and has since become well established in Ontario. This beetle is known to feed on more than 300 plant species.
Field scouting for Japanese beetles can occur while scouting for aphids and hornworms. The beetles are easy to detect in the field starting in late June through to August because of their bright metallic green and coppery brown coloured bodies, and by the damage they induce on leaves while feeding. Their feeding on tobacco creates ragged holes and removes the leaf lamina leaving only the leaf veins giving the leaf a lacy appearance.
There are no economic thresholds established for Japanese beetles on tobacco in Ontario and there are no registered products for their control.
Information about Japanese beetles can be found in the technical bulletin entitled “Tobacco Pests and Disorders in the Field – Insect Pests: White Grubs” posted in the Plant Protection area of this website.
Post prepared and sent by Canadian Tobacco Research Foundation
Mailing Address: CTRF, P.O. Box 322, Tillsonburg, ON N4G 4H5
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