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Growers may be seeing a lot of aphids in some tobacco fields. At this point in the season, the time left until topping is the major factor to consider when making a decision on aphid control. Yield and quality losses to aphids occur gradually – from the onset of infestation, about six weeks after transplant, until topping. Most of the loss is due to reduced leaf size and weight from heavy infestations. There is little chance for return from attempting to control heavy infestations just before harvest. The loss has already occurred.
If the field will be topped within about a week or has been topped, aphids have already done their damage; there is no benefit to an insecticide application that is limited to aphid control. Topping the plants not only removes the greatest concentration of aphids but also the tender tissue on which they prefer to feed and where they cause the most new damage. Aphids present on the discarded tops will remain there and will continue to feed until the leaves dry completely. Few, if any, will find their way back to a plant. Aphids present on tobacco that has been topped are not a cause for concern at this time in the season.
Aphids can be a problem in late set fields that are still several weeks from being topped. Foliar sprays can be effective if applied when about 20 percent of the plants in the field are infested with aphids. Judge the benefit of control by looking at new foliage at the top of the plant. If aphid build ups are not seen on these leaves, then the sprays are working – they are more effective at protecting uninfested foliage than at killing aphids lower down on the stalks.
Hornworms and grasshoppers are the most common insect pests that are active in tobacco that has been topped. They can be very destructive during the weeks the crop remains in the field before harvest. Most insecticides used on tobacco should be effective for about a week. After that time, weekly field checks should be made to detect any late pest buildups.
Hornworm moths fly over a relatively long period of time so worm season can occur over an extended interval.
Grasshoppers can cause extensive leaf loss in tobacco fields. They usually occur in grassy pastures and can be driven into crop fields as hay is cut and cured. Check for grasshoppers in hay fields and pastures adjacent to tobacco before clipping it. An average of 10 or more hoppers per square yard can mean trouble if they move to tobacco. It is much better to control these insects before they get into the crop than to try a rescue application later. Cythion (malathion) and Sevin are labeled for grasshopper control in pastures and hay fields. See the label for rates and restrictions.