NC Small Fruit & Specialty Crop IPM: Mid season tobacco pests & the burning question of tank mixes

Tobacco budworm moth. Moths lay their eggs in or near tobacco buds and flowers and are more attracted to flowers. Photo: HJB

Tobacco budworm populations have been booming in our research plots this summer, and as they often are, tobacco budworm larvae are present in fields at or near topping. The logical questions that follow these tobacco budworm populations is whether or not they need to be managed. In general, if plants are within 2 weeks of toppping or large enough that contact sucker controls are being applied, tobacco budworm treatments are unnecessary. This is because budworm larvae prefer to feed on flowers and seed capsules when given a choice between these and leaves. These are the same plant parts (along with the youngest, smallest leaves) that are removed during topping. Tobacco budworm feeding during the interval between buttoning (when the unopened flowers are visible outside of the bud) and flower does not damage plant tissue that will be harvested and has no economic impact. Therefore, any insecticide applied at this stage for tobacco budworm is a waste. Tobacco budworm will not reinfest tobacco after topping because the attractive buds and flowers are no longer there.

Tank mixing insecticides and sucker controls
Among some North Carolina tobacco growers, it has been standard practice to include an insecticide (most commonly acephate) in the first few applications contact sucker control. Contact sucker control materials are pesticides that, as their name implies, need to come into contact with a leaf axil in order to inhibit growth. Contacts are typically fatty alcohols, or soaps, which are commonly made from petroleum or, in the case of organic contacts, palm oil. Fatty alcohols are also used in a number of industrial and cosmetic applications.

Foliar (sprayed on) insecticides in tobacco should only be used when insects are present at damaging levels (the economic threshold). Acephate is a broad spectrum insecticide, so it could potentially be used against a wide range of insect pests. The insect pests most commonly present at or near topping are tobacco budworm, the "tobacco form" of the green peach aphid, and, less commonly, tobacco & tomato hornworms. We already know that pretopping treatments for tobacco budworms are not needed, but what about aphids and hornworms?

Aphids
Aphid management in tobacco has shifted dramatically in the last 15 years. Aphids used to be frequent pests which often reached treatment threshold (10% or more of plants with 50 or more aphids in the upper third) several times in a season. The widespread (over 90%) use of systemic neonicotinoid insecticides (imidacloprid & thiamethoxam) has rendered aphids infrequent early season pests who typically only appear near topping. If aphids are present in a field when contacts are being applied, it may be tempting to include an insecticide in with these materials, but this is totally unnecessary. Contacts (soaps) can be excellent aphid control materials, because they dry out their soft bodies, and the application method for contacts actually results in decent coverage for aphid control. In addition, aphids are less attracted to tobacco leaves as they "harden off" following topping and will rarely reinfest. In an organic aphid management trial we conducted in 2009, none of the organic insecticides were effective against the large aphid populations present, but as soon as the grower started spraying organic contact and topping, the aphids disappeared. Timely topping and good sucker management can eliminate an aphid problem, so insecticides in the contact do not provide any additional benefit.

Tobacco & tomato hornworms

Tobacco or tomato hornworm eggs. Hornworm eggs are laid on the upper surface of tobacco leaves. Photo: HJB

Tobacco hornworm (Manduca sexta) larva, note the red posterior "horn". Tomato hornworm larvae will have a dark blue horn. Photo: HJB

Tobacco and tomato hornworms typically occur in their highest numbers just after topping and, from a timing stand point, are perhaps the most logical insects to be targeted by an insecticide tank mixed with contacts. However, I do not recommend tank mixing insecticides with sucker control materials even for hormworms for two important reasons:

1. Phytotoxicity may occur when insecticides are tank mixed with oils and soaps, and more importantly,

2. Contact sucker controls and insecticides should be applied to different parts of the plant and will be less effective with different coverage patterns.

The only time I have seen phytotoxicity associated with some of our newer, caterpillar active insecticides in tobacco is when they have been used in combination with contacts. Although I have never seen truly damaging phytotoxicity associated with either Belt or Coragen, it doesn't make sense to combine these with sucker controls.

Because contacts need to coat leaf axils to be effective, they are applied in a coarse spray, run down spray, sometimes using modified hoods, as described in the 17 June issue of the Tobacco Connection Newsletter. This application method drives contacts down the stalk, but hormworms are leaf feeding insects and could be missed with a stalk spray or hood application. If an tank mixed insecticide application fails, it means yet another treatment across the field. I would rather see one insecticide trip made, done correctly.

Stink bugs

A midseason tobacco ecosystem, tobacco budworm larva and adult brown stink bug. Hoke County, NC. Photo: HJB

The last tobacco insects that may pop up in the mid season are stink bugs, both brown and green. Stink bugs feed on the stalk and mid veins of tobacco leaves, which can cause them to wilt. This damage is rarely, if ever, economically significant, and insecticide treatments for stink bugs are not recommended. In fact, so few insecticides are effective for stink bugs, that is unlikely that any of the materials with workable preharvest intervals would provide control regardless of whether treatment was needed.