Insects – Green Peach Aphid
The most common aphid species found in tobacco is the green peach aphid (Myzus persicae). Tobacco is just one of many green peach aphid hosts. In cold climates, green peach aphids overwinter as eggs laid on woody perennial hosts, Prunus species, hence the common name. In the southeast, however, eggs are not produced, and only female aphids are present. These female aphids give live birth to new, female aphid nymphs, which quickly develop to adults. They overwinter on weeds and other crop hosts, including strawberry. It has been suggested that tobacco-feeding green peach aphids are more restricted in their host range than green peach aphids feeding on other crops and feed mainly on tobacco and related plants. Because they reproduce so easily and quickly, green peach aphids can rapidly increase their populations and may have up to 30 generations per year.
Because of their short generation time and reproductive strategy, aphids are a high risk insect for the development of insecticide resistance. Resistance to organophosphate, organochlorine, and pyrethriod insecticides resulting in insecticide failure has been documented in some aphids (see below), and neonicotiniod resistance has been detected in field populations of green peach aphids, although field failures of these materials have not been observed.
There are 3 color morphs of the green peach aphid in the southeast, but all of these are still the same species. The green color morph is often most common in spring and early summer, while the red color morph dominates in the late summer and fall. The red morph is more tolerant of higher temperatures and is also more prone to develop resistance to insecticides. This relationship between red color (due to the adoption of a gene from fungi allowing them to synthesize caratiniods) and resistance to insecticides is common among several aphid species, including potato aphids. There are also differences in predator preference between green and red aphids, with green aphids preferred by parasitoid wasps and red aphids preferred by lady beetles and their larvae. In tobacco, red morphs resistant to organophosphate insecticides were noted in the early 1990s. Other researchers have noted that red color morphs are more tolerate to dimethoate (an organophosphate), endosulfan (an organochlorine), and lamda-cyhalothrin (a pyrethriod). A third, orange, color morph began to appear in North Carolina in summer 2007. The orange color morph appears intermediate between the green and red morphs, but its biology has not been compared to the other morphs.
Aphids are a pre topping pest and prefer to feed on succulent new growth. Because of this feeding preference, aphids are typically localized at the top of the plant. They are also more attracted to plants grown in soil with high nitrogen levels, as this results in succulent, green leaves. After topping, tobacco leaves harden off and mature and are no longer attractive to aphids. In addition to being a less attractive substrate, contact treatments (fatty alcohols) applied to tobacco to limit sucker growth are also toxic to aphids.
Thresholds and Management
As the use of systemic neonicotiniod insecticides (ie. Admire Pro and Platinum) in tobacco has become the standard, the need to foliar aphid treatments has diminished. Virtually all conventional grown tobacco in North Carolina is treated with a systemic neonicotiniod insecticide in the greenhouse, or less often, at transplant. These materials provide long term control of aphids, typically through topping, although there is long term concern about resistance management with these materials (see Biology).
Because systemic treatments are applied at or near transplant, they are necessarily preventative. However, aphid populations in North Carolina are variable and may not require treatment each year. Therefore, systemic treatments may be unnecessary. Systemic treatments are attractive because of their simplicity and efficacy, but their widespread use has the potential to encourage resistance development in aphids.
Rescue treatments for aphids are justified when 10 percent or more of plants have as many as 50 aphids on any upper leaf before topping. Because of their small size, it is useful to calibrate your eye by counting several leaves to get a sense of what 50 aphids look like. Do not wait until hundreds of aphids are present to count a plant infested. This threshold should be used carefully. Before topping, populations can increase rapidly beyond 10 percent infestation. Do not delay initiating treatment. There are several foliar materials affective against aphids registered in tobacco, many of which include the same active ingredients as the systemic neonicotiniods used at or near transplant. If these materials are to be used, be sure to following label resistance management strategies.
Organic aphid management is challenging. The registered organic insecticides in tobacco include soaps, oils, and pythrethum (the natural version of synthetic pyrethriod insecticides). All of these are non persistent, meaning that coverage is very important. This requires a fairly high volume of water per acre (a minimum of 50 gal) and drop nozzles to ensure good under leaf wetting. Limited efficacy data is available for organic aphid materials registered in tobacco.
See the North Carolina Agricultural Chemicals Manual for registered materials and use recommendations.
Aphid biological control agents include parasitoid wasps, syrphid fly larvae, lacewing adults and larvae, and lady beetle adults and larvae. These beneficial insects occur naturally in North Carolina and can contribute to control. Organic tobacco growers will often seed skip rows with sunflowers and/or buckwheat. These flowering plants are thought to serve as nectar sources for wasps, flies, lacewings, and lady beetles. It is unclear how important these beneficial insects are to the tobacco agroecosystem, but they often do not appear until high populations of aphids are already present, limiting their economic significance.
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