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Time Again for Summertime Pests

December 11, 2020 |

Some cycles are never far from a tobacco farmer’s mind: the cycle of plant growth, from seed to plant to harvest; the changing seasons, bringing longer, warmer days when tobacco thrives; the water cycle’s clouds and rain that quench thirsty fields. These cycles create a rhythm and consistency in the uncertain business of farming. But there are other, equally important but undesirable cycles that threaten a farmer’s livelihood—the life cycles of insect pests, such as hornworms, which can ravage tobacco plants overnight.

Even unwanted cycles have a beauty to their predictability. Use this predictability to your advantage by regularly scouting fields for pests. Hannah Burrack, professor of entomology and Extension specialist at North Carolina State University, recommends tobacco farmers start scouting three to four weeks after transplant. Before you do, arm yourself with knowledge of how to identify pests, when they are active and where to find them on tobacco plants.

 

Tobacco Flea Beetles

Tobacco flea beetles emerge in tobacco fields from immediately after transplanting to about six or seven weeks later. When small, distinct, shot-like holes appear on leaves during scouting rounds, tobacco flea beetles are feeding.

At around 2 mm, these brown-and-black beetles use their powerful hind legs to propel themselves when disturbed, making them easy to recognize but difficult to count. They are less active in cool, early mornings, allowing more accurate numbers. When scouting, inspect tops and the undersides of leaves, counting only live beetles.

Because flea beetles can damage entire transplants, causing defoliation and stunted growth, they are managed primarily through preventive greenhouse treatments. Should flea beetles emerge in the greenhouse, the treatment threshold is four beetles per transplant.

Mature tobacco plants tolerate much greater numbers. Sixty beetles per plant warrants treatment, and typically you will find them on the lower one-third of plants. “Late season flea beetles are much, much less common than early season, and they are related to tobacco maturity,” says Burrack.

Mature leaves’ chemistry attracts flea beetles, she explains. “The best thing growers can do is harvest these leaves rather than trying to treat them with insecticides.”

 

Tobacco Budworms

Another early summer pest is the tobacco budworm. They munch on tobacco foliage in and around the bud and can cause extensive leaf damage and destroy a plant’s apical growth point. This causes the plant to top early, which leads to more costly sucker control.

To scout for budworms, gently peel leaves back to check inside buds. Next, check tops and undersides of leaves immediately surrounding buds. Budworms are more likely to crawl out of buds on cool mornings and hide deep within buds during hot afternoons.

Signs of budworms are frass (worm droppings) and small holes in the small leaves that enclose buds. However, only larvae count as an infestation. Tiny, young larvae have yellow-green bodies while older ones range from shades of green, red or yellow with whitish bands on their backs and sides.

Tobacco budworms look similar to corn earworms. Distinguishing the two is important because not all treatments are effective for both. Using a hand lens, inspect the bumps on the larva’s back for short bristles. If you see bristles, it’s a tobacco budworm.

Before topping flowers, treat for budworms when at least 10 percent of plants are infested. Within two weeks of topping, budworm damage will be limited, so there is no need for treatment, and scouting can stop. Budworms prefer new growth, so topping eliminates them as a pest.

 

Green Peach Aphids

Don’t be fooled by the name; green peach aphids can also be red or orange. All green peach aphids quickly multiply, making them prone to developing insecticide resistance, but red ones are even more likely to do so.

Green peach aphids are most common one month to two months following transplant. At 1.5 mm, they feed on tender new growth and flowerheads, which causes thin, lightweight leaves that may ripen prematurely. Aphids also excrete “honeydew,” a sticky, sugary substance that can lead to black, sooty mold, cause leaves to stick together and result in poor quality curing. In addition, green peach aphids may transmit tobacco viruses.

When scouting for green peach aphids, check the undersides of leaves on the upper stalk for wingless aphids. Avoid counting winged aphids (that may not colonize plants), cast skins or “mummies” (parasitized aphids that are dull, light brown, larger and more rounded than other aphids and do not move if touched). Continue scouting through topping.

If at least 10 percent of plants have 50 or more aphids, don’t delay treatment. If you find aphids below the treatment threshold, scout more frequently. Their short reproductive cycles can quickly overwhelm plants. However, after topping, mature plants no longer appeal to the aphids. Post-topping treatments to limit sucker growth also kill aphids, according to https://ipm.missouri.edu/pestmonitoring/tbw/identification.cfm.

Hornworms

Two species of hornworms found on tobacco emerge in mid-season heat: tobacco hornworms and tomato hornworms. Biologically similar, they are managed the same way, and at a glance, they look the same: bright green with a horn protruding up from their hind ends. The difference is in the details; tobacco hornworms have seven diagonal stripes on each side and a red horn while tomato hornworms have eight chevrons on each side and a blue-black horn.

Hornworms are heavy feeders. Just two can defoliate a tobacco plant seemingly overnight. With that type of appetite, it doesn’t take many to create significant damage in a field.

When scouting, barrel-shaped frass and smooth-edged holes in leaves are tip offs that hornworms are near. Start treatment for hornworms when you find one or more per 10 plants. When you see hornworms covered in tiny white cocoons, the telltale sign they are parasitized, count each as one-fifth a healthy hornworm. It takes about five of them to eat as much as a healthy hornworm.

 

Insect Scouting Tips

Scouting allows farmers to collect data that indicates if fields have reached the economic thresholds to treat for pests. The basics of scouting based on North Carolina State University Extension guidelines are:

  • Make a set number of stops randomly throughout the field based on field size.
  • At each stop, inspect five plants.
  • Record the number of insects per plant or the percentage of infested plants for each field and keep these records.

 

For a detailed sampling scheme, see the article “Tobacco Insect Scouting Methods,” updated in 2016: https://tobacco.ces.ncsu.edu/tobacco-insect-scouting-methods/.

 

Category: Diseases & Pests

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