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Wednesday, September 1, 2010

Tobacco splitworm damage spiking throughout NC

Tobacco leaf with damage from tobacco splitworm larvae (Phthorimaea operculella). This leaf had at least 20 galleries. Photo: HJB
Damage due to tobacco splitworm larvae has dramatically increased at sites in Johnston, Columbus, and Chowan Counties in the past two weeks. Masters student Monique Rivera has been studying the seasonal biology and behavior of tobacco splitworm for the last 2 years and is wrapping up her thesis research this fall--just in time to see the damage this insect can do in the field.

The accepted ESA common name for the tobacco splitworm is the potato tuber moth, which reflects the fact that this insect is also a pest of potatoes, feeding on the tubers. For at least the past 5 years, however, the tobacco splitworm has been present in tobacco plantings where no potatoes have been found. This suggests that the insects are coming from other sources into tobacco fields. Although little work has been done on tobacco splitworm as a pest of tobacco, research conducted in the early 1900s in Maryland suggested that dry, hot years resulted in the greatest amount of tobacco splitworm injury. This observation appears to hold true for our recent NC infestations, with widespread injury in 2007, 2008, and now 2010.

Tobacco splitworm larvae are leafminers in tobacco, creating distinctive galleries. Female moths lay eggs at the base of tobacco plants, on the soil surface. After the larvae hatch, they move up the stalk and begin feeding on leaves. Damage first appears on lower stalk leaves, but the larvae are capable of feeding on leaves throughout the plant if lower stalk leaves are not present. Newly hatched larvae are roughly 2 mm long, and the galleries they create are also small. Splitworms typically spend their entire larval lifespan in a single gallery, although they may feed internally on leaf veins, making it appear as though they create multiple galleries. In total, a single mature gallery takes up roughly 2 square inches (see above image).

Sites in Johnston, Columbus, and Chowan Counties have had in excess of 50 galleries per plant. Work conducted in the NCSU Entomology Department in 2006 through 2007 found that very high numbers of splitworm larvae like this may reduce total yield. Yield, however, was not the primary concern of growers in Johnston County. It is possible that even fairly low (5 or more full size galleries per leaf) could reduce leaf quality.

Tobacco splitworm gallery (circled). Photo: HJB
Late instar (nearly mature) tobacco splitworm larva in opened gallery. Photo: HJB

Partially cured tobacco leaf with tobacco splitworm injury. Photo: HJB
Growers first noticed the infestations in Johnston County after harvested leaf had been placed in barns. Large numbers of larvae began to flee through the base of the barns and were killed in yellowing leaf.

Tobacco splitworm larvae outside curing barn. Photo: Bryant Spivey, Johnston County NCCE
Dead tobacco splitworm larvae near the door of a bulk barn. Photo: HJB
Curing leaf without tobacco splitworm damage. Photo: HJB
Curing leaf with high levels of tobacco splitworm infestation. Photo: HJB
Although they can be potentially devastating, tobacco splitworms are an occasional pest of tobacco, and preventative treatment is not recommended. It is also not recommended to treat adult moths because they likely do not spend the majority of their time in tobacco. Instead, management strategies should be targeted to the larval stages.

Scouting is key to effective splitworm management in tobacco. Based on trapping and field monitoring conducted since 2007, it appears we have 2 to 3 splitworm generations during the tobacco growing season. Years where 3 generations occur are likely to result in the greatest crop damage because each successive generation will attack the same tobacco. Damage from the early generations indicates that a population is present in a field and that this field should be observed carefully. Early generation damage will appear on the lowermost leaves, and scouting before and after topping should focus on these parts of the plant. Pesticide treatments should be applied if damage begins appearing up the stalk and live larvae are present in these mines.

Treatments should be made when new mine formation begins. Treating mature larvae in mines will likely not be economically beneficial, because large larvae have already caused most of the damage they will do at this stage.

Adult moths can monitored using pheromone baited sticky traps, which are attractive to males and appear to be relatively specific. This means they do not appear to attract large numbers of non target moths which could be confused with tobacco splitworms. Monitoring data conducted in North Carolina for the last 3 years suggests that adult moth trap captures peak 3 to 4 weeks before infield damage. This suggests that adult trap capture peaks could be used to time in field applications of pesticides to control larval infestation. Moths are also caught in fields where no damage occurs, however, so relying only on trapping is not advisable. It is possible that traps could be deployed in fields where early season injury is noted in order to effectively time late season, potentially economically damaging, populations.

Male tobacco splitworm moth captured in pheromone baited trap. Moths appear as similar to many other species to the naked eye. Photo: HJB

Two insecticides, Belt (flubendiamide) and Coragen (chlorantroniliprole) are registered for tobacco splitworm in North Carolina. However, there is very little data on either of these materials against tobacco splitworms, and I cannot recommend one over the other for control at this time. We are working to generate more data on these materials and their appropriate rates.

Why now?
Historical data suggest that tobacco splitworm injury is most problematic in hot, dry years. I suspect that the fields where we are seeing injury now may have received less rainfall than other areas. The marked decrease in the use of broad spectrum, foliar insecticides may also have contributed to increases in splitworm injury. A behavioral shift in the larvae or adults may have resulted in preference for tobacco over other hosts, which may explain why injury is occurring in areas where no other commercial hosts (ie. potatoes) are present.

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