Splitworms have become a more frequent pest of tobacco in recent years, and in 2007 the hard-to-control leaf miners appeared to be a bigger problem on burley than on flue-cured tobacco in North Carolina and Virginia.
Once considered a threat primarily to flue-cured tobacco, splitworms have increased in severity as more acreage has been converted to burley, especially in eastern, North Carolina.
In August of 2007, growers in Pitt County, N.C., reported the first ever outbreaks of the insect.
The tobacco splitworm is unique compared to other tobacco insects in that it tunnels into the foliar tissue creating transparent areas frequently referred to as mines. If plant populations reach about 15-20 per plant, reduced leaf quality frequently occurs from splitworm damage.
On burley tobacco, the splitworm, which is also known as the potato tuber worm, often causes minimal damage to the leaf in the field. However, the insect continues to feed on burley leaves and stalks in the curing barn.
During its long exposure to burley leaves, the splitworm feeds between upper and lower surfaces of the plant, and especially on older leaves. It causes gray to brown splotches on the tobacco leaf. During curing these splotches deteriorate and weight is lost due to tunneling in the leaves.
Splitworms are true leafminers, feeding on the inside of the tobacco leaf, which makes them difficult to manage with chemicals. Combating the problem is a lack of insecticides labeled for use on these insects. Control, at best, requires high volumes of water.
In 2007, splitworms, which usually attack the lower half of the tobacco stalk were a bigger problem on burley than on flue-cured tobacco in eastern North Carolina.
No insecticides are specifically labeled for this pest, and those that provide the best control can't be used due to lengthy pre-harvest interval requirements. Given this, the simplest approach taken by some North Carolina growers was simply to harvest and destroy affected leaves.
Splitworm damage tends to slowly work its way up from the bottom of the stalk over a period of weeks and worm generations. By harvesting all affected leaves, plus a couple above that level, in most cases growers were able to stay ahead of the problem. Though once considered a late season, or even a post-harvest pest, tobacco splitworm can also damage transplants.
Typically, splitworms tunnel between the upper and lower surfaces of leaves on mature tobacco plants, causing gray blotches that frequently turn brown. However, on young transplants, these insects can tunnel into the bud of the plant, destroying the transplant is a short period of time.
North Carolina State Entomologist Hannah Barruck, who was appointed just prior to the 2007 cropping season, says most of the calls she received from growers had to do with splitworm damage to tobacco.
She says populations of 30 larvae per leaf and up to 200 per plant were not uncommon. This level is considerably higher than in past years — the highest ever in some areas — and may be reflective of the eastern and southern movement of burley tobacco into historical flue-cured tobacco areas.
Orthene and Lannate gave the best control in grower trials, but neither provided adequate protection under heavy splitworm pressure. There are issues with both materials and they are not consistently effective enough to warrant recommendation.
Two new, experimental insecticides did provide good control, but these materials may be several years away from the marketplace, Barruck says.
Barruck says she will be part of a four-state project with entomologists in Virginia, South Carolina and Georgia. They will be tracking splitworm populations at four sites in each state.
“We want to get an idea of number of generations per year and how these tie into weather patterns and changes. Data that goes back more than 50 years indicates that a warm, dry year, it results in additional generations of the insect.”
“We will also take a look at adult dispersal and larval movement, which will tell us more about how potatoes, tomatoes, eggplant and other host plants near tobacco will affect splitworm population buildups. However, people who have worked a long time in tobacco say that sometime the insect builds up in locations where no other host crops are located within many miles of tobacco.”
“Clearly there are other non-crop hosts, a leading suspect is jimsonweed, but we need to know more about potential host plants to help us better develop management strategies for splitworm in tobacco,” Barruck contends.
With the increased demand for burley tobacco, and the continued movement of the crop into areas in which flue-cured tobacco has been grown for many years, the potential is great for increasing problems with splitworm. The good news is that researchers recognize the potential threat to tobacco and are looking for tools to help growers better manage the problem.