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Why does splitworm damage begin at the bottom of tobacco plants?

By Hannah Burrack, North Carolina Extension Entomologist, in Southeast Farm Press

One of the first projects my lab tackled when I began working in North Carolina was attempting to understand the biology and behavior of tobacco splitworm.

Our goal was to use this information to develop more effective scouting and management strategies for this occasional, but potentially significant late season tobacco pest.

Our first scientific publication from this work has just come out: Host utilization is mediated by movement of pre-feeding larvae in Nicotiana tabacum (subscription required to view to whole article, but reprints are available from me upon request).

In this study, masters student Monique Rivera determined that tobacco splitworm larvae are first observed on the lower stalk tobacco leaves.

She then tested why this might be by first caging newly hatched larvae on upper, middle, and lower stalk leaves to see how quickly they developed (faster development is better for insects) and how large the pupae produced on each leaf type were (bigger is better for pupae).

She repeated this early in the growing season and late in the growing season.

Monique found that larvae grow faster and bigger on younger plants (earlier in the growing season) and on upper and middle stalk leaves when compared to lower stalk leaves.

Remember, we find the most splitworm larvae in lower stalk leaves at the middle and end of the growing season. This suggests that larvae in the field were selecting leaves for other reasons than because they were the best possible host.

Monique then removed different amounts of leaves, from the bottom of the plant up, similar to what occurs during tobacco harvest. She found that, while larvae could reach the top of tobacco plants even when only the very tip leaves remained, fewer of them successfully established.

This suggested that larvae were selecting sub-optimal, lower stalk leaves because traveling to upper stalk leaves was riskier.

What does this means for tobacco splitworm management?

First, we learned that plants should be scouted bottom to top. To detect infestations before they become damaging, scouts and growers must carefully look at bottom stalk leaves, starting at the beginning of July.

Next, we learned that harvest may reduce, but likely not eliminate splitworm infestations. Harvest timing may play a roll in mitigating splitworm damage when severe.

Additional work on splitworm in our lab has suggested there are two, perhaps three, generations during the tobacco growing season, that moths can be captured in pheromone traps even in fields where no damage is observed (suggesting there are other things they may prefer to eat), and that moth trap captures may be useful in timing potential crop infestations.  We will be summarizing and publishing this information over the winter.

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