"We know that the tannins in oak leaves inhibit gypsy moth growth, but that they also provide protection against the nuclear polyhedrosis virus," says Dr. Heidi Appel, research associate in entomology in the College of Agricultural Sciences.
Oak trees make more tannins when gypsy moths chew on them and the tannins slow caterpillar growth, reduce the number of eggs and lead to less successful young. The trees' response should be inhibiting the caterpillar, but it takes much more virus to kill the caterpillars when they are eating tannins. The oak trees' defense -- tannin -- actually lets the gypsy moth caterpillar survive.
Nuclear Polyhedrosis virus, LdNPV, is commonly called wilt disease because the caterpillar turns into a sack of virus and slumps over. This naturally occurring virus is acquired when caterpillars eat the dormant virus left on leaves by previous victims. When the caterpillar eats the virus, together with the oak leaf and its tannin, the virus becomes activated.
"We thought the tannin acted by itself, that it did something to the viral proteins," Appel told attendees at the Ecological Society of American Conference, today (Aug. 14) in Albuquerque. "It turns out that enzymes in the leaves may be the important factor."
Appel looked at two enzymes; polyphynol oxidase (PPO), which helps tannin work better and peroxidase (POD), which inhibits the action of tannin. Both naturally occur in oak leaves.
"Peroxidase just chews up everything it comes in contact with," says Appel. "It probably inactivates the tannin. PPO, on the other hand, probably oxidizes the tannin into the products that actually inhibit the virus."
The Penn State researcher is looking at caterpillars in the field and in the laboratory, and she is interested in the chemical reactions that take place in the caterpillar gut. The wilt virus needs the very basic environment of the gut to shed its crystalline coat and become active. This same alkaline environment does oxidize some of the tannin, but Appel has found a strong relationship between PPO and virus inhibition. The trick is to put this knowledge to work in controlling the caterpillars.
The constant sound of chewing at the height of a gypsy moth infestation is more than just annoying. Besides damaging the leaf canopy of northeastern forests, these voracious eaters cost the hard woods industry millions of dollars. Luckily, from an economic point of view, gypsy moths usually do not eat coniferous trees, which are the basis of the multimillion dollar pulp paper industry.
"However, there is another threat, the Asian gypsy moth, which eats conifers and has been introduced at several seaports by ships from the Far East," says Appel. "But so far, these introductions have been eradicated."
While gypsy moth mothers do not fly, the Asian gypsy moth females do, which means the caterpillars will spread faster and could pose a greater threat to the forests if these caterpillars are not controlled.
"The ideal approach for controlling both pests would be to add something to the virus spray that would disarm the tannin and make the caterpillars susceptible to the virus," says Appel. "This could either stop the oxidation of tannin or inhibit the action of PPO." Further research into the chemistry of the gypsy moth caterpillar gut and the interaction of caterpillar, oak tree and virus, might eventually lead to a control method.
EDITORS: Dr. Appel may be reached at (814) 863-3380 or at email@example.com by email.
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