Lymantria dispar Multienveloped
Nuclear Polyhedrosis Virus (LdMNVP)

LdMNPV is thought to have entered the gypsy moth populations in the U.S. at the time of, or soon after, the introduction of the gypsy moth itself, probably in the form of latent infections or by parasitoids. The LdMNPV is a relatively virulent species. It pervades the tissues of the host and is usually lethal. Infected larvae often show the classic climbing behavior known to pathologists as "summit disease" and die hanging by their prolegs from a stem or other object. The fragile, infected dermal tissue breaks, releasing a brown fluid containing hundreds of thousands of polyhedra.

Epizootics are obvious and frequent in high density gypsy moth populations, but usually occur after the build-up of the host population and after serious defoliation has occurred. This pathogen does, however, regulate gypsy moth populations (Campbell and Podgewaite, 1971), and early workers considered the possibility of manipulating the virus as a microbial insecticide.

The USDA Forest Service selected LdMNPV as one of two disease agents to study for use as microbial insecticides (the other was Bacillus thuringiensis kurstaki). The reasons NPV was chosen were the following (Lewis, 1981):

1. NPV is naturally occurring and has been implicated in causing natural collapse of the insect.
2. NPV is environmentally safe and, of the known natural pathogens of the insect, presented the least problems in registering a product based on the organism.
3. NPV is selective for the gypsy moth.
4. NPV gave evidence of carryover in natural populations, indicating long-term control potential.
5. Because NPV was a naturally occurring agent, it presented the possibility of enhancement or manipulation within the environment.
6. Laboratory evaluations and early field tests gave support to the use of the NPV as a direct control agent.

Thus began a long period of work on the virus. First, bioassays were conducted to determine which of many strains were the most virulent. Synergistic compounds were investigated and storage effects were studied. Techniques were worked out for large scale production and quality control; unfortunately, LdMNPV must still be produced in living gypsy moth larvae. Early efforts to produce the virus in tissue culture were not suitable for large-scale production and the virus cannot reproduce outside living cells. Industry and the Forest Service developed several products; the Forest Service formulation was called Gypcheck. Early products, including Gypcheck were erratic in performance and the formulations fell out of favor- and most out of production. The Forest Service continued to produce Gypcheck, however, and new isolates and new formulations improved the product significantly. NPV continues to be produced by the Forest Service and is used particularly for spot eradication in environmentally sensitive areas. In addition, as the importance of natural enemies in established exotic insect populations is emphasized, LdMNPV is recognized once again as a vital part of the gypsy moth natural enemy complex.