AN EPIZOOTIC OF NUCLEAR POLYHEDROSIS VIRUS IN ARMYWORMS (PSEUDALETIA UNIPUNCTA) IN CONNECTICUT

Northeastern Naturalist, 2004 by Magnarelli, Louis A, Andreadis, Theodore G

ABSTRACT - An outbreak of armyworms, Pseudaletia unipuncta, occurred in widely separated sites in New England during the summer of 2001. Heavy mortality of caterpillars was noted in severely damaged suburban lawns and destroyed hayfields. Microscopic examinations of 90 moribund or recently expired caterpillars collected in Connecticut revealed a nuclear polyhedrosis virus (NPV) in 59 (66%) individuals from 5 of 6 sites. Virus development occurred in the nuclei and cytoplasm of the fat body. Based on ultrastructural studies, the NPV observed in Connecticut appears to be similar to a strain described earlier for armyworms in California and Hawaii.

INTRODUCTION

The armyworm, Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidac) inhabits natural and managed grass-dominated ecosystems in the northeastern United States. Local populations in Connecticut probably do not overwinter there, and thus, annual populations can be spotty depending on the dispersal of moths from the southern areas of the country. Populations are usually low in most years, but when numbers of caterpillars rise to high densities, there can be extensive damage to corn, barley, oats, and grasses (e.g., hayfields and lawns). Crop losses can occur over broad geographic areas (Sheppard and Wcinzierl 2002).

Outbreaks of P. unipuncta have been recorded periodically in eastern United States and Canada (Chapman and Glaser 1915, Guppy 1961). High populations of armyworms have been noted to collapse following epizootics of nuclear polyhedrosis virus (NPV) or granulosis virus as separate or combined infections (Chapman and Glaser 1915; Tanada 1959a, b). Epizootics frequently occur in outbreak populations of caterpillars and play important roles in the decline of pest populations. During the summer of 2001, P. unipuncta infestations were reported at scattered locations throughout New England (New England Agricultural Statistics Service, 2001). High mortality of caterpillars was observed in different habitats in Connecticut, and subsequent microscopic examinations of moribund or recently expired larvae revealed infection with a nuclear polyhedrosis virus (NPV). In this report, we document a natural epizootic of NPV infection in larval populations of P. unipuncta in Connecticut, provide ultrastructural characterization of the virus, and note differences in viral infections reported earlier in western United States.

METHODS

Caterpillars, predominantly fourth and fifth instars, were collected from vegetation in North Canaan and New Milford (Litchfield County), Eastford (Windham County), and 3 sites in Hamden (New Haven County), CT, during the period july 5 through july 16, 2001. Collection sites, which were all severely damaged by caterpillar feedings, included hayfields in North Canaan, Eastford, and New Milford and lawns at 3 suburban locations in Hamden. Healthy and moribund caterpillars, resting on blades of grass or other vegetation, were placed into separate plastic cups (volume = 30 ml) and transported to the laboratory in styrofoam containers.

All 344 caterpillars were kept separately for observation in sealed plastic cups with fresh blades of grass. The cups were placed into plastic containers with lids and held for a maximum of 18 days at room temperatures between 27 and 31 0C. Larvae were checked for changes in color or behavior every 2 to 3 days. Live larvae that were near death in a listless state and unresponsive to touch - were considered moribund and were microscopically examined for viral infection along with caterpillars that had expired within 6 hours. Wet mount preparations of selected tissues and hemolymph samples obtained from caterpillars were examined under phase contrast at � 1000 for viral occlusions (polyhedra). Measurements of live viral occlusions (n = 100) were made with a Zeiss Axioplan 2 Imaging Microscope.

For the ultrastructural studies, infected tissues (1 mm^sup 3^) were dis sected from diseased larvae and fixed overnight at 4 �C in a 2.5% (v/v) glutaraldehyde solution containing 0.1% (w/v) CaCl^sub 2^ and 1% (w/v) sucrose in 100 mM Na cacodylate (pH 7.3). Specimens were post-fixed in 1 % (w/v) OsO^sub 4^, dehydrated through an ascending ethanol and acetone series and embedded in a LX-112/Araldite (Ladd Research Industries) mixture. Thin sections were post-stained with 5% (w/v) uranyl acetate in 50% (v/v) methanol followed by Reynold's lead citrate and examined in a Zeiss EM 1OC electron microscope at an accelerating voltage of 80 kV.

RESULTS

Caterpillars infected with NPV were collected from 5 of 6 sampling sites (Table 1). Of the 90 P. unipuncta larvae examined, 59 (66%) were infected with NPV. In those instances when tissues from at least 11 caterpillars were examined, infection ranged from 17% to 100%. Latestage caterpillars infected with NPV were creamy white to tan in color, in contrast to their natural olive green, and were noticeably lethargic. Moribund caterpillars became flaccid as the viral infections advanced. The integument was easily ruptured, liberating large numbers of viral occlusion bodies in hemolymph samples. Infected caterpillars darkened rapidly after death.