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

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

Viral occlusion bodies were seen as irregularly shaped polyhedra in light microscopy (Fig. 1). They ranged in size from 1.29 to 2.62 �m in diameter with a mean of 1.89 �m � 0.03 SE (n = 100). Virus development was detected within the nuclei and cytoplasm of the fat body (Fig. 2) or in nuclei. Numerous virions, typical of a NPV, were embedded in a crystalline protein lattice within the occlusion body (Fig. 3). The nucleocapsids were rod-shaped and bundled in multiples with up to ten nucleocapsids per envelope (Figs. 4, 5). Nucleocapsids ranged from 260 nm to 290 nm in length and averaged 30 nm in diameter in ultrathin section. The entire virion was 320 nm to 370 nm in length (avg. 340 nm).

DISCUSSION

Outbreaks of P. unipuncta occur sporadically in states and provinces in eastern North America (Britton 1938, Guppy 1961, Chapman and Glaser 1915). The last major outbreak of P. unipuncta in Connecticut happened in 1937 (Britton 1938). There was widespread damage to crops. Although the sudden rises in armyworm populations are unpredictable and may be due to adult moths being blown into the northeast from southern states, viral infections occur in widely separated areas when these insect populations reach high densities during the summer. For example, polyhedra were found in armyworms from Nantasket, MA; Hagerstown, MD; and Norfolk, VA during August (Chapman and Glaser 1915).

The NPV described herein from local populations of P. unipuncta in Connecticut appears to be very similar to the "typical strain" of NPV described by Tanada (1959a) from army worm populations in California and Hawaii and is likely to be the same virus reported by Chapman and Glaser (1915). The average size ranges of the viral occlusion bodies reported by these authors (0.86 �m-2.69 �m, avg. 1.5 �m � 0.4 SE; Tanada 1959a), and 1.88 �m-2.3 �m (Chapman and Glaser 1915) were very similar to those obtained from our isolate. Furthermore, the lengths of the "naked virus rods" reported by Tanada (1959a) (333.9 nm-372 nm, avg. 347 nm) were virtually the same as those calculated for virions in the present study (320 nm-370 nm, avg. 340 nm). Electron micrographs of this virus published in a later study by Tanada et al. (1969, Fig. 5) also reveal multiple enveloped nucleocapsids within the occlusion bodies with similar ultrastructural morphology to that reported here. Comparative genetic analysis of the viral DNA from each isolate would be required to definitively ascertain the conspecificity of these viruses. However, there was no evidence in the present study of the atypical hypertropy strain of NPV, which infects the tracheae and hypodermis (Tanada et al. 1969, 1982) or of the synergistic granulosis virus reported by Tanada (1959b).

Although localized damage to crops and lawns was caused by P. unipuncta caterpillars in Connecticut during 2001, NPV caused mortality of caterpillars at 5 sites. Amounts of virus inoculum present in the environment and population densities of caterpillars were unknown. Sixth-instar larvae are highly resistant to NPV infection (Tanada 1959a), but all other larval stages can be infected and usually die in 13 days or less. Armyworm outbreaks and population collapses occur rapidly. Therefore, we were unable to determine when the caterpillars we examined were initially infected with NPV or if infection rates for first, second, or third instars were comparable to our results for caterpillars in later stages of development.