Vertical transmission of two baculoviruses infectious to the tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae) using an autodissemination technique. (1990)

Transovum transmission efficiencies of the multiply-embedded form of Au tographa californica nucleopolyhedrosis virus (AcMNPV) and the singly-embedded form of Heliothis zea nucleopolyhedrosis virus (HzSNPV) were comparatively evaluated against the tobacco budworm, Heliothis virescens (F.) using an autodissemination technique. The corrected mean viral-induced mortality for progeny arising from mating pairs in which only the male was surface contaminated with virus was 68.8% for AcMNPV and 53.0% for HzSNPV. AcMNPV, however, induced an LT50 response approximately one day sooner than HzSNPV. Scanning electron microscopy (SEM) revealed that polyhedra were abundant on the chorion of eggs laid by females indirectly contaminated with virus. Although much variation occurred, all the eggs observed had some degree of viral contamination. Polyhedra were concentrated on the upper hemisphere of the eggs and along the ridge-like features of the chorion; particularly at the rosette where they would most likely be ingested during eclosi?n. SEM also revealed abundant polyhedra on the wing and body scales of adults. The size and shape of polyhedra conformed to the linear depressions of the scales. Naturally occurring nucleopolyhedrosis viruses (NPV), Baculovirus, Subgroup A (Wildy, 1971), have long been recognized as important factors in the eventual collapse of lepidopterous and hymenopterous pest populations (Burges, 1981; Granados and Federici, 1986; Smith, 1976). Consequently, these viruses have received much attention as microbial control agents (Allen et al., 1978; Burges, 1981; Granados and Federici, 1986; Smith, 1976). Most attempts to utilize these viruses in microbial control have deployed them as microbial insecticides although alternative deployment strategies, such as autodissemination, have been proposed (Allen et al., 1978). Autodissemination, as defined by Ignoffo (1978), is the pur poseful use of insects to spread an entomopathogen in the environment. This includes 1) the use of a stage or stages of the usual host that are refractive to the entomopathogen or 2) the use of unusual or non-host insect species to disperse the entomopathogens for subsequent encounter with susceptible hosts. As a first step toward developing a sex attractant-mediated form of autodis semination to facilitate dispersal of viruses infectious to the tobacco budworm, Heliothis virescens (TBW), we comparatively evaluated the transovum transmis 1 Scientific paper no. 89-7-166 of the Kentucky Agricultural Experiment Station, Lexington. This research has been conducted (in part) with funds from the USDA, ARS Cooperative Research Agree ment No. 58-43YK-8-0025. The contents do not necessarily reflect the views and policies of USDA, ARS. 2 Professor and Associate Professor, respectively, Department of Entomology, University of Ken tucky, Lexington, Kentucky 40546. 3 Research Entomologist, USDA, ARS, Crops Research Laboratory, Oxford, North Carolina 27565. Accepted for publication 23 October 1989. This content downloaded from 157.55.39.45 on Tue, 19 Jul 2016 05:40:08 UTC All use subject to http://about.jstor.org/terms 394 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY sion efficiencies ofAutographa calif or nica multiply-embedded nucleopolyhedrosis virus (AcMNPV) and the Heliothis zea singly-embedded nucleopolyhedrosis virus (H3SNPV) derived from Elcar?. The objective of this study was to investigate opportunities for initiating early instar Baculovirus-induced mortality with viral dust formulations under controlled laboratory conditions with the longterm ob jective of utilizing the mating and oviposition behavior of the TBW to initiate early mortality among F, progeny in the field. The following information on transmission of these baculoviruses to progeny resulting from surface-contami nated adults represents some of our early laboratory research toward evaluating this strategy. Materials and Methods We initiated pair mating studies in the laboratory to determine transovum transmission efficiency under controlled conditions. Inocula used were a plaque purified AcMNPV isolate obtained as a gift from Dr. Edward M. Dougherty (USDA, ARS, Beltsville, MD) and HzSNPV derived from Elcar? sample obtained from Sandoz, Inc., San Diego, CA. Both viruses were increased by passage through a disease-free colony of tobacco budworm (TBW) maintained in the Insect Pa thology Laboratory, Department of Entomology, University of Kentucky. Newly molted fourth-instar TBW were individually inoculated per os by dispensing 2 ?xl of a suspension containing 7.0 x 103 PIB's of either virus onto the surface of 5mm diet discs in 24-well tissue culture dishes (Boucias and Nordin, 1977). Harvested virus was purified by differential centrifugation, washed in acetone, air-dried, then ground into a dust using a mortar and pestle. Polyhedra were then mixed 1:1 wt/wt with Day Glo? A-14-N Fire Orange fluorescent dye (2.8 x 1011 PIBVgram) to allow visualization of moth coverage. An adult male of the pair was contaminated with viral dust by confining it in a one pint Fonda? cardboard carton containing one gram of powered viral polyhedra and fluorescent dye marker and shaken for 30 sec. After contamination, individual males were transferred to a separate carton so that excess dust could fall off. The control treatment involved dusting males with only the Day Glo marker dye. Contaminated males were then paired with uncontaminated female partners in new cartons covered with Saran? film. The pairs were allowed to mate for two days, and only the females were transferred to fresh oviposition cartons. A total of 48 eggs were taken from each female with a 24-egg sample taken one day after transfer and another 24-egg sample taken 3 days later. Eggs were individually transferred and subsequently held in 24-well tissue culture plates partially filled with a wheat germ agar medium (Yearian et al., 1966). Eclosi?n and viral-induced mortality were monitored daily. Progeny from five replicates of 10 pairs were sampled from treatments where the male partner was initially treated with either virus. The control treatment involved dusting males with only the Day Glo marker dye. All test subjects in treatments were held at 23?C in darkness except for periodic assessment of treatment effects. Mortality over time was corrected for control mortality (Abbott, 1925) and analyzed using the SAS probit analysis procedure (SAS Institute, 1985). In order to visualize the extent of viral contamination of the chorion of indi vidual eggs laid by contaminated females, we set up additional eggs for scanning electron microscope (SEM) examination. Eggs oviposited by viral-contaminated This content downloaded from 157.55.39.45 on Tue, 19 Jul 2016 05:40:08 UTC All use subject to http://about.jstor.org/terms