Introduction of NPV for Suppression of Anticarsia gemmatalis in Coastal legumes Prior to Emigration to Soybean
J. R. Fuxa, F. Peng, A. R. Richter, and S. J. Johnson
Department of Entomology
Louisiana Agricultural Experiment Station
Louisiana State University Agricultural Center
Baton Rouge, Louisiana 70803

    The overall goal of the project was to introduce a nucleopolyhedrovirus (NPV) into Louisiana coastal legumes for long-term suppression of early-season velvetbean caterpillar (Anticarsia gemmatalis) populations, thereby reducing infestations of the pest in soybean. A. gemmatalis cannot overwinter in Louisiana but immigrates with each new growing season. The A. gemmatalis NPV (AgNPV) is not indigenous to the United States but is a naturally occurring biotic control agent in Brazil and other countries. Previous research had indicated that the AgNPV could control A. gemmatalis populations for 3-4 years after only one spraying in soybean. Thus, there was every reason to expect that the virus could permanently reduce populations of the pest in wild, coastal legumes.
    Persistence of AgNPV was significantly lower in marshland soil than in agricultural soil. AgNPV was inactivated significantly faster in soil at -0.3 bar water potential (normal moisture) than at 0 bar (very wet soil) or -5 bar (very dry). AgNPV lost activity more quickly in non- sterilized soil than in soil that had been autoclaved. AgNPV in non-autoclaved marsh water lost almost all of its activity within 50 d, whereas AgNPV in distilled water or autoclaved marsh water was almost 100% active at 50 d. Thus, these soil and water experiments indicated that some heat-sensitive agent, quite possibly a microorganism, was destroying the viability of AgNPV. Over 80% of AgNPV activity was lost within 7 d when sprayed on upper leaf surfaces of soybean Glycine max or the wild legume Vigna luteola. AgNPV sprayed on the underside of the foliage did not decline to 20% original activity until 14 d or more. Thus, AgNPV persisted as well on the wild legume as on soybean. Also, AgNPV and similar viruses will persist longer and probably control insect pests more effectively if sprayed on the underside of leaves.
    Median lethal doses (LD50s) of AgNPV in 3rd-instar Ag were 22.3, 3 5.2, 14.6, and 4 1.0 occlusion bodies/insect when reared on G. max, Pueraria lobata, Rhynchosia minima, or V luteola, respectively. Based on lack of overlap of 95% confidence limits, insects reared on V luteola were less susceptible to AgNPV than those reared on G. max or R. minima. Mean larval development time of Ag reared on P. lobata was significantly longer than for insects reared on the other three plants. Pupal weights of Ag reared on G. max and V luteola were significantly greater than those of insects reared on R. minima or P. lobata. Larval development times and pupal weights of survivors of AgNPV treatment were not significantly different from those of control insects reared on the four plants. The percentages of larvae pupating for Ag that survived exposure to AgNPV were similar to those for controls. However, insects surviving AgNPV exposure had a higher percentage of deformed pupae than control insects. The LD50 results were not encouraging for biological control by AgNPV in wild legumes, because the virus was less virulent to insects feeding on V luteola than on the other plants, and V luteold is the most important wild host-plant of A. gemmatalis in Louisiana.
    The overall conclusion of the research was that AgNPV probably would not be successful for classical biological control of A. gemmatalis in wild legumes, because it did not persist very well in marsh water or soil (the major reservoir for NPVs) and because it had reduced virulence in the insects feeding on the wild legume most important to the insect's population dynamics in Louisiana.

Publications

Peng, F. 1996. Environmental Factors Affecting Anticarsia gemmatalis Nuclear Polyhedrosis Virus. M.S. Thesis, Department of Entomology, Louisiana State University, Baton Rouge, 99 pp.

Peng, F., J. R. Fuxa, S. J. Johnson, and A. R. Richter. Susceptibility of Anticarsia gemmatalis, reared on four host plants, to a nuclear polyhedrosis virus. Environ. Entomol. 26: 973- 977.

Peng, F., J. R. Fuxa, S. J. Johnson, and A.R. Richter. Effects of soil type, moisture, host plant, and application method on persistence of Anticarsia gemmatalis nucleopolyhedrovirus. In preparation for Environ. Entomol.