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.