Giving Baculoviruses a Better Edge
Cell lines from budworms, Heliothis virescens,
are being used to grow highly infective
baculoviruses like the one that killed
the smaller Caterpillar.
|Scientific advancements built on an
understanding of what nature already provides are leading to environmentally
friendly crop-pest control by either biological agents or specifically designed
synthetic anti-insect compounds. A case in point: research on baculoviruses.
Baculoviruses are rod-shaped DNA viruses, many of which begin their life cycle
reproducing inside cells. In the nuclei of caterpillar cells infected with
baculoviruses, viral progeny multiply and are incorporated into protective
polyhedron-shaped protein structures called occlusion bodies. Infected
caterpillars die and contaminate the leaf surfaces with the occlusion bodies.
Then, healthy caterpillars ingest the occlusion bodies and release the virus
when feeding on contaminated leaves, thus continuing the life cycle of
infection and replication.
Healthy cells from cabbage looper.
|Each year, natural baculovirus
epidemics nearly wipe out populations of some caterpillar pests, such as corn
earworm, cotton bollworm, tobacco budworm, and cabbage's nemesisthe
diamondback moth, says ARS
microbiologist Arthur H. McIntosh, at the Biological Control of Insects
Research Laboratory (BCIRL), Columbia, Missouri. If that assertion doesn't have
a familiar ring to most growers of major U.S. crops, it's probably because the
epidemics occur after crop damage has already been done.
Might routine sprays of baculovirus while caterpillars are still young be an
ideal way to avert extensive crop damage?
Not yet, because of the economics of present biopesticide-versus-chemical
control technology in most U.S. agriculture, says McIntosh. But in some
countries, like Thailand, subsistence farmers spray suspensions of last year's
diseased pest-insect larvae on crops, giving the biological control agent a
jump-start. And in Brazil, velvetbean caterpillars are reared to produce the
baculovirus known as AgMNPV, which is marketed to control the caterpillars on 2
million acres of soybean fields.
Infected cells from cabbage looper.
The infected cells have occlusion
bodies of the baculovirus AcMNPV
With an eye to worldwide marketing, several companies are developing technology
to mass-produce baculoviruses in cultures of insect cells rather than in whole
insects. Such technology is expected to save on labor costs and to keep
bioinsecticides free of microbial contaminants.
Besides serving as growth media for baculoviruses, insect cell
culturesparticularly those from embryonic and nerve tissuemay
someday be used to screen natural or synthetic chemical compounds for their
potential as environmentally friendly anti-insect compounds. For example, the
cultures might help to determine whether an insect growth regulator effectively
disrupts development of pest insects without harming other insects.
Microbiologist Art McIntosh examines cells
from cabbage looper, Trichoplusia ni, infected
with a strain of baculovirus that is highly
effective at killing caterpillar pests.
| Cell cultures may also help
scientists learn why certain chemicals paralyze or kill a particular pest
insect or cause it to stop eating. One neuronal cell line that shows promise
for helping find answers to such basic and applied research questions was
developed by ARS entomologist Cynthia L. Goodman, graduate student Jennifer
Wittmeyer, and McIntosh in a cooperative research and development agreement
between ARS and Aventis Crop Science (formerly Rhone-Poulenc).
In hundreds of biomedical laboratories around the world, scientists are
bioengineering baculoviruses grown in cell cultures to produce proteins that
may be used to diagnose or prevent disease.
Someday, pending sufficient research on environmental safety, proteins from
genetically modified baculovirusesor even the viruses themselvesmay
be produced as bioinsecticides in bioreactors, which are vats for growing large
quantities of virus-infected insect cells.
Recently, entomologist James J. Grasela and McIntosh developed recombinant
baculoviruses containing colored fluorescent protein markers. These markers may
help researchers determine more quickly if the recombinants possess desired
traits, such as the ability to kill pests with low levels of virus.
For baculovirus production, the researchers have established eight insect cell
lines from embryos and ovaries from members of the Helicoverpa and
Heliothis species (for example, cotton bollworm and corn earworm) that
damage cotton, corn, tobacco, tomato, and many more crops. Given proper
nutrients, one of the cell lines produced 10 times more of a baculovirus known
as AcMNPV than did the other lines. This virus infects a wide range of
caterpillar pests, but it is named after the host it was isolated from,
Autographa californica (alfalfa looper).
In experiments with a baculovirus called PxMNPVCL3, which McIntosh recently
discovered in samples from infected diamondback moth caterpillars (Plutella
xylostella), the caterpillars' infection levels were up to 2,000 times
greater than rates of infection in caterpillars exposed to either AcMNPV or to
another baculovirus discovered by BCIRL scientists.
Besides being a nemesis of cabbage, diamondback moth is also a major worldwide
pest of other cole crops. Diamondback moth has developed resistance to
several intensively used chemical insecticides and to toxins produced by the
bacterium Bacillus thuringiensis, or Bt. So the newly isolated
baculovirus stands out as a potential control agent of this pest, says
Despite having environmentally friendly advantages, use of baculoviruses
against pests may entail some dilemmas, which McIntosh, Goodman, Grasela, and
ARS entomologist Carlo M. Ignoffo (retired) are trying to solve. The viruses'
strong points, when contrasted with chemical insecticides, can also be their
greatest weaknesses. For example, baculoviruses pose no direct threat to
beneficial insects, but most of these viruses work only against a narrow
spectrum of pest insects. And baculoviruses don't kill pest insects as rapidly
as chemical insecticides.
Add another wrinkle: Baculoviruses don't persist well in the environment,
mainly because they are inactivated by ultraviolet-B (UV-B) rays of sunlight,
which probably cause DNA damage. If they could be made a little more
persistent, they might become much more practical alternatives to conventional
chemical insecticides. Further research on the UV-B phenomenon may provide
insights into developing better bioinsecticides through genetic engineering or
even through improved production and handling techniques.
In cabbage looper and corn earworm cells, the scientists found that genes like
those known to be involved in repair of DNA damage in noninsect organisms may
be operative, say McIntosh and Grasela. Cabbage looper cells were less
sensitive to UV-B and produced more baculovirus on infection than did
virus-infected corn earworm cells. Now the scientists are trying to identify
the genes that help protect the cells and perhaps the virus.
Each new discovery may help lead to more economical control of the
bollworm/budworm pests that attack more than 30 food and fiber crops worldwide.
In the United States alone, the damage these pests inflict and the costs for
controlling them add up to more than $1.25 billion each year.By Ben
Hardin, formerly with ARS.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Arthur H. McIntosh, Carlo M.
Ignoffo, Cynthia L. Goodman, and
James J. Grasela are with the
USDA-ARS Biological Control of
Insects Research Laboratory, 1503 South Providence Rd., Research Park,
Columbia, MO 65203; phone (573) 875-5361, fax (573) 875-4261.
"Giving Baculoviruses a Better Edge" was
published in the January 2002
issue of Agricultural Research magazine.