BY: Bo Simons
What is biological control?
Biological control is the use of natural enemies to reduce the
damage caused by a pest population. Biological control practitioners
note that their method differs from natural control.
Natural control occurs much of the time without human intervention
as natural enemies keep populations of potential pests in check.
Biological control involves active human intervention, introducing
new or increasing populations of a pests enemies. These
may include parasites, predators, and pathogens.
Biological control is often just one part of a total integrated
pest management (IPM) solution to a pest situation. Additional
control methods deployed in conjunction with biological control
include chemical applications, usually in lower amounts and concentrations
than without biological controls, cultural practices (soil management,
pruning), hygienic practices, strategic timing of controls, and
crop selection strategies.
Biological control success stories
Biological control is used with practically all citrus grown in
Southern California. The industry was threatened back in the 1800s
by cottony cushiony scale (Icerya purchasi), an insect
native to Australia. In response, growers introduced a predatory
insect, the vedalia beetle (Rodolia cardinalis), and the
parasitic fly (Cryptochaetum iceryae), both from Australia.
Within a few years the cottony cushiony scale was completely controlled
by these natural enemies. Today, the citrus industry controls
another scale insect, red scale (Aonidiella aurantii),
with assistance of parasitic wasps (Aphytis melinus and
Aphytis lingnanensis) that lay their eggs on the insect.
Besides citrus, other biological control success stories include
alfalfa and walnuts. The alfalfa weevil (Hypera postica)
posed a serious threat to the forage industry in the northern
U.S. until it was substantially reduced by the introduction of
several natural enemies, including a parasitic wasp (Bathyplectes
curculionis). Control of walnut aphid (Chromaphis juglandicola)
involves another parasatic wasp (Trioxys pallidus) that
lays its eggs within the walnut aphid.
Is there hope for biological
control in managing GWSS?
Not if your goal is to eradicate GWSS, replies Dr.
David Morgan, the entomologist in charge of the California Department
of Food & Agricultures GWSS biological control effort
in the six southernmost counties of California. Biological
control works best by reducing populations to lower levels, rarely
by wiping them out completely. If the goal is to reduce populations,
yes it is working already.
Preliminary research undertaken by University of California, Riverside,
indicates that egg parasitoids (insects that kill eggs by reproducing
within them) can cause over 90% egg mortality from June through
the remainder of the growing season.
We hope that by introducing natural enemies early on in
the season, notes Morgan, we can achieve control earlier.
Once this is accomplished, we hope to reduce our reliance on more
aggressive control methods. In many ecosystems where insecticide
use is limited (organic farms, urban, and wild land areas), biological
control remains the most important control strategy available.
Are there existing parasitoids
for GWSS in California?
Native parasitoids of GWSS already exist in California,
answers Morgan, such as the closely related smoke-tree sharpshooter
(Homalodisca lacerta). The most common parasitoid is Gonatocerus
ashmeadi, followed by Gonatocerus morrilli in coastal
areas. At least three other species are found at low levels.
Gonatocerus parasitoids are tiny stinger-less wasps. They
lay their eggs within the eggs of sharpshooters. The developing
Gonatocerus consume the contents of the sharpshooter eggs.
What emerges from these parasitized eggs are adult Gonatocerus
not more sharpshooters.
What are their impacts on
In most areas where it is established, GWSS produces two generations
each year, in spring and summer. The eggs of the summer generation
are largely killed by the parsitoids. The critical factor in combating
GWSS with biological controls involves getting a parasitoid that
can dramatically reduce the spring generation.
Gonatocerus can successfully kill over 90% of eggs
laid by GWSS, explains Morgan, but only later on in
Is part of the problem with
of the native wasps that they do
not work very well with the early
or spring generation?
Thats right, replies Morgan. This is true
of most biological control systems. There is frequently a lag
between population change of the pest and the natural enemy. If
you have a very good natural enemy, then that lag is closed pretty
In many cases, by the time the natural enemy has built up
its population, so has the pest, to such huge amounts that the
enemy is never going to catch up with the pest. Even though we
get a high level of control later on in the year, by the time
that occurs, there are so many sharpshooters out there, even if
youre killing 90% in the eggs with the wasps, that 10% youre
not killing still represents a huge number of insects.
What is the status of explorations
for other GWSS parasitoids?
Morgan describes the large coordinated effort: Collaborators
from university, state, and federal agencies from U.S. and have
been searching and continue to search for natural enemies in the
southeastern U.S. and Mexico where the GWSS is from.
Populations of GWSS in its native area are less than 1/1000th
of those found here, and we are keen to find the agents that suppress
it. When in the area, we search for eggs of the sharpshooter or
closely related insects. These are catalogued and returned to
quarantine at University of California, Riverside. There, eggs
are observed daily for emergence of egg parasitoids.
Any emerging insects are offered fresh GWSS eggs to see
whether they accept them as hosts. If they do, we rear them through
at least three generations to ensure they are not carrying any
During this time, we are able to accurately identify them
by sending specimens to taxonomists that specialize in the group
of insects involved. We also screen the insect against the eggs
of a number of closely related insects found in California to
ensure that they have a sufficiently narrow host range; we do
not want a new parasitoid to adversely affect our native fauna.
Once these studies are complete, we apply to federal and
state agencies for a permit to release. If this is obtained, the
insect is taken out of quarantine and reared at one of the states
two mass-rearing facilities dedicated to control of GWSS. The
insects are then released at strategic locations throughout the
The two mass-rearing facilities are Mount Rubidoux Field Station
in Riverside County, headed by Morgan, and the Oswell Street Biological
Control Facility in Kern County, headed by Dr. Greg Simmons, USDA-APHIS.
The mass-rearing facilities ultimate goal is to produce
as many high quality natural enemies as possible, continues
Morgan. To do this, we capture adult GWSS from the field
or rear them in cages and collect their eggs just like
collecting eggs in a hen house.
How do the production facilities operate? Describe the production
In order to produce parasitoids we need to raise lots of
plants for the insects to feed on and lay their eggs in. The eggs
are then offered to the wasps that parasitize them. Less than
two weeks later, the offspring emerge. These wasps are collected
for release in GWSS-infested areas in Southern California.
We also use the wasps and sharpshooters for research purposes,
for instance, for testing newer and less aggressive insecticides,
and in trying to develop more economical rearing techniques.
In certain areas you cannot pursue aggressive chemical control,
explains Morgan. We release into organic, urban, and native
areas where chemical control is limited and GWSS populations can
be high. In this way, we hope to develop a viable control method
for use in areas that may otherwise be considered as GWSS refuges.
What are the distribution plans
for the wasps?
We contact all sorts of people, explains Morgan. Farmers,
PCAs (Pest Control Advisors), and government representatives at
the county and state levels who know about the area. We ask for
advice about areas where we could release wasps where they are
not going to be sprayed and killed immediately. Ultimately we
end up targeting organic areas, public areas, and nature reserves.
We try to locate close to threatened crops. Those are primarily
nursery products or grapes. Most releases are made into citrus
because this is the primary host plant of the pest year-round.
Is it a measure of success that populations be self-sustaining?
We hope they would be, says Morgan. It would
be nice if it were from year to year, but at least for one year.
Frequently, biological control producers augment populations that
are already present. That is a very viable alternative. Its
been done for over 50 years in California with great success.
Basically they just add extra numbers at the beginning of the
year, so the wasp has a head start on these pests.
Is there a goal in numbers?
How many thousand Gonatocerus
of a specific species are needed to manage the GWSS in Temecula?
A lot of people like doing that, notes Morgan. They
like counting. That is not the sole priority. Obviously the more
the merrier, but theres also the aspect of quality, for
instance how long the insect lives, how many eggs it lays, and
how well it can find its host. Often by increasing production
you can actually reduce the quality. We also aim to keep the quality
Its basically a case of do you want 10 wasps that
look for insects for one day or do you want one wasp that looks
for insects for 10 days? By dealing with different temperatures
and different parameters you may be able to alter the setup so
that you get different qualities, as opposed to different numbers.
Thats one angle.
The other angle is the right insect. Its not just
numbers, its the right insect. We could be producing billions
of insects, but if they dont do what were expecting
them to do, then we might as well be producing none.
Is there a percentage in the
spring generation you are hoping
No, we dont have a target, responds Morgan.
We are hoping to optimize, to get as many as possible. I
havent looked at data from 2002 yet. In the past, it started
off at 15% to 20% parasitism at the beginning of the year. Then
it builds up so that by mid-year, its in excess of 90%.
We are obviously hoping to increase that 15% to 20% upwards.
Are there other biological control options that have been considered
for GWSS besides parasitoid wasps?
There are other control methods that occur in the southeast
U.S. that weve been unable to make use of, says Morgan.
We noticed bacterial and fungal pathogens, especially fungi.
Now, those dont work very well in the desert. Those agents
really get cooked with ultraviolet light, low humidity, and high
temperatures. So we cannot use that methodology.
Another method is using predators that feed directly on
GWSS adults, nymphs, or eggs. There are some good predators in
the southeast, but the trouble is they are not very specific.
They eat a wide range of insect species. Its very difficult
to do quality tests and proper screening and be assured that a
particular predator is specific to the sharpshooter. On the whole,
these predators arent specific. They eat what comes their
We cannot really introduce predators from another part of
the world or another part of the country without potentially disrupting
the ecology of California. However, we are looking into the predators
that already exist in California.
How will success be measured?
Morgan differentiates between ultimate and short-term success
criteria: Success is ultimately measured as an area-wide
reduction in GWSS populations, elucidated by intensive monitoring
by the CDFA using yellow sticky traps. In the short-term, we measure
success by recovering offspring of wasps originally released.
This shows that our wasps have been able to successfully search
for, parasitize, develop, and reproduce on GWSS in California.
Every wasp of this sort that we capture not only represents one
dead GWSS, but also reflects the many insects we do not capture
that are continuing to control the pest.
What will be the role of biological
control agents in areas where exclusion and eradication are
the goals, such as the North Coast
of California where GWSS has yet to establish?
The first line of defense in a new area is eradication,
responds Morgan. In these areas, parasitoids have a limited
role. Their importance lies in areas where widespread and stable
GWSS populations already exist, where eradication is not feasible.