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 pest’s 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 & Agriculture’s 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
GWSS populations?
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 the year.”

Is part of the problem with some
of the native wasps that they do
not work very well with the early
or spring generation?
“That’s 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 quickly.

“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 you’re killing 90% in the eggs with the wasps, that 10% you’re 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 diseases.

“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 state’s two mass-rearing facilities dedicated to control of GWSS. The insects are then released at strategic locations throughout the infested area.”
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 facilities:
“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. It’s 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 there’s 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 up.

“It’s 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. That’s one angle.

“The other angle is the right insect. It’s not just numbers, it’s the right insect. We could be producing billions of insects, but if they don’t do what we’re expecting them to do, then we might as well be producing none.”

Is there a percentage in the
spring generation you are hoping
to achieve?
“No, we don’t have a target,” responds Morgan. “We are hoping to optimize, to get as many as possible. I haven’t 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, it’s 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 we’ve been unable to make use of,” says Morgan. “We noticed bacterial and fungal pathogens, especially fungi. Now, those don’t 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. It’s 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 aren’t specific. They eat what comes their way.

“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.”