Mechanized Delivery Methods for Field Release of Beneficial Insects

These days natural enemies are readily available in large quantities from numerous suppliers for control of lots of different pests. For the backyard gardener or small greenhouse owner, it's easy to shake the beneficials out from the little containers they arrived in, or hang small cards containing wasps in host eggs onto individual plants. But getting buckets of green lacewing eggs or parasitic wasps evenly distributed over your 160 acres of corn or a 20 acre cabbage field is another story.

Certainly beneficials can be applied by hand over larger areas, but this is time-consuming and expensive. For those few crops that require intensive hand labor anyway (such as greenhouse tomatoes), workers can distribute beneficials throughout the crop during the normal course of their activities without a significant increase in operational costs. But this is just not practical for most crops.

Many beneficial organisms have always been applied mechanically--bacteria, fungi, viruses and nematodes are "released" into the field by spraying with conventional pesticide application equipment. Utilization of these beneficials has been facilitated by reliable spray technology that is familiar and available to operators. Larger organisms, including most predatory and parasitic arthropods, cannot be delivered as easily through conventional equipment. Development of efficient mechanical systems for delivery of natural enemies could encourage greater acceptance and utilization of these natural enemies.

Adoption of biological control is often impeded by lack of quality control, and problems of storage, shipment, and application. These hindrances to widespread use reduce demand, which reduces the economy of scale for producers and maintains higher cost. A 1994 study estimated that improving field application techniques for Trichogramma could reduce treatment costs by 96% and subsequently increase demand and lower production costs by a factor of 24. In addition to reduced cost of both the product and labor, other advantages of mechanical delivery of biological control organisms are the potential for improved timing of releases, greater uniformity of deposition, enhanced survivorship and ease of application.

Much of the initial work with mechanical delivery systems was done with green lacewing eggs or Trichogramma wasps inside host eggs. Eggs are inexpensive, making releases of large numbers practical. They are more readily handled and shipped than other stages because they are not mobile, large quantities require little space, and they can also be delivered in dry or liquid media.

Mechanical devices were first developed to apply beneficials, particularly lacewing eggs, in a dry solid carrier such as bran, rice hulls, corn grit, or vermiculite. Dry carriers provide bulk to ease handling and provide cushioning, but there are disadvantages: plugging or cavitation of mechanical devices during distribution, potential for mechanical damage to the eggs, difficulties with calibration, moisture affecting the flowability of the carrier material. The concentration of the organisms in the carrier must be kept constant to achieve uniform distribution in the field. With most mechanical application devices, this requires agitation--which can be difficult to achieve and may damage the organisms. The machines spread both the carrier and natural enemy over the crop, but do not necessarily deliver consistent, uniform rates. Problems with cannibalism result when lacewing eggs are deposited in clusters. However, physically attaching the eggs to the carrier or good design of the distribution device can overcome these potential problems. In the late 1970's, a method for attaching Trichogramma-parasitized caterpillar eggs to wheat bran flakes was developed by Texas scientists, using a mixture of mucilage and water. A modified CycloneŽ seeder attached to a backpack-mounted, power-driven fan placed the Trichogramma in a favorable location in the plant canopy for parasitization of pest eggs. One prototype mechanical distributor of a lacewing egg/vermiculite mixture tested in California consistently delivered a uniform concentration of eggs with no detrimental impact on hatching rate. As a rotating metering plate moved below a cylindrical reservoir, the mixture filled cells in the plate. When each cell passed an opening on the bottom of the stationary plate, a brief burst of compressed air cleared out each cell as the contents fell downward. Release rates could be changed by varying the rotational speed of the metering plate and the size of the cells on the plate. A similar mechanical system was developed for distribution of predaceous mites in strawberries. The initial design was unacceptable due to the severe injury to the mites it produced--highlighting the need for biological evaluation of handling effects on individual organisms. Because the highly mobile mites were easily damaged by agitation of the commercial vermiculite-mite mixture, the mixture was placed in an insulated storage reservoir that kept the mixture stationary and chilled. Uniformity was better than with manual release and application was achieved more quickly and with less labor. (A single distributor was twice as productive as human workers on a per acre basis; machine productivity could be increased by installing more units on the vehicle.)

But one of the biggest problems with applications using a dry carrier is that few of the biocontrol agents remain on the foliage, instead ending up on the ground. This may not be of much consequence when the release is a mobile stage on a low crop--such as predatory mites on strawberries. But when eggs are released or the crop is taller--such as fruit trees--this may greatly reduce the efficacy of that natural enemy release.

Liquid carriers can alleviate some of these problems. Adhesives added to the liquid eliminate losses due to eggs falling to the ground since the eggs are attached to the leaf surface. Damage to the eggs during delivery is usually reduced, although the carrier must not be toxic to the natural enemy. Another advantage of liquid systems is that sprayer technology is familiar to the agricultural industry--growers already have liquid application equipment, calibration procedures are well developed, and application is relatively simple.

Keeping the biocontrol organisms uniformly suspended in the liquid carrier before application is one of the challenges of developing successful liquid application devices. Floating or settling of organisms in suspensions can be affected by various factors such as vehicle vibration or differing densities of the organisms and carriers. Many different additives have been investigated for maintaining specific organisms in solution without affecting viability. A 0.125% agar solution was a successful carrier for green lacewing eggs, but may not be applicable to Trichogramma inside host eggs.

Many different liquid delivery systems have been developed. One liquid delivery system immersed green lacewing eggs and caterpillar eggs parasitized by Trichogramma pretiosum in water. Eggs were kept suspended by pneumatic agitation and the spray was discharged through large-orifice nozzles without affecting hatch of the natural enemies. The nozzles in this simple, inexpensive system were large enough to avoid plugging and excessive shear on the relatively large organisms. The concentration of eggs in sprayed suspensions varied over time, as no attempts were made in this study to optimize agitation. Another recent system used an electronically controlled mechanical system to meter and discharge liquid suspensions of green lacewing eggs. The pneumatic system intermittently discharges a liquid jet to produce 2mm droplets that can be projected up to 3 feet horizontally. This results in a uniform concentration of eggs in the emitted liquid and there is no effect on egg viability.

One of the most successful of the mechanical devices to date is the Bio-Sprayer™. The sprayer was developed over a period of about 10 years by Dr. Louis Tedders of the USDA-ARS (and patented by the USDA) for distribution of eggs of beneficial insects onto pecan trees and grape vines. He first attempted to use conventional sprayers, but none worked because the pressure used to generate the spray destroyed eggs as the liquid mixture went through the pressure regulator. The Bio-Sprayer uses air atomization to shear liquid into small droplets and propel them to target foliage: the liquid mixture is forced through a simple tube, not a narrow nozzle, and the air stream then disperses the eggs. A nontoxic spray adhesive (BioCarrier™), which adheres eggs firmly to foliage was developed by Beneficial Insectary (Oak Run, CA). So far, the insects successfully applied are Trichogramma wasps and green lacewings, but research continues to investigate the potential of the machines for mite and whitefly parasite applications. Beneficial Insectary worked for several years with the USDA in developing mechanical distribution of eggs, and is a partner in commercialization of the Bio-Sprayer which was introduced in 1996. Three sprayer models are being manufactured by Smucker Mfg., Inc. (Harrisburg, Oregon) under license from the USDA. Beneficial Insectary continues to support research on mechanical delivery, not only as an avenue of improving use of the beneficials the company markets, but for the benefit of everyone producing and selling natural enemies and growers who are currently using these products or contemplating adding them to their pest management arsenal.

This discussion so far has focused on ground application, but in many situations aerial application is far more practical. A variety of delivery systems have been explored for use on aircraft. The earliest efforts dispensed packets of Trichogramma-parasitized caterpillar eggs in a grid pattern. Adult wasps emerged from the packages through an opening formed as they were released from the aircraft. This method was impractical for large-scale commercial use because of excessive costs and labor required to prepare the packages, as well as the non-uniformity of the releases. Then a conventional dry material dispersal system on an agricultural aircraft was modified for broadcast release of Trichogramma-parasitized caterpillar eggs attached to wheat bran flakes. This gave satisfactory results, but the bran was more uniformly distributed than the host eggs or adult Trichogramma. This method was questionable for large-scale commercial applications because of the quantity of bran required and the logistics of preventing premature emergence of the wasps. Further attempts to eliminate the bran and improve accuracy of distribution and wasp viability were only marginally successful. In other experiments, the predatory mite Phytoseiulus persimilis was released by conventional light aircraft onto field corn for the control of spider mites. The predators were mixed with corncob grits, and released about 50 feet above the plant canopy with a mechanized, refrigerated delivery system in a near uniform distribution. This study demonstrated the feasibility of aerial release and inoculation of predatory mites in corn fields. Continuing research by scientists in universities, government agencies and private industry will improve methods of aerial release of natural enemies.

The latest slant on aerial application is the utilization of small-scale, remote controlled aircraft, both fixed wing and helicopters. One initial impetus for the use of such aircraft was for pesticide spraying in narrow spaces where both ground and conventional air access was difficult, such as terraced rice fields. The Japanese are promoting several types of utility remote control aircraft for this use, but the granular chemical application devices offered for use with such aircraft can be modified or replaced for dispensing natural enemies. Advantages include lower initial cost of the aircraft itself, reduced maintenance costs, operator safety, small storage space, and applications in inaccessible locations. As with any other application method there are drawbacks. Electronic wave operation limits aircraft use where strong electric signals (such as radio stations) are being generated. Windy conditions dramatically affect the ability of the aircraft to operate and apply natural enemies uniformly. Experiments in Wisconsin cranberries for application of green lacewing eggs yielded less than satisfactory results because of windy conditions. One model of aircraft was recommended, but frequently could not fly because of stiff breezes that were not atypical for the region. A combination of weather and pilot availability resulted in applications not being made in a timely fashion. Ultimately, the problem was resolved by using a larger model of airplane.

In summary, there are many challenges in the development of mechanical methods to safely and effectively deliver natural enemies over large areas. Mechanical delivery of pathogens (microbial pesticides) and insect-parasitic nematodes by conventional pesticide delivery equipment has been a reality for decades. Recent developments for the delivery of lacewing eggs, Trichogramma in host eggs, predatory mites, and other small arthropods have resulted in the commercialization of both air and ground equipment for the delivery of both liquid and dry formulations of these natural enemies. Although further refinements may be necessary, we are entering a period when large-area releases of natural enemies is becoming a reality.

- Susan Mahr, University of Wisconsin - Madison

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