Written by Rebecca Schmidt-Jeffris, USDA-ARS, 3/6/2023
Many different species of natural enemies are available for purchase from various commercial insectaries. In orchards, these natural enemies can be used to control aphids, mealybugs, mites, and thrips. Research on how to successfully release natural enemies is ongoing and this article compiles both research results and grower experiences on what they have found works for them. Because orchard releases are only recently increasing in popularity, recommendations are still in development and will change as new information is learned.
General Natural Enemy Release Best Management Practices
- Store natural enemies properly when they arrive – read the directions from the insectary, avoid extreme heat and cold. Use immediately if possible.
- Do not release if nighttime freezes are possible or during exceptionally hot days.
- Release flying stages of natural enemies, especially ladybeetles, when it is dark.
- Avoid pesticide applications before and after releases.
- Avoid other potential disruptions to natural enemies immediately after releases, such as mowing and heavy overhead irrigation.
- Monitor pest levels before and after release to determine if a release worked.
Currently, there are no scientifically-based rate recommendations for releasing any species of natural enemy in orchards. Rate recommendations found on insectary websites are typically for use in greenhouse crops and are likely not relevant to tree fruit. Crop consultants familiar with releases can provide recommendations based on their experiences, but it is important to note what rate you choose and to monitor pest levels before and after releases to determine if the rate chosen was effective. Unlike a pesticide application, the rate of natural enemy releases needed may vary dramatically based on pest pressure. It is very difficult to find released natural enemies after application, but that does not mean the release was not effective.
Note the quality of natural enemies when they arrive. Examine the material to confirm that the natural enemies are alive, under magnification if necessary. Natural enemies should be brought to the field in cool storage, such as a cooler with some ice packs. If natural enemies cannot be released on the day of receipt, consult with the insectary regarding storage temperature.
Avoid pesticide applications (even organic pesticides) immediately before and after a release. The longer the window between a natural enemy application and a spray, the better chance the natural enemy has at establishing. High pressure sprays of water alone may knock less mobile stages of natural enemies out of the trees.
In general, natural enemy stages that can fly should be released in the dark, or at dusk/dawn. Many adult natural enemies have a behavioral tendency to fly towards the sun, away from the orchard. This is especially true if the natural enemy is being released above the canopy by drone. Drone releases are still being evaluated for their efficacy and should be monitored to determine if pest levels decrease. Application by drone will result in a more even dispersal of the natural enemy and reduce labor, but a higher release rate may be needed as the number of natural enemies making it into the tree will be reduced.
Lacewings (Chrysoperla rufilabris and Chrysoperla carnea)
Lacewings are available in the greatest variety of release methods. Sprinkler containers of adults, larvae, and eggs can all be purchased. Eggs are also available glued to cards or laid on strings, which can be hung on the tree, or in hex-cell cardboard dispensers. Eggs are the least expensive stage to purchase, followed by larvae. It is likely not cost effective to release adults and adults are not predatory; you will need to wait for adults to lay eggs and for those eggs to hatch to see pest control. Some growers prefer egg cards due to ease of application, but there are concerns that cards may increase predation of the lacewing eggs by ants or other lacewings. Research has not yet determined whether release methods differ in efficacy. Releases of lacewings and other natural enemies may not be effective in orchards with high populations of ants. The ants will “tend” aphids and other honeydew producing insects, attacking natural enemies that try to feed on the aphids.
Some growers evaluate lacewing eggs prior to releasing them to determine if they are about to hatch. Eggs near hatch turn greyish, whereas younger eggs are bright green. It may be beneficial to hold the container at room temperature and wait until the majority of eggs turn grey before releasing. This decreases the number of unhatched eggs that might be predated. Do not store containers of hatched larvae at room temperature, they will rapidly cannibalize each other.
Currently, there are two main species of lacewings advertised in the U.S., but identification of lacewings can be confusing. First, some sellers may use “Chrysopa” instead of “Chrysoperla” when describing what species of lacewing they have available. Many species of lacewings used to be called “Chrysopa” that are now no longer assigned to this genus, but this name has persisted in some cases. The species name “rufilabris” or “carnea” is a more reliable indicator of what you are ordering. In general, C. rufilabris is correctly advertised, as this species is relatively easy to identify. However, purchased “C. carnea” are sometimes actually other species. This can be problematic if the substituted species is not suited to arid climates. Consult with an Extension entomologist if you are in doubt of which species you have received. Chrysoperla carnea is also a species group and it is impossible to distinguish species without obtaining either genetic sequences or multiple live adult specimens. Therefore, while there are native members of this species group in Washington, it cannot be determined with certainty whether what you have ordered is related to the Washington-native lacewings.
There are several key points to consider in choosing between the two available species. Chrysoperla carnea is more expensive than C. rufilabris. However, C. rufilabris is less tolerant of arid climates and cannot overwinter in Washington. In one study we recently completed, releasing 100,000 C. carnea eggs per acre reduced aphids by about 50%, but releases of C. rufilabris eggs did not. Work is ongoing to determine whether C. carnea is effective enough to justify the higher price point.
Ladybugs (Hippodamia convergens)
The most commonly sold ladybug is the convergent ladybeetle. These are sold as adults, which have been collected from overwintering locations where they are in diapause, and sometimes kept in cold storage until the point of sale. Prior research by the USDA has shown that the quality of these insects can be highly variable depending on when they are harvested and how long they are kept in cold storage. Because they are harvested and not reared, availability can be inconsistent. There are also concerns that released individuals may spread disease to resident ladybeetles. The efficacy of convergent ladybeetle for controlling aphids in orchards has not been formally assessed.
Mealybug destroyers (Cryptolaemus montrouzieri)
These are smaller ladybeetles that are specialist predators of mealybugs. As specialists, they require higher populations of the pest to remain in the orchard and will disperse or starve when mealybug numbers are low. Because of this, these natural enemies only appear to work in situations where mealybug pressure is high. These are highly dispersive natural enemies and may not remain in the crop long enough to achieve the desired level of pest control. Release when daytime temperatures are consistently above 50°F, because this insect is not cold-tolerant. It does not overwinter in Washington. Running irrigation prior to releases to wet the leaves may increase the short-term retention of mealybug destroyers. If mealybug destroyers establish, larvae can be found in the orchard 6-8 weeks after releasing adults. We found that releases of 2,000-5,000 mealybug destroyers per acre decreased mealybug numbers by nearly 50% in one trial, but follow-up trials showed no establishment or pest control.
Minute pirate bugs (Orius insidiosus)
Orius insidiosus is related to our native minute pirate bug (O. tristicolor), but it does not appear to be able to overwinter in Washington. It is currently being assessed for its ability to control pear psylla and thrips. Preliminary work has not shown that it can reduce pear psylla populations, but releases reduced thrips in apples by 30-50%. It may also have efficacy against pest mites, but this has not yet been studied in orchards.
Predatory mites (Galendromus occidentalis and other species)
Many species of predatory mites are available. Each species has a different range of pests that it feeds on and climate preferences. For control of pest mites, G. occidentalis (“typhs”) is a native species and therefore the most likely to establish in an orchard. Other species, such as Phytoseiulus persimilis, require much higher humidity and are not likely to be effective. If G. occidentalis is not naturally present in an orchard, it may be that current pesticide applications are disruptive. If an orchard has recently switched to an organic or soft IPM program, releases may help re-inoculate the orchard with G. occidentalis, but this has not yet been tested. To maximize the chance of success, predatory mites will need to be released into the orchard early, well before traditional pesticide-based thresholds. Other spider mite predators that are commercially available include Neoseiulus californicus, Amblyseius andersoni, and Neoseiulus fallacis. Of these, A. andersoni and N. fallacis have been found “naturally” in orchards and may therefore establish if released. However, none of these species has been formally evaluated.
Stethorus punctillum
Stethorus punctillum is a very small, black ladybeetle that feeds on spider mites. What is sold commercially may not be the same species as our native Stethorus, as this genus is under revision and they are difficult to identify at the species level. Stethorus can be difficult to consistently obtain due to supply issues. While releases of this species have not been formally evaluated, it can anecdotally be very effective. However, like all ladybeetles, it is highly dispersive and may not remain in the orchard after release.
Contact
Rebecca Schmidt-Jeffris
USDA-ARS
rebecca.schmidt@usda.gov
509-454-6556
Funding and acknowledgements
I would like to thank Teah Smith (Zirkle Fruit), Aaron Avila (G.S. Long), and Tianna DuPont (WSU Extension) for reviewing this article prior to submission. Funding for natural enemy release projects was provided by grants from the Washington Tree Fruit Research Commission, Fresh and Processed Pear Committee Research, and the Western IPM Center. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
Additional information
Lebeck and Leppla. 2021. Guidelines for purchasing and using commercial natural enemies in North America. https://anbp.org/
For a complete list of natural enemies available in the United States
Schmidt, R.A., Beers, E.H., Unruh, T.R., and Horton, D.R. 2013. Releases of insectary-reared Galendromus occidentalis (Acari: Phytoseiidae) in commercial apple orchards. J. Econ. Entom. 106: 1996-2005. DOI: 10.1603/EC13153
For additional information on releasing predatory mites for spider mite control
Henry, C.S., Wells, M.M. 2007. Can what we don’t know about lacewing systematics hurt us? A cautionary tale about mass rearing and release of “Chrysoperla carnea” (Neuroptera: Chrysopidae). Am. Entomol. 53: 42-47.
Unruh, T., Beers, E., Horton, D. 2012. Pest practices for predator releases: lacewings, beetles, and mites. Washington Tree Fruit Research Commission Final Report. https://treefruitresearch.org/report/best-practices-for-predator-releases-lacewings-beetles-and-mites/
For information about quality of purchased convergent ladybeetles
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