Cranberry growers are always interested in diversifying their pest management strategies to reduce inputs, increase sustainability, and improve environmental stewardship of the land. One Integrated Pest Management (IPM) strategy we do not talk about enough is biological control. Biological control is a pest management strategy that can be integrated within an IPM program with care for, and understanding of, all the natural enemies that are present in agroecosystems. Natural enemies are represented by very diverse groups of insects and other arthropods that feed on pest insects and thereby provide pest management in a natural and environmentally-sound approach. Natural enemies include predators (e.g. spiders, ladybugs, praying mantis), parasites who do not kill the host (e.g., flies, mites, and lice), parasitoids (e.g., wasps, flies), and pathogens (fungi, bacteria, viruses, and nematodes). Biological control can be implemented according to three strategies: Importation, Augmentation, and Conservation biological control. 

The first strategy, importation biocontrol (aka classical biocontrol), is of importance in the case of new invasive pest species. In this strategy, researchers return to the native range of the invasive pest to identify and collect the natural enemies of the pest species in its original location. This is a very lengthy process of about 10 years that requires quarantine, mass rearing, evaluations of non-target species in the invaded range, and regulatory permits. For the most part in cranberry, we deal with native insect pests and therefore there has not been a historical need for importation biocontrol in cranberry. This strategy is currently being implemented for other fruit insects, such as spotted-wing drosophila and brown marmorated stink bug, with increasing success in reducing these pest populations.  

The second strategy is called augmentation biocontrol. Here, naturally-occurring natural enemies that have been determined to be effective biocontrol agents and can be reared in mass in insect-rearing facilities are released in the area that needs to be controlled. One example would be the entomopathogenic nematodes that are mass reared and released into cranberry beds to kill several pests, including flea beetle, cranberry fruitworm, and sparganothis fruitworm (Steffan 2022). Another example would be the minute parasitic wasps of the genus Trichogramma that lay eggs inside butterflies and moths’ eggs, thereby preventing them from hatching.  

Finally, the third strategy is conservation biocontrol in which the goal is to capitalize on the naturally occurring natural enemies by providing habitat and resources for them to thrive in our agroecosystems and by reducing pesticide applications that would negatively impact them.  

The case of cranberry 

Natural enemies 

Cranberry is a great example of a native crop embedded into a natural ecosystem that harbors natural enemy populations which can be fostered to help reduce pest populations. A diversity of natural enemies has been documented to attack our pest complexes, including syrphid flies (Fig. 1), parasitoid wasps (Fig. 2), predators, nematodes, and others. Natural parasitism rate varies widely among pest species, with larval parasitism rates of ~60% for the false armyworm and sparganothis fruitworm, ~20% for the green spanworm, ~10% for the blackheaded fireworm, and almost no parasitism for the brown spanworm (Drolet et al., 2019).  

Augmentation 

Some nematode species and strains can be purchased from different companies in the US or are reared by the Steffan’s Lab at USDA-ARS. They are either mass reared in bioreactors or using an alternative host (also called factice host), which are usually mealworm larvae. They can then be applied at rates around a billion per acre. Nematodes are microscopic and relatively resistant, so they can be easily mixed with clear water and sprayed into beds. The key to their efficacy is to get them washed down into the ground with water, where they can survive for multiple days and move around up to 1 m, looking for a host. Nematodes parasitize pests when they are at their larval stage, so their application should be synchronized when the targeted pest is at this stage. They have proven to be effective biocontrol agent against the cranberry fruitworm, sparganothis fruitworm, red-headed flea beetle and the cranberry girdler. Similarly, multiple species of parasitic wasps in the genus Trichogramma are available commercially and can be easily bought by the billions.  Different indigenous species, such as T. pretiosum (Fig. 2) in Wisconsin, have been found naturally parasitizing different cranberry pests. They are also typically reared on a factice host (usually sterilized eggs of the Mediterranean flour moth) in insect rearing facilities. They are applied as pupae parasitizing their factice host egg, and they emerge as adults in the days following their application. Freshly emerged adults then mate and go on looking for moth eggs. Therefore, their application should be synchronized with the targeted pest’s flight, which is also approximately their egg-laying period. They can either be applied manually (stuck on cardboard pieces spread across the fields), or mechanically. Direct injection spraying devices are currently being developed in Quebec, CA to allow Trichogramma pupae application in water (just like any other pesticide), which make it possible to apply them at a large scale quickly and with limited human resources. Their application rate is around 1 million per acre, but their adult lifespan is around 2 or 3 days, so they must be applied two to three times during the pest’s flight to maximize results. Neither of these two augmentative biocontrol agents have a detrimental effect on the crop, but their establishment on the marsh is expected to be limited. Thus, they must be reapplied year after year, similar to insecticides, minus the risks to human health and the environment. 

Conservation 

Implementing conservation biocontrol can be accomplished via the establishment of flower patches (such as pollinator gardens) or allowing weeds on marsh edges to flower, as these would provide nutritional resources, in the form of nectar and pollen, and habitat that would provide nesting areas, refugia from pesticide sprays, and hibernation sites. Reducing the number of insecticide applications and using reduced risk pesticides would also greatly benefit natural enemies and should be considered throughout the season. For example, implementing a spring flood helps reduce the use of insecticides. A 30hrs spring flood in late May was shown to be as effective as a broad-spectrum insecticide application of Chlorpyrifos in reducing the numbers of black headed fireworm, cranberry fruitworm, and sparganothis fruitworm, while supporting more biocontrol agents in the form of spiders and parasitoids in flooded beds compared to sprayed beds (van Zoeren et al., 2018). Parasitism rates were also shown to be higher on neglected marshes where they reached close to 50% of cranberry fruitworm egg parasitism by the parasitic wasp T. pretiosum (Fig. 2; Simser, 1995), with similar observations in British Columbia with blackheaded fireworm eggs (Li et al., 1993). These high parasitism rates can be linked to the fact that very little to no insecticides were used on those marshes since most broad-spectrum insecticides not only kill pest species, but also natural enemies. Marshes implementing IPM strategies were observed to foster natural enemy communities thereby benefiting from natural biocontrol and minimizing crop losses. For more information on how to select pesticides, please see this Oregon State publication or download the app “Reduce bee poisoning from pesticides”. 

In summary, the two biological control strategies that are the most relevant to cranberry at this time are augmentation and conservation. Augmentation biocontrol would require purchasing commercially-available natural enemies such as parasitoid wasps or nematodes and releasing them at the appropriate times and in the right amounts to best target the pest species. Conservation biocontrol could be implemented with minimal input by providing floral resources and habitat, reducing pesticide usage, and using reduced-risk pesticides to foster naturally-occurring populations of natural enemies. 

References 

Drolet, I., Guay, J. F., Fournier, V., & Cloutier, C. 2019. Biodiversity of lepidopteran pests and their parasitoids in organic and conventional cranberry crop. Biological Control, 129, 24-36. 

Li, S. Y., Sirois, G. M., Luczynski, A., & Henderson, D. E. 1993. Indigenous Trichogramma (Hym.: Trichogrammatidae) parasitizing eggs of Rhopobota naevana (Lep.: Tortricidae) on cranberries in British Columbia. Entomophaga, 38(3), 313-315. 

Simser, D. 1995. Parasitism of cranberry fruitworm (Acrobasis vaccinii; Lepidoptera: Pyralidae) by endemic or released Trichogramma pretiosum (Hymenoptera: Trichogrammatidae). The Great Lakes Entomologist, 27(4), 2. 

Steffan S. 2022. New targets and timings for our WI nematodes. Cranberry School Proceedings Vol 30. https://d31n3wj3oi4lt9.cloudfront.net/wp-content/uploads/sites/36/2022/03/2022-Cranberry-school-Proceedings-Complete-FINAL.pdf  

Van Zoeren J., Guedot C., and Steffan S.A. 2018. Conserving carnivorous arthropods: an example from early-season cranberry (Ericaceae) flooding. Canadian Entomologist 150: 265–273.  

This article was posted in Cranberry, Insects and tagged agroecosystem, augmentation biocontrol, Biological Control, Christelle Guédot, conservation biocontrol, Cranberries, Didier Labarre, importation biocontrol, insects.