BUT WHAT ARE THEY?

Beneficial nematodes are microscopic roundworms that live in the soil. They are used for pest control because they are excellent hunters of soil pests (and occasionally aboveground pests, as well). Their main tactic is to parasitize insects in their larval and pupal stages. Nematodes move within moist soil and enter a suitable host. The nematodes, alongside an associated bacteria, kill the host within a few days. It is still relatively unresearched in the horticultural and scientific community but is gaining traction as an another low-risk, chemical-free method for controlling pests.

There are a few different types of nematodes: entomopathogenic nematodes, plant pathogenic nematodes, and saprophytic nematodes. The entomopathogenic nematodes are the beneficial nematodes used by orchardists. They are the natural enemies of many insects, including some key orchard pests like codling moth, peach tree borers, and plum curculio. Entomopathogenic nematodes infect only insects and related arthropods and are harmless to humans, other animal life and plants.

Most soil nematodes are actually semi-aquatic. They find refuge in the water that surrounds soil particles. This is why nematodes need to be applied in a liquid suspension and why the soil needs to be moist before and after application.

HOW THEY OPERATE

Beneficial nematodes invade the body of insect pests and release a pathogenic bacterium which kills the pest. Entomopathogenic nematodes are not themselves parasites because they do not feed on their host directly. Instead, it’s the symbiotic bacteria which the nematodes release that are the true parasites that kill the insect. The relationship between the nematodes and bacteria is a true obligate mutualism because the bacterium needs the nematode to carry it into the insect body cavity. In return. the nematode needs the bacterium to create conditions in the insect suitable for its reproduction and growth, and as food.

The two most common beneficial nematodes are Steinernema carpocapsae and Heterorhabditis bacteriophora. These nematodes have different strategies for finding their insect host. Heterorhabditis bacteriophora actively searches for their host while Steinernema carpocapsae tends to sit backand wait for passing insects. All nematodes rely on chemical cues, temperature cues, and touch or vibration to detect insect host. Once nematodes have found a host, they enter and kill it.

EXCITING RESEARCH ON NEMATODES FOR CONTROLLING PLUM CURCULIO

Many of our POP orchards have been experiencing increased issues with plum curculio (C. nenuphar). POP has historically and continues to recommend kaolin clay and pyrethrin as forms of mechanical and chemical control at our sites, but there might potentially be an alternative form of biocontrol: parasitic nematodes! Using parasitic nematodes as a form of biological control in orchard management pest management strategies is currently under research. Specifically, the incorporation of nematodes in controlling plum curculio is seen as a potentially effective strategy.

In laboratory studies conducted at Cornell University in New York, parasitic nematodes, specifically strains of Steinernema riobrave, S. feltiae, S. carpocapsae, and Heterorhabditis bacteriophora have shown to be an effective way to control the larval stage of plum curculio following fruit drop. These nematode strains were able to reduce the plum curculio larval populations as much as 70–97%, depending on various factors.

The commercially available nematode strains that were studied had less of an effect after 6 months. This means that an annual application of nematodes would be necessary. However, in the studies, an application of nematode strains native to NY were shown to persist in the field for many years. Additionally, the native nematode strains (native to NY in this particular study) were able to attack the overwintering adult plum curculio before they emerged in the spring, as well as continually attack any time the adults were in contact with the soil during colder temperatures with low insect activity. This potentially shows the importance of sourcing nematodes native to the region where you are farming in terms of increased effectiveness. This study is a positive outlook into using parasitic nematodes, a form of (natively sourced) biocontrol, to reduce the number of plum curculio larvae residing in the orchard.

HOW TO APPLY BENEFICIAL NEMATODES

It is important to know what pest you are targeting and to match it with the appropriate strain of nematode. Nematodes don’t target all pests. They prey on many different insects, but most commonly caterpillars and grubs. In terms of orchard pests, this would include oriental fruit moth, peach borer, apple maggot, plum curculio and coddling moth. Make sure you purchase nematodes from a reputable supplier and select a strain of nematode specific to the pest your are attempting to control.

Because nematodes are living organisms, their successful use is influenced by environmental conditions. Nematodes need adequate moisture, temperatures within the tolerance levels for the specific nematode, and protection from UV radiation (direct sunlight) during application

Before applying, make sure:

• Do not apply nematodes when soil is below 60 degrees F or on a very hot day (nematodes are killed by the cold and extreme heat).
• They should be applied early in the morning or in the evening when air temperatures are between 60 and 85°F.
• Irrigate the soil before application to a point of moisture, making sure it is not waterlogged.
• Nematodes tend to settle in the tank, so agitation must be provided for uniform application. However, nematodes can also be killed by excessive tank agitation, so agitate with gentleness.
• Most nematode labels suggest two to six gallons of spray per 1000 square feet (87-260 gallons per acre)

How to apply:

• Mix up a solution of fresh nematodes in cool, distilled water, pour the solution into a sprayer, and apply to the soil at the base of the infected tree.
• Irrigate after application. Several irrigations may be needed.
• Follow up a week or so after each application with same spraying regiment. Look for red or yellow-brown infected larvae or pupae.

Storage:

• Shelf life of nematode products is reduced when stored at high temperatures. Most products should not be used after the first year.

SOURCES

Healthy Lawns—Manage Pests and Beneficial Nematodes

Insect-Parasitic Nematodes for the Management of Soil-Dwelling Insects- Penn State Extension

Beneficial Nematodes– UMass Extension Landscape, Nursery and Urban Forestry Program

Nematodes for Pest Control– National Collaborating Center for Environmental Health

Nematodes for Pest Control- Cornell University

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3547350

DISCLAIMER

The information presented on this website is for informational, reference, and educational purposes only and should not be interpreted as a substitute for diagnosis and treatment by a health care professional. Always consult a healthcare professional or medical doctor when suffering from any health ailment, disease, illness, or injury, or before attempting any traditional or folk remedies. Keep all plants away from children. As with any natural product, they can be toxic if misused. 

This POP Blog was written by Orchard Assistant Simone Shemshedini with help from Co-Executive Director Phil Forsyth.

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