A provincial Health Ministry has been asked by the Ministry of the Environment to advise on the health risks associated with use of nematodes to control the cranberry girdler, an insect that has been affecting the economic viability of provincial cranberry production. You are assigned the development of a response as part of your practicum with the Ministry of Health. Here is an outline of how you could structure your response.
Nematodes are simple un-segmented roundworms, usually microscopic in size. There are many subspecies, with estimates ranging from 1 to 20 million. Different nematodes inhabit almost every environment on earth, ranging from the ocean floor to swamps, deserts, and polar ice caps.1-3 Some nematodes feed on bacteria, some on plants, and others can be animal parasites or insect parasites.1 Some nematode species are human parasites, such as ascarids (Ascaris), filarias, hookworms, pinworms (Enterobius) and whipworms (Trichuris trichiura).4
Nematodes used for insect control are of a very different subspecies than the human parasites. They are also called entomopathogenic, insect pathogenic, beneficial nematodes or biological control nematodes.2,3 The term nematodes will be used throughout this document to refer to entomopathogenic nematodes. Strictly speaking, entomopathogenic nematodes are not themselves parasitic 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.2,3
Entomopathogenic nematodes are soil-dwelling organisms that occur naturally in the water film that surrounds soil particles.1 The most promising nematodes for the control of insect pests are nematodes in the genera Steinernematidae and Heterorhabditidae.1,3 Steinernematidae contain the insect-parasitic bacterium Xenorhabdus, while Heterorhabditidae possesses Photorhabdus.1 Nematodes are effective against a wide range of insect hosts in the laboratory,3,5 but field effectiveness is somewhat more restricted because environmental conditions cannot be controlled in the field.3
Commercially available nematodes are: Steinernema carpocapsae, Steinernema feltiae, Steinernema riobrave, Heterorhabditis bacteriophora, Heterorhabditis marelatus and Heterorhabditis megidis1,3; most commonly used for control of insect pests in crops, such as home lawns and gardens, turf, nurseries, citrus, cranberries, and mushrooms.1,3 Several species of soil-dwelling immature insects, mainly belonging to Lepidoptera (cranberry girdler) and Coleoptera (oriental beetle, black vine weevil, strawberry root weevil, cranberry rootworm), are common pests in cranberries.6 Steinernema carpocapsae and Steinernema glaseri have been found effective against black vine weevil on cranberries in Washington State; the presence of black vine weevil was reduced by 96% and 100%, respectively.7 In another field trial, Heterorhabditis bacteriophora and Steinernema carpocapsae suppressed black vine weevil populations by 56-100%.6 Steinernema carpocapsae was also found to be effective against cranberry girdler; effectiveness ranging from 44% to 92% in different field studies in the Pacific Northwest.6 Strawberry root weevils are not as sensitive to nematodes as black vine weevils.6 The effectiveness of nematodes on strawberry root weevils has not been studied as extensively as the effects on black vine weevils; field trials show 32-90% mortality by Heterorhabditis bacteriophora and Steinernema carpocapsae. The table below summarizes the most important nematode species that can be used against insect pests in cranberries.1-3,7,8
| Target pest | Nematode species |
|---|---|
| Strawberry root weevil | Hb, Sc |
| Black vine weevil | Hmeg, Hb, Hm, Sg |
| Cranberry girdler | Hb, Sc, Hm |
| Webworms | Sc |
| Cutworms | Sc |
| Armyworms | Sc |
| Wood-borers | Sc |
Hb= heterorhabditis bacteriophora, Sc= steinernema carpocapsae, Hmeg= heterorhabditis megidi, Hm= heterorhabditis marelatus, Sg=steinernema glaseri
Since many nematodes are highly temperature-dependent, not all are equally suitable for use in Canada. Cooler soil temperatures slow nematode activity and pest mortality.8In all climates, control of black vine weevils must be achieved before the temperatures drop below 11°C because larvae can still develop below this temperature and may escape control by nematodes.6 Heterorhabditis megidis can infect insects below 15°C.3,8 Steinernema carpocapsae are most effective at temperatures between 22-28°C,3 although they have been effectively applied against root weevils in cranberry fields in western Washington State when soil temperatures were as low as 14°C.7 Efficacy of Heterorhabditis bacteriophora is reduced when the soil temperature is below 20°C.3 Steinernema riobrave is warm-temperature adapted8 and may not be as suitable for use in some regions of Canada.
Nematodes have different strategies for finding their insect host. Heterorhabditis bacteriophora uses an active cruiser strategy to search their host while steinernema carpocapsae tend to sit and wait for passing insects (ambushers).1,3,8,9 Nematodes rely on chemical cues, temperature cues, and touch or vibration to detect insect hosts.9 Once nematodes have found a host, they enter and kill it.
Nematodes go through several life stages1-3,5:
- Infective third-stage juveniles (0.4-1.5 mm in length) in soil enter an insect through natural openings, such as mouth, anus or breathing holes.
- Nematodes enter insect body cavity and release their bacteria. Toxins produced by the bacteria kill the insect within a couple of days.
- Nematodes feed on the liquifying host and the bacteria; developing into adults.
- Nematodes reproduce and generate offspring.
- Infective juvenile nematodes leave the dead insect and seek a new insect host.
Under optimal conditions, nematode-infected pest stages should be present 5-7 days after application. Insects killed by Steinernematidae become brown or tan; ones killed by Heterorhabditidae turn red.1,3,8 Dead insects, however, are not always visible.2
In agriculture, nematodes can be sprayed using standard agrochemical equipment.1,5,8 Nematodes are sensitive to high pressure systems,3 sunlight (UV), and high temperatures; therefore, they should be applied early in the morning or in the evening.3,5 Pre- and post-irrigation are recommended to moisten the soil and wash the nematodes into the soil.1,3,5 Nematodes can withstand short-term exposure (2-24h) to many chemical and biological insecticides, fungicides, herbicides, fertilizers and growth regulators.10
Nematodes used for control of cranberry pests are an alternative option (i.e., biocontrol agent) to chemical pesticides often used for the control of these pests. Insecticides commonly used include acephate, azinphos-methyl, carbaryl, diazinon, malathion and phosmet.11
Nematodes contain bacteria, but these bacteria cannot live independently and they cannot infect humans.1,2,5 Nematodes are also safe for animals and plants.2 Human-parasitizing nematodes are not used for insect-control.
Since nematodes cannot live above the soil, they cannot infect non-target insects that live above the soil, such as foliar-inhabiting insects. Mortality among soil-inhabiting non-target arthropods can occur but it is temporary, spatially restricted, and will affect only part of the population.12 One study in Washington State found that nematodes did not disperse beyond the treated plot.7 Introduction of exotic nematodes can cause a reduction in native nematode species (biological pollution), but since nematodes generally have low mobility, large parts of the native nematode populations will survive.12
Cranberries can be affected by several insect pests, such as cranberry girdler, root weevil, black vine weevil, webworms, cutworms, armyworms and wood-borers. For some of those (cranberry girdler, armyworms, root weevils) no chemical insecticides are available,11 which makes biological control necessary. Using nematodes as biocontrol agents, instead of pesticides to control common cranberry pests, can have the following advantages and drawbacks:
Advantages
- They are considered safe for humans, plants, and animals and relatively safe for the environment, especially when compared to traditional pesticides. Unlike pesticides, the use of nematodes does not require personal protective equipment, such as safety masks and re-entry intervals and there are no residues and groundwater contamination.3,5
- Cranberry fields often have good environmental conditions for nematodes. The soil around the roots of cranberries is moist and humid, protected from harmful UV light and temperatures; often suitable for use of nematodes.6,8
Drawbacks
- Nematodes are sensitive to environmental conditions, such as UV and high temperatures5; conditions that may be hard to control.
- Nematodes may not be compatible with some chemical pesticides used to control other pests.
- Compared to traditional chemical insecticides, nematodes are generally more expensive.13
- Nematodes need to be reapplied. In a natural system, the number of insect hosts and nematodes is in balance. If the natural balance is overloaded with nematodes, they will kill the insects but will then die off themselves until insect populations start to build again.2
- Nematode formulations have a short shelf-life.
For registration purposes, nematodes are generally considered macroorganisms; therefore, do not require registration in many countries.12 In Canada, biological control agents are regulated through the Plant Protection Act administered by the Canadian Food Inspection Agency,14 but there is currently no registration required for use of nematodes.12
Acknowledgment
University of British Columbia community medical residents Raina Fumerton and Piotr Klakowicz developed an initial response; Erna van Balen provided additional content. We would like to thank Mary Mitchell, John Worgan, Peter Chan, Yadvinder Bhuller, Michele Wiens and Helen Ward for their input and review of this document.
References
- Barbercheck M. Insect-parasitic nematodes for the management of soil-dwelling insects University Park, PA: Penn State University, College of Agricultural Sciences; 2011; Available from: http://ento.psu.edu/extension/factsheets/parasitic-nematodes.
- Crow WT. Using nematodes to control insects: Overview and frequently asked questions. Gainesville, FL: University of Florida, Institute of Food and Agricultural Sciences; 2009. Available from: http://edis.ifas.ufl.edu/in468.
- Cornell University. Biological control. Ithaca, NY: Cornell University, College of Agriculture and Life Sciences, Department of Entomology; 2011; Available from: http://www.biocontrol.entomology.cornell.edu/pathogens/nematodes.html#Top.
- Troeh FR, Donahue RL. Dictionary of agricultural and environmental science. New York, NY: Wiley-Blackwell; 2003.
- Natural pest control with beneficial nematodes. Tucson, AZ: Garden Insects; 2011; Available from: http://www.gardeninsects.com/beneficialNematodes.asp.
- Cowles RS, Polavarapu S, Williams RM, Thies A, Ehlers R-U. Chapter 12: Soft fruit applications. In: Grewal PSE, R-U, Shapiro-Ilan DI, editors. Nematodes as biocontrol agents. New York, NY: CABI; 2005.
- Booth SR, Tanigoshi LK, Shanks CH. Evaluation of entomopathogenic nematodes to manage root weevil larvae in Washington state cranberry, strawberry, and red raspberry. Environ Entomol. 2002 Oct;31(5):895-902.
- Polavarapu S. Insecticidal nematodes for cranberry pest management. Chatsworth, NJ: Ohio State University; 1999; Available from: http://www.oardc.ohio-state.edu/nematodes/cranberry_fact_sheet.htm.
- Fitzpatrick S. Understanding beneficial nematodes. Cranberry web - Newsletter of the BC Cranberry Marketing Commission [serial on the Internet]. Agassiz, BC: Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre; 2006; 1(1): Available from: http://www.bccranberries.com/pdfs/BCCMC_News_vol1_issue1.pdf.
- Koppenhöfer AMG, P S. Compatibility and interactions with agrochemicals and other biocontrol agents. In: Gaugler R, editor. Entomopathogenic nematology. New York, NY: CABI; 2005. p. 363-81.
- Agriculture and Agri-Food Canada. Crop profile for Cranberry in Canada - insects and mites. Ottawa, ON: Government of Canada; 2011; Available from: http://www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1299255467972&lang....
- Ehlers R-U. Chapter 6. Forum on safety and regulation. In: Grewal PS, Ehlers R-U, Shapiro-Ilan DI, editors. Nematodes as biological control agents. Cambridge, MA: CABI Publishing; 2005.
- Wright D, Peters A, Schroer S, Fife J. Chapter 5: Application technology. In: Grewal PS, Ehlers RU, DI S-E, editors. Nematodes as biocontrol agents. Oxfordshire: CABI Publishing; 2005.
- Kuhlmann U. International regulation practise for macrobials. Wallingford, UK: CABI; 2006 (?). Available from: http://www.rebeca-net.de/downloads/ASG1%20Kuhlmann.pdf.
February 2012
