Insect-killing nematodes

Relative Effectiveness and Application Parameters

Growers will not adopt biological agents that do not provide efficacy comparable with standard chemical insecticides. Technological advances in nematode production, formulation, quality control, application timing and delivery, and particularly in selecting optimal target habitats and target pests, have narrowed the efficacy gap between chemical and nematode agents. Nematodes have consequently demonstrated efficacy in a number of agricultural and horticultural market segments. 

Entomopathogenic nematodes are remarkably versatile in being useful against many soil and cryptic insect pests in diverse cropping systems, yet are clearly underutilized. Like other biological control agents, nematodes are constrained by being living organisms that require specific conditions to be effective. Thus, desiccation or ultraviolet light rapidly inactivates insecticidal nematodes; chemical insecticides are less constrained. Similarly, nematodes are effective within a narrower temperature range (generally between 20 °C and 30 °C) than chemicals, and are more impacted by suboptimal soil type, thatch depth, and irrigation frequency (Georgis and Gaugler, 1991; Shapiro-Ilan et al., 2006). Insect-killing nematodes may be inactivated if stored in hot vehicles, cannot be left in spray tanks for long periods, and are incompatible with several agricultural chemicals. Chemicals also have problems (e.g., mammalian toxicity, resistance, groundwater pollution, etc.) but a large knowledge base has been developed to support their use. Accelerated implementation of nematodes into IPM systems will require users to be more knowledgeable about how to use them effectively.

Therefore, based on the nematodes’ biology, applications should be made in a manner that avoids direct sunlight, e.g., early morning or evening applications are often preferable. Soil in the treated area should be kept moist for at least two weeks after applications. Application to aboveground target areas is difficult due to the nematode’s sensitivity to desiccation and UV radiation; however, some success against certain above-ground targets has been achieved and recently approaches have been enhanced by improved formulations (e.g., Shapiro-Ilan et al., 2010). In all cases, the nematodes must be applied at a rate that is sufficient to kill the target pest; generally, 250,000 infective juveniles per m2 of treated area is required (though in some cases an increased or slightly decreased rate may be suitable) (Shapiro-Ilan et al., 2002). Additionally, it is important to match the appropriate nematode species to the particular pest that is being targeted (see the table below for species effectiveness).
 

Appearance

Nematodes are formulated and applied as infective juveniles, the only free-living and therefore environmentally tolerant stage. Infective juveniles range from 0.4 to 1.5 mm in length and can be observed with a hand lens or microscope after separation from formulation materials. Disturbed nematodes move actively, however sedentary ambusher species (e.g. Steinernema carpocapsae, S. scapterisci) in water soon revert to a characteristic "J"-shaped resting position. Low temperature or oxygen levels will inhibit movement of even active cruiser species (e.g., S. glaseri, Heterorhabditis bacteriophora). In short, lack of movement is not always a sign of mortality; nematodes may have to be stimulated (e.g., probes, acetic acid, gentle heat) to move before assessing viability. Good quality nematodes tend to possess high lipid levels that provide a dense appearance, whereas nearly transparent nematodes are often active but possess low powers of infection. 

Insects killed by most steinernematid nematodes become brown or tan, whereas insects killed by heterorhabditids become red and the tissues assume a gummy consistency. A dim luminescence given off by insects freshly killed by heterorhabditids is a foolproof diagnostic for this genus (the symbiotic bacteria provide the luminescence). Black cadavers with associated putrefaction indicate that the host was not killed by entomopathogenic species. Nematodes found within such cadavers tend to be free-living soil saprophages. 

Habitat

Steinernematid and heterorhabditid nematodes are exclusively soil organisms. They are ubiquitous, having been isolated from every inhabited continent from a wide range of ecologically diverse soil habitats including cultivated fields, forests, grasslands, deserts, and even ocean beaches. When surveyed, entomopathogenic nematodes are recovered from 2% to 45% of sites sampled (Hominick, 2002).

Pests Attacked

Because the symbiotic bacterium kills insects so quickly, there is no intimate host-parasite relationship as is characteristic for other insect-parasitic nematodes. Consequently, entomopathogenic nematodes are lethal to an extraordinarily broad range of insect pests in the laboratory. Field host range is considerably more restricted, with some species being quite narrow in host specificity. Nonetheless, when considered as a group of nearly 80 species, entomopathogenic nematodes are useful against a large number of insect pests (Grewal et al., 2005). Additionally, entomopathogenic nematodes have been marketed for control of certain plant parasitic nematodes, though efficacy has been variable depending on species (Lewis and Grewal, 2005). A list of many of the insect pests that are commercially targeted with entomopathogenic nematodes is provided in the table below. As field research progresses and improved insect-nematode matches are made, this list is certain to expand.

Conservation

Conservation strategies are poorly developed and largely limited to avoiding applications onto sites where the nematodes are ill-adapted; for example, where immediate mortality is likely (e.g., exposed foliage) or where they are completely ineffective (e.g., aquatic habitats) (Lewis et al., 1998). Minimizing deleterious effects of the aboveground environment with a post-application rinse that washes infective juveniles into the soil is also a useful approach to increasing persistence and efficacy. Native populations are highly prevalent, but, other than scattered reports of epizootics, their impact on host populations is generally not well documented (Stuart et al., 2006). This is largely attributable to the cryptic nature of soil insects. Consequently, research and guidelines for conserving native entomopathogenic nematodes are in need of advancement. 

Compatibility

Infective juveniles are compatible with most but not all agricultural chemicals under field conditions. Compatibility has been tested with well over 100 different chemical pesticides. Entomopathogenic nematodes are compatible (e.g., may be tank-mixed) with most chemical herbicides and fungicides as well as many insecticides (such as bacterial or fungal products) (Koppenhöfer and Grewal, 2005). In fact, in some cases, combinations of chemical agents with nematodes results in synergistic levels of insect mortality. Some chemicals to be used with care or avoided include aldicarb, carbofuran, diazinon, dodine, methomyl, and various nematicides. However, specific interactions can vary based on the nematode and host species and application rates. Furthermore, even when a specific chemical pesticide is not deemed compatible, use of both agents (chemical and nematode) can be implemented by waiting an appropriate interval between applications (e.g., 1 – 2 weeks). Prior to use, compatibility and potential for tank-mixing should be based on manufacturer recommendations. Similarly, entomopathogenic nematodes are also compatible with many though not all biopesticides (Koppenhöfer and Grewal, 2005); interactions range from antagonism to additivity or synergy depending on the specific combination of control agents, target pest, and rates and timing of application. Nematodes are generally compatible with chemical fertilizers as well as composted manure though fresh manure can be detrimental. If you use a pesticide, you must read and follow the label and also comply with any other local regulations.

Commercial Availability

Of the nearly eighty steinernematid and heterorhabditid nematodes identified to date, at least twelve species have been commercialized. Comparison-shopping is recommended as prices vary greatly among suppliers. Additionally, caution is again advised with regard to application rates. One billion nematodes per acre (250,000 per m2) is the rule-of-thumb against most soil insects (containerized and greenhouse soils tend to be treated at higher rates). A final caveat is that, just as one must select the appropriate insecticide to control a target insect, so must one choose the appropriate nematode species or strain. Ask suppliers about field tests supporting their recommended matching of insect target and nematode.