producers in the U.S. During the 1940s and 1950s, soil fumigation was commonly used to

manage nematodes in cotton, but difficulty in application, expense, and environmental and

human health risks resulted in the removal of most of these chemicals from the market.

During the next 30 years, carbamates and organophosphates largely replaced soil

fumigants as nematicides in cotton. These materials were attractive because they were low

in phytotoxicity and could be applied at planting. In addition, many of these chemicals

were also effective against insects, so producers could accomplish both nematode and insect

suppression with the same pesticide. Recently, however, human health and environmental

concerns have resulted in the suspension of many of insecticide/nematicides, and today only two remain labeled for use on cotton. Few new chemicals that can be used for nematode control in

cotton have been forthcoming, and growers now face increasingly severe nematode

problems in the absence of many of their historically effective nematicides.

Environmental concerns and focus on safety have resulted in nematode management

strategies that either utilize existing nematicides more efficiently or rely on

chemicals used at lower rates than those presently registered.

KEYWORDS: nematode, nematicide, nematode control, abamectin, harpin, thiodicarb

Innovations in Chemical Nematicides for Use in Cotton Control

T.L. Kirkpatrick and W.S. Monfort

Nematicides – A Brief History

Fumigant nematicides. Prior to WW II, research on chemicals specifically targeted to

plant-parasitic nematodes was focused on high value crops such as pineapple (Ananas comosus) (Godfrey, 1935; Johnson and Godfrey, 1932) where nematodes had been “discovered” to be a contributing factor to yield and quality reduction. At this point, the impetus for using chemicals to control nematodes was driven mainly by the availability of large quantities of surplus chloropicrin that remained at the close of WW I rather than by yield and quality concerns alone (Johnson and

Feldmesser, 1987). The need to dispose of chloropicrin and the fact that it was a relatively

effective soil biocide fueled considerable study of nematodes as plant pathogens, basic soil

ecology, and the technology needed to apply volatile chemicals to soil. Fortunately, by the time

government stores of surplus chloropicrin were exhausted by the late 1930s, the concept of

nematicide application as a means of mitigating nematode damage to crops had become

established, and a search for new, effective chemicals to replace chloropicrin was well underway.

In 1941, a practical method for using another volatile biocide (methyl bromide) was

reported (Taylor and McBeth, 1941). Methyl bromide was extremely effective across a range of

soil borne organisms including nematodes, fungal pathogens, and many weeds, but it was highly

toxic and rather expensive to use, limiting its use to only the highest value crops. Within the

next few years, however, two new chemicals that were less expensive and much easier to apply

were discovered. The first of these was a product obtained from the Shell Chemical Corporation

known as D-D (a mixture of 1,3-dichloropropene and 1,2-dichloropropane) (Carter, 1943). The

second was a related material, ethylene dibromide (EDB) that was marketed by both Shell and

Dow Chemical Companies (Christie, 1945). Both materials were much less volatile than methyl

bromide and were highly effective against nematodes, but were less efficacious against soil

borne fungal pathogens and weeds. In addition to providing an effective means of managing

nematode damage to various crops, these materials were perhaps most important in

demonstrating to the general public the role of nematodes as plant pathogens (Johnson and

Feldmesser, 1987).

In 1954, 1,2-dibromo-3-chloropropane (DBCP) was reported as having nematicidal

properties (McBeth, 1954; Raski, 1954). This material provided a considerable improvement

over previously used nematicides because it was effective at relatively low application rates

(making it less expensive to use), and was much less phytotoxic and less volatile than either D-D

or EDB. DBCP launched a new era of nematicide use because it could be applied either

before or after planting on an array of crops . The introduction of DBCP into the

marketplace as Nemagon® (Shell Chemical Co.) or Fumazone® (Dow Chemical Co.) solidified

the concept of nematicide application as a primary strategy in nematode management in

agronomic crops including cotton. Unfortunately, environmental and human health concerns

resulted in its elimination from the marketplace in the mid-1970s. A second material with

nematicidal properties, sodium methyl dithiocarbamate (metam sodium), was reported in 1956

(Lear, 1956). Metam sodium (Vapam®) releases methyl isothiocyanate as it degrades, a

chemical that is active against nematodes as well as certain soil borne fungal pathogens and

weeds. All of the fumigant nematicides mentioned above were labeled for use in cotton. Today

only Vapam and a related product, K-Pam® (potassium N-methyldithiocarbamate), chloropicrin,

and 1,3-dichloropropene (Telone II®), remain on the market, although limited use of methyl

bromide still occurs with a few high value crops under a special critical use

exemption.

Non-fumigant nematicides. A second category of chemicals with toxicity to nematodes was reported during the 1960s and 1970s primarily as a result of evaluations to

discover more effective insecticides. Two general chemical classes: organophosphates and

carbamoyloximes (carbamates) were recognized as having toxicity against nematodes as well as

insects (Christie and Perry, 1958; Jenkins and Guengrich, 1959; Weiden et al., 1965). These

insecticide –nematicides were less volatile than the nematicides that were being used at the time.

They were also easier to apply, and they were less phytotoxic. The first of these

products to be used were the organophosphates thionazin and dichlofenthion, VC-13, which were

marketed for nematode control in turf and ornamentals. In the ensuing 30 years, numerous

organophosphates and carbamates were registered as nematicides, and many were labeled for use

on cotton. Today, however, only two of these chemicals, aldicarb (Temik®) and

oxamyl (Vydate®), remain available for nematode control in cotton.

In the U.S. three nematodes are considered of major economic importance in cotton: the

southern root-knot nematode (Meloidogyne incognita), the reniform nematode (Rotylenchulus

reniformis), and the Columbia lance nematode (Hoplolaimus columbus) (Kirkpatrick and

Rothrock, 2001). Root-knot nematodes can be found associated with sandy soils throughout the

Cotton Belt. The reniform nematode is found throughout most of the southeastern and southern

states, and appears to favor soils containing greater amounts of silt and clay (Koenning

et al., 1996; Robinson et al., 1987; Still and Kirkpatrick, 2006). Columbia lance nematodes are

restricted in incidence currently to extremely sandy soils in the southeastern coastal plain.

Although generally one species is predominant in a particular field, it is not uncommon to find

two or, in some cases, all three species at economic levels in the same field. Regardless of the

species, when population densities are above economic thresholds, yield suppression ranging up

to 50% in some areas within individual fields can occur (Koenning et al., 2004).

At present, no cotton cultivars are available commercially with appreciable resistance to

either R. reniformis or H. columbus (Robinson and Percival, 1997; Bowman and Schmitt, 1994).

Although numerous breeding lines with high levels of resistance to M. incognita have been

reported, only a few cultivars with moderate levels of resistance have been released

commercially (Koenning et al., 2004). Consequently, nematicide application is currently the

main strategy for nematode suppression in fields where population densities exceed economic

thresholds. Almost 50 years ago, Hollis (1958) suggested that the ideal nematicide should be

efficacious in the soil for several months, non-phytotoxic and low in mammalian toxicity, and

be free from residual properties that might render food crops unfit for consumption. To date,

this ideal chemical has not been discovered. In the U.S. only a limited number of chemicals are

labeled for use in cotton.

Current Nematicide Use Strategies

Conventional strategies. In the southeastern states, and to a limited degree in the mid

southern and western regions, preplant soil fumigation with Telone II has been a standard

practice among cotton growers in fields where severe nematode damage is expected.

Disadvantages of this strategy include the difficulty and relatively high per hectare cost for

fumigation and the fact that the fumigant must be applied at least two weeks pre-planting, many

times under unfavorable soil and weather conditions. In addition, air quality concerns in some

areas limit the quantity of Telone II that may be applied in a given year. When Telone II is

applied properly, significant yield responses are generally seen (Figure 1).

The second and more widely used strategy is the application of the non-fumigant insecticide-nematicide, Temik, in the furrow at the time the seeds are planted. Temik is more convenient to apply and less expensive on a per hectare basis than soil fumigation, but yield responses may be less consistent (Figure 2). Likely due to its rather broad spectrum of efficacy that includes both nematodes and certain early-season insect pests, Temik usage has dramatically increased in U.S. cotton over the last 15 years, and it is used on 20 to 30% of the acreage across the Cotton Belt each year (Koenning et al., 2004).

Less conventional strategies. Although nematicides have historically been applied

either pre-planting or during the planting operation, the low degree of phytotoxicity of both

Temik and Vydate allows them to be applied post-planting. While the high mammalian toxicity

of Temik precludes foliar applications, some growers have found that nematode suppression can

be improved by a second soil application, applied as a side dressing four to six weeks after

emergence (Baird, et al., 2000). Generally, this additional application of Temik is delivered into

furrows (4 to 6 cm deep) that are parallel to the row 10 to 15 cm away from the plants. Vydate,

which is marketed both as a foliar insecticide and as a nematicide, has been shown to be

basipetally translocated in a number of crop species (Radewald et al., 1970; Santo and Bolander,

1979). Recent investigations indicate that foliar applications to cotton early in the growing

season in combination with at-planting application of Temik may improve nematode suppression

above that achieved with Temik alone (Lawrence and McLean, 2000; Lawrence and McLean,

2002).

Biorational approaches to nematode control have received only limited attention.

Recently, a protein from Erwinia amylovora (harpin) with the reported ability to elicit a resistant

response in certain plant species (Wei and Beer, 1996) has been suggested as a means to enhance

crop growth, development, and pest resistance. Studies in cotton indicate that reproduction of

Meloidogyne incognita may be slightly lower in harpin-treated plants (Table 1), but yield

responses have been disappointing (Bednarz et al., 2002). A second interesting

biorational approach involving genetic engineering of the cotton plant has been

suggested (Atkinson et al., 2003). Aldicarb in the soil solution at rates that are usually applied results in cholinesterase inhibition leading to paralysis and ultimately nematode mortality (Spurr, 1982). However, at much lower concentrations (21 pM), aldicarb acts as a chemoreception

inhibitor in nematodes (Winter et al., 2002). Certain peptides have been shown to elicit this

same response, and development of plants that produce these proteins in roots has been

suggested as a way to suppress nematode infection. Unfortunately, the concept of developing

plants that produce aldicarb-like substances is likely not prudent from a human or animal health

perspective.

Very recently, a strategy that utilizes low dosage application of nematicidal chemicals to

cotton seed as a seed treatment has shown considerable promise for providing a limited degree of

nematode control. Avermectins, macrocyclic lactones derived from Streptomyces avermictilus,

are extremely toxic to nematodes and certain insects (Putter et al.,1981). These chemicals are

widely used as anthelminthics in animals, and as foliar insecticides, and they have been studied

as nematicides (Sasser et al., 1982; Garabedian and Van Gundy, 1983). Unfortunately, their

utility as soil-applied nematicides has been limited due to expense, low water solubility, and their

tendency to adsorb to soil particles (Bull et al., 1984). However, application of certain

avermectins (abamectin) as a seed dressing has shown promise for nematode suppression in

cotton seedlings (Monfort et al., 2006). This product (Avicta Complete Pak®) was labeled for

use in U.S. cotton in 2006 as a component of a seed treatment that also includes an insecticide

and a fungicide. A second seed treatment that utilizes a carbamate (thiodicarb) as the

nematicidal component (Aeris®) will be available for cotton in 2007. Seed treatment may

suppress nematode infection for a limited time early in the growing season (Figure 3; Table 2),

a critical growth period for the cotton crop (Penteado et al., 2005). Although the concept of

applying nematicides to seed holds considerable promise as a part of an overall strategy, seed

treatment alone will likely not be sufficient to avoid yield loss in fields where nematode

population densities are high.

Improved efficiency in the application and placement of existing nematicides has also

been suggested as a means of lowering production costs and environmental risks. Precision

farming technology holds promise as a means of applying nematicides in a site-specific manner within individual fields rather than using the current approach of one rate applied field wide

(Evans, 2002). Since the spatial variability of nematodes within fields is generally high, identification of areas within fields that are at a high risk for sustaining economic crop damage based on nematode population density and other edaphic factors would allow focused nematicide treatments to be applied (Monfort et al., 2007). Unfortunately, attempts to develop this approach have shown only limited practicality largely due to the difficulty and expense required for nematode

sampling (Wheeler et al., 1999; Wyse-Pester et al., 2002; Wrather et al., 2002). Current

investigations into using aerial imagery or mobile soil electrical conductivity meters to define

potential problem areas appears promising and may lead to improved strategies for nematicide

placement in the near future (Kirkpatrick et al., 2006; C. Overstreet, personal communication).

Nematicides continue to be the primary means of managing nematode-induced yield

suppression in cotton in the U.S. Growers are facing a dilemma. While the number of highly

effective nematicides labeled for use in cotton has declined dramatically over the past 30 years,

nematode incidence and severity across the Cotton Belt are at an all-time high. Production costs

continue to increase, and environmental and health concerns have restricted or eliminated the use

of numerous products that were both economical and effective. In the face of this change, it will

be imperative that more sustainable, environmentally appropriate approaches to nematode

management in the crop be developed.

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Table 1. Reproduction of Meloidogyne incognita on harpin-treated plants in a growth chamber.
	Eggs/Root System	Eggs/Adult Femalez
Harpin (Messenger®)	1,690 b	149 b
Control	22,152 a	318 a
zBased on egg counts from 10 individual egg masses.

Table 2. Lint yield and Rotylenchulus reniformis population density 30 days after planting.
	Nematodes/500 cm3	Lint
		-- kg/ha --
Insecticide alone	2,989 a	1,199 b
Aeris	2,556 a	1,245 ab
Temik (840 g/ha)	2,500 a	1,357 a

Figure Legends

Fig. 1. Cotton lint yield in Arkansas cotton field trials (2000-2004) with and without soil fumigation with Telone II at 29 liters/ha.

Fig. 2. Cotton lint yield in small plot field tests (2005 and 2006) with or without Temik application at planting at 5.6 kg/ha. Letters indicate individual field sites.

Fig. 3. Comparison of abamectin seed treatment and Temik soil treatment in microplots. A. Number of Meloidogyne incognita in roots of cotton at 12 days after planting. B. Gall ratings at harvest.

Figure 1.

Figure 2.

2006

2005

F

A igure 3.

B