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Cotton

Potential Interaction of Pendimethalin and Systemic Insecticides for Thrips Control in Cotton

^a Univ. of Georgia, College of Agric. and Environ. Sci., P.O. Box 748, Tifton, GA 31794
^b Univ. of Georgia, College of Agric. and Envrion. Sci., 1109 Experiment St., Griffin, GA 30223

^* Corresponding author (tgrey{at}uga.edu)

Received for publication May 16, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Pendimethalin (N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine) can be preemergence (PRE) applied to cotton (Gossypium hirsutum L.) up to 2 d after planting (DAP). Delayed application in combination with excessive moisture (rainfall or irrigation) can result in seedling injury. The systemic insecticides aldicarb [(2-methyl-2-(methylthio)propanal O-[(methylamino)carbonyl]oxime] and imidacloprid [(EZ)-1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] are used at planting for thrips (Frankliniella fusca Hinds) control. Experiments were conducted in Georgia to determine if pendimethalin injury to cotton would reduce insecticide efficacy and thus lead to increased thrips injury and subsequent reduced cotton growth or delayed maturity. Treatments included three insecticide treatments; none, imidacloprid at 0.044 kg ha^–1 a.i., and aldicarb 15G at 0.8 kg ha^–1 a.i., in combination with five herbicide applications; none and pendimethalin at 1.1 and 2.2 kg ha^–1 a.i. applied either PRE or 2 DAP. Variables measured included thrips infestation, stand count, leaf area, root length, stem length, dry weights, nodes above white flower, first position boll retention, and seed cotton yield. Results indicate significant differences in early season cotton growth relative to herbicidal injury and lack of insecticidal thrips control. However, there was no significant insecticide x herbicide interactions, indicating no apparent loss of insecticide efficacy after pendimethalin injury.

Abbreviations: DAP, days after planting • 2 DAP, 2 days after planting preemergence • NAWF, nodes above white flower • PRE, preemergence

	INTRODUCTION

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COTTON PRODUCTION in the USA includes the use of preemergence dinitroaniline herbicides and at-plant systemic insecticides as independent pesticide applications. However, autonomous effects from these pesticides may lead to simultaneous influences that are not anticipated. Pesticide interactions for cotton have been previously noted. Fungicide x insecticide treatments caused an interaction that decreased cotton boll counts (Monks et al., 1996). However, that research indicated that herbicide, fungicide, and insecticides did not interact to change overall cotton production strategies in terms of pesticide inputs or cotton maturity and yield. In contrast, herbicide x fungicide interactions reduced plant stand for combinations of the herbicides norflurazon, pendimethalin, fluometuron, prometryn, fomesafen, and oxyfluorfen with the fungicides tolclofos-methyl, pencycuron, carboxin, flutolanil, metalaxyl, and chloroneb (Moustafa-Hahmoud et al., 1993). Herbicide injury reduced fungicide effectiveness for control of seedling diseases resulting in cotton stand losses.

Disulfoton and phorate applied in combination with aldicarb protected cotton from clomozone injury (York and Jordan, 1992). This was attributed to reduced clomazone metabolite formation, which was associated with cotton injury (Culpepper et al., 2001). Pyrothiobac x aldicarb interaction was not significant for any measured cotton parameter including height, leaf area, or nodes above white flower (Costello et al., 1999). Significant interactions occurred for early season herbicide injury and tobacco thirps injury resulting in peanut (Arachis hypogaea L.) yield and quality loss (Brecke et al., 1996). Previous research has investigated potential pesticide interactions in cotton for residual herbicides and nutrient uptake (Gordon and Green, 1999) and dinitroaniline herbicides and soil-applied insecticides (Treacy et al., 1989). The dinitroaniline herbicidal effect on cotton seedlings has been widely documented and reviewed in earlier research (Keeling et al., 1996; Keeling and Abernathy, 1989).

Trifluralin, a dinitroaniline herbicide, causes increased cotton injury as compared with pendimethalin in the form of reduced cotton root dry weight and lateral root development (Pavlista, 1980). This is attributed to the greater ability of cotton to detoxify pendimethalin; consequently, less pendimethalin is present in the plant to cause inhibition (Shaner et al., 1998). Varying the rate of trifluralin or pendimethalin resulted in no significant differences for P concentration in cotton plants under field conditions (Gordon and Green, 1999). Cathey and Sabbe (1972) determined that P concentration in cotton was placement dependent when trifluralin and P fertilizer were simultaneously applied to different levels of the soil. Reductions in cotton P concentration occurred when the trifluralin was placed at the same level as the fertilizer (Cathey and Sabbe, 1972). Because variations in absorption of soil-applied fertilizers have been documented with dinitroaniline herbicide use, absorption of systemic insecticides applied at planting could also be influenced. Thus, root suppression by dinitroaniline herbicides could result in reduced uptake of soil-applied insecticides.

Aldicarb and imidacloprid are systemic insecticides applied at planting for control of early season thrips, aphids (Aphis gossypii Glover), and other insects. Aldicarb is most commonly applied in-furrow at planting whereas imidacloprid is applied as a seed treatment. The water solubility of aldicarb is 4.93 g L^–1 (Tomlin, 2003a) and imidacloprid is 0.61 g L^–1 (Tomlin, 2003b). Both are absorbed by the developing seedling, thus providing systemic control of insects; however, imidacloprid at currently labeled rates is not as effective as aldicarb (Duyn et al., 1998; Faircloth et al., 2002).

In a review about early season thrips in cotton, Lohmeyer et al. (2003) indicated that feeding injury causes yellowing, stunting, and overall plant decline. On an annual basis, control measures for thrips cost $81 million with more than 1 million kg of aldicarb being applied at planting on 3 million ha of cotton in the USA. Furthermore, 93% of the crop was infested with thrips causing an estimated loss of 235 996 bales in the USA (Lohmeyer et al., 2003). Variable plant establishment, yield, and quality results are also associated with thrips injury in cotton (Sadras and Wilson, 1998; Faircloth et al., 2002). Cotton stand reduction from thrips injury also has a variable effect on yield loss (Micinski et al., 1990).

Pesticide interactions can be variable with respect to measures of plant growth and yield. In a factorially arranged study for aldicarb and trifluralin plus diruron, Micinski (1985) reported no interactions of these pesticides on stand, plant weight, or cotton yield. In another study, injury from trifluralin resulted in decreased uptake of phorate and terbufos by seedling cotton (Treacy et al., 1989). Significant interactions have been observed for early season vernolate and paraquat injury and tobacco thrips injury in peanut, resulting in yield and quality loss (Brecke et al., 1996).

Pendimethalin is applied preemergence (PRE) or preplant incorporated to approximately 30% of Georgia cotton for control of grasses and small seeded dicot weed species (NASS, 2003). In 2001 the annual estimate of pendimethalin applied for agricultural uses in the USA was 6.8 to 8.6 million kg of active ingredient (USEPA, 2004). Among the dinitroaniline herbicides, pendimethalin has greater water solubility [0.275 µg mL^–1; WSSA, 2002)] and lower volatility (Wilcut et al., 1988), which makes it more conducive for conservation tillage crop production systems. Conservation and reduced tillage cotton has increased in the southeastern USA in recent years (Johnson et al., 2001), and comprised 29.3% of the U.S. cotton land area in 2002 (CTIC, 2002).

Cotton tolerance to pendimethalin is thought to be due to differences in metabolism and sequestration of pendimethalin in the lysigenous glands (Shaner et al., 1998). Pendimethalin mode of action in susceptible species is inhibition of mitotic cell division in developing root systems (WSSA, 2002). Row crops either grow through (Gordon and Green, 1999) or are planted below the treated zone of soil while susceptible weed species are controlled (Keeling et al., 1996; Keeling and Abernathy, 1989). Pendimethalin is registered for PRE application up to 2 d after cotton planting. However, delayed application in combination with excessive moisture (rainfall or irrigation) can result in injury to seedling cotton (Brown and Culpepper, 2000). Pendimethalin injury to cotton seedlings results in delayed hypocotyl development and can also cause abnormal root growth. This injury is commonly associated with enlarged lower stems and "bottle brush" root development, which may inhibit insecticide uptake.

The objectives of this research were to determine if pendimethalin induced cotton injury could lead to reduced insecticide efficacy and quantify the effects these factors have on cotton growth and development. The intent was to determine if the rate and timing of pendimethalin application in combination with different insecticides affected the crops ability to compensate for thrips and/or herbicide detrimental effects.

	MATERIALS AND METHODS

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Experiments were conducted during 2000 and 2001 at the Southwest Georgia Branch Experiment Station located near Plains, GA, in separate areas of the same field. Soil was a Faceville sandy loam (clayey, kaolinitic, thermic Typic Kandiudults) with 71% sand, 13% silt, and 16% clay. Organic matter and pH were 1.1% and 6.1 for 2000 and 1.0% and 6.0 for 2001. The cotton cultivar Roundup Ready-Bollgard ST 4892 RRBG was planted 3 cm deep at 20 seed rows m^–1 using a vacuum planter. Planting dates were 9 May 2000 and 3 May 2001. Individual plots were 4 rows 91-cm wide by 7.6 m long. Irrigation was applied when necessary after trial initiation (Table 1). Plots were maintained weed free by as-needed applications of glyphosate either sprayed topically or post-directed, depending on the cotton growth stage, cultivation, and weeding by hand. Otherwise, standard culture practices for cotton were followed using University of Georgia Extension recommendations (Bridges, 2001). Treatments included three insecticide treatments; none, imidacloprid at 0.044 kg ha^–1 a.i., and aldicarb 15G at 0.8 kg ha^–1 a.i., in combination with five herbicide applications; none and pendimethalin at 1.1 and 2.2 kg ha^–1 a.i. applied either PRE or 2 DAP. The standard use rate of pendimethalin on these soils is 1.1 kg ha^–1.

Thrips were sampled 14, 21, 28, and 35 DAP by randomly selecting 5 plants per plot and immediately immersing and swirling plants in a specimen container containing 70% ethyl alcohol. Adult and immature thrips were counted using a dissecting microscope. Beginning 2 wk after planting, stand counts were taken on the entire length of the second row in each plot, and 10 consecutive plants from each plot were excavated by hand from rows one or four. Plants were excavated by using water from a wash bottle to wet a 5 cm area of soil (to slurry) around the plant, then extracting the entire wetted area with a spatula. Roots were washed free of soil. Leaves were removed by hand and total leaf area per plant determined with an area meter (LI-3100; LICOR, Lincoln, NE) (Bednarz et al., 2000). Stem and root lengths were measured on seedlings cut at the soil line. Stems, leaves, and roots from each plant were dried at 50°C for 48 h and dry weight for each determined. This entire plant sampling routine was conducted at 14, 28, and 42 DAP.

In mid-August at 80 DAP, nodes above white flower, and plant height measurements were taken. First position boll retention was also determined. Cotton was defoliated at optimum maturity and the center two rows were mechanically harvested and seed cotton yield determined.

Data were subjected to analysis of variance appropriate for the 3 (insecticide) x 5 (herbicide) factorial treatment arrangement. Analysis of variance procedures were conducted with the PROC GLM procedure in SAS (SAS Institute, 1999). Means for significant main effects and interactions were separated using Fisher's protected LSD method test at P 0.05.

	RESULTS

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Significant treatment x year interactions prevented the data from being analyzed across tests. Therefore, data are presented separately for each year. The two-way interactions between insecticide and herbicide were not significant for any variable either year. Therefore, data for the main effects of insecticide were combined and analyzed across herbicide treatments, and data for the main effects for herbicide were combined and analyzed across insecticide treatments.

Plant growth was less in 2001 compared with 2000 for the same time frame after planting due to environmental differences between years and thrips infestation levels. Total rainfall plus irrigation for May were similar in 2000 and 2001 with 11.2 and 9.3 cm, respectively (Table 1). From 27 to 30 June 2001 rainfall exceeded 12 cm. However, this was after the 42 DAP sampling date. The remaining 18 cm of rainfall was distributed across the other 26 d of June 2001 (Hoogenboom, 2005).

For all tests, more than 95% of the adult thrips sampled and counted were tobacco thrips [Frankliniella fusca (Hinds)]. Fewer than 5% of the adult thrips collected were western flower thrips [Frankliniella occidentalis (Pergrande)] (Lohmeyer et al., 2003). Soil samples taken at the time of the study indicated there were no nemotodes present.

Insecticides
Only immature thrips data are presented (Table 2). The presence of immature thrips is a more accurate measure of insecticide failure or loss of efficacy compared with adults (Slosser, 1993). Thrips populations exceeded the recommended threshold of 2 to 3 thrips plant^–1 each year in the nontreated plants (Bridges, 2001). Immature thrips populations were lower in both aldicarb and imidacloprid treatments compared with the no insecticide treatments at 14 and 21 DAP. Additionally, aldicarb reduced immature thrips compared with imidacloprid at 14 DAP during 2000 and 21 DAP during 2001, similar to previous research (Faircloth et al., 2002). No differences were observed between insecticide treatments at 28 and 35 DAP except for aldicarb reducing immature thrips compared with no insecticide and imidacloprid at 28 DAP during 2001.

View this table: [in this window] [in a new window]   Table 2. Number of thrips larvae per plant as influenced by herbicide and insecticide application conducted for 2 yr at Plains, GA.

View this table: [in this window] [in a new window]   Table 3. Root length and dry weight as influenced by herbicide and insecticide application conducted for 2 yr at Plains, GA.

View this table: [in this window] [in a new window]   Table 4. Leaf area and dry weight as influenced by herbicide and insecticide application conducted for 2 yr at Plains, GA.

View this table: [in this window] [in a new window]   Table 5. Stem length and dry weight as influenced by herbicide and insecticide application conducted for 2 yr at Plains, GA.

View this table: [in this window] [in a new window]   Table 6. Nodes above white flower, boll number, and yield as influenced by herbicide and insecticide application conducted for 2 yr at Plains, GA.

Herbicides
Pendimethalin PRE at either rate on the day of planting did not affect thrips numbers at 14 or 21 DAP as compared with the nontreated check for either year and for 28 and 35 DAP in 2001 (Table 2). Although there were differences in thrips numbers in 2000 at 28 and 35 DAP between herbicide treatments, there was no apparent trend for pendimethalin rate or timing.

No significant differences in plant populations occurred between herbicide treatments at 14 or 28 DAP (data not shown). Typical pendimethalin injury was observed, but this did not effect plant establishment as has been previously noted (Keeling et al., 1996).

At 14 DAP, root length was not affected by the PRE treatments. Pendimethalin at 2.2 kg ha^–1 at 2 DAP reduced root length in 2000 (Table 3). However, at 28 and 42 DAP root lengths did not differ for pendimethalin treatment vs. the nontreated. There were no differences for root length for pendimethalin rate or timing in 2001.

In contrast to root length, root dry weight reductions occurred with pendimethalin treatments (Table 3). During 2000, the 2.2 kg rate of pendimethalin reduced root dry weight by 7 and 32% for the PRE treatments and 21 and 45% for the 2 DAP treatments as compared with the nontreated plants at 14 and 28 DAP, respectively. For 2001, all pendimethalin treatments, except 2.2 kg ha^–1 applied PRE, reduced root dry weight at 28 DAP compared with the nontreated. At 42 DAP root weight was not different among treatments in 2000, but in 2001, pendimethalin at 2.2 kg ha^–1 applied 2 DAP reduced root dry weight as compared with the other treatments. These data indicate that delaying pendimethalin at 1.1 or 2.2 kg ha^–1 until 2 DAP can result in significant reduction in root dry matter production.

Leaf number (data not shown), area and dry weight were not affected by treatments at 14 DAP in either year (Table 4). In 2000 and 2001, leaves per plant were reduced by pendimethalin at 2.2 kg ha^–1 compared with no herbicide and the 1.1 kg ha^–1 rate when at 28 DAP (data not shown). All pendimethalin treatments had fewer leaves per plant compared with no herbicide at 42 DAP in 2001. Leaf area per plant and leaf dry weight per plant reflected reduction in leaf production (Table 4). Leaf area and weight in 2000 were lower in the 2.2 kg rates of both the PRE and 2-DAP treatments at 28 DAP, but these variables were not significantly different at 42 DAP. In 2001 leaf area and weight of only pendimethalin at 2.2 kg ha^–1 applied 2 DAP was significantly less than the nontreated when measured at 28 and 42 DAP.

Stem length differences were further reflective of pendimethalin treatment injury. Beginning at 14 DAP, 2 DAP treatment resulted in plants that were 0.5 to 1.0 cm shorter as compared with the nontreated in 2000 and 2001 (Table 5). By 28 DAP for both experiments the PRE pendimethalin treatments were equivalent to the nontreated. At 28 DAP pendimethalin at 2.2 kg ha^–1 applied 2 DAP stem length was 7.8 and 3.9 cm plant^–1 in 2000 and 2001, respectively. The cotton grew very little from 14 to 28 DAP in 2001 because of cloudy, rainy conditions. It is interesting to note that stem length increased by 1.5 to 2 cm plant^–1 for the nontreated and pendimethalin PRE treatments. In contrast, the pendimethalin 2.2 kg ha^–1 rate 2 DAP application remained similar at 4.8 to 3.9 cm plant^–1 at 14 to 28 DAP, respectively. This indicates that pendimethalin at 2.2 kg ha^–1 applied after germination initiation resulted in reduced hypocotyl growth (Brown and Culpepper, 2000). Stem dry weight per plant was reflective of stem length responses (Table 5).

The NAWF measurements at 80 DAP were similar for all treatments in both years (Table 6). The percentage of first position bolls retained on Nodes 4 to 10 and 11 to 16 were similar for all treatments (Table 6). No differences were detected for height in 2000, whereas height of cotton 80 DAP in 2001 reflected early season stunting. All pendimethalin-treated cotton had height reductions of 8 to 12 cm vs. the nontreated in 2001 (Table 6). No differences in seed cotton yield occurred among herbicide treatments in 2000 (Table 6). In 2001 cotton yield of the PRE and low rate of the 2 DAP treatments were not lower than the nontreated plants. Cotton yield at the 2.2 kg rate at 2 DAP was lower (by 29.3% less) than the nontreated plants and also was lower than the PRE-treated plants. For the 1.1 kg rate at 2 DAP, cotton yield was intermediate between the nontreated and 2.2 kg rate at 2 DAP treatment.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
These results suggest that the use of systemic insecticides reduces growth and development associated with thrips feeding injury. Aldicarb provided improved control of thrips up to 4 wk after planting when compared with imidacloprid seed treatment, and this prevented significant yield loss. Imidacloprid at the currently labeled rate used in this study was less effective in preventing thrips injury and did not prevent yield losses due to thrips injury. Higher rates of imidacloprid are currently being evaluated, which may provide better thrips control than the currently labeled rate (unpublished data, 2004). Previous research has indicated that cotton can compensate following insect damage and defoliation (Lei and Wilson, 2004; Sadras and Wilson, 1998; Terry, 1992). For the study reported herein, cotton could not overcome the detrimental effects of early season thrips injury where no insecticide was applied, which has been previously noted (Duyn et al., 1998; Slosser, 1993).

As with previous studies investigating pyrithiobac x aldicarb interactions (Costello et al., 1999), there were no pendimethalin x insecticide interactions for any measured cotton parameter including leaf area, stem and root length, dry weight, nodes above white flower, and yield. Pendimethalin applied as a PRE treatment up to two times the labeled rate had little effect on cotton seedling establishment, growth, and yield in either year. When delaying application until 2 DAP, higher rate (2.2 kg ha^–1) caused substantial seedling injury as measured by reduced root and plant weight, leaf area, and plant height. This injury resulted in reduced cottonseed yield in 1 yr. The recommended rate of 1.1 kg ha^–1 applied 2 DAP also reduced seedling growth and dry matter accumulation but did not consistently reduce yield. Nevertheless, these results indicate that even when applied at the recommended rate, early season injury from applying pendimethalin 2-DAP could result in cotton yield reductions. Delays beyond 2-DAP presumably would increase the likelihood of seedling injury from pendimethalin.

Stand establishment was not significantly affected by insecticide and herbicide applications. In contrast to interaction studies of herbicides with fungicides where stand reduction occurred (Moustafa-Hahmoud et al., 1993; Sumner et al., 1995), our data indicate that pendimethalin x insecticide interactions did not occur. Thus, pendimethalin use and injury did not affect the efficacy of either aldicarb or imidacloprid in controlling thrips nor did pendimethalin affect the seedling response to thrips injury. Thrips injury in cotton is often noted as visually dramatic deformed leaves (Lei and Wilson, 2004). Thrips injury would be expected to cause proportional reductions in seedling growth and yield loss in healthy and pendimethalin-injured cotton. This information will be useful in making decisions concerning post-emergence thrips control if at-planting insecticide treatments fail to provide acceptable levels of control.

	ACKNOWLEDGMENTS

 
We recognize the technical contribution of W. Robert Slaughter, Jr. and statistical analysis by Jerry W. Davis.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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