Synthetic Auxin Herbicides Research Articles

Physiological Basis for Differential Selectivity of Four Grass Species to Aminocyclopyrachlor

Aminocyclopyrachlor (AMCP) is a synthetic auxin herbicide used for broadleaf weed control in pasture and rangeland. The tolerance and fate of AMCP within pertinent grass species is not well understood. Research was conducted to establish the tolerance of four grass species to AMCP application and observe their absorption, translocation, and metabolism. Results indicate that tall fescue is the most tolerant of AMCP at rates required for weed control. Bahiagrass and bermudagrass are marginally tolerant, and cogongrass is the most sensitive. Tall fescue and bahiagrass absorbed more AMCP than bermudagrass and cogongrass, but cogongrass absorption is the most rapid and complete within 2 days after treatment (DAT). Cogongrass and bermudagrass translocated the least amount out of the target area, whereas bahiagrass and tall fescue translocated the most. Radioisotope imaging revealed that tall fescue may sequester absorbed AMCP in leaf tips. This sequestering may be the basis of the greater tolerance to AMCP by tall fescue relative to the other species evaluated. No metabolism of AMCP was detected in any grass species out to 42 DAT.

Herbicide Program Approaches for Managing Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus tuberculatusandAmaranthus rudis) in Future Soybean-Trait Technologies

Herbicide-resistantAmaranthusspp. continue to cause management difficulties in soybean. New soybean technologies under development, including resistance to various combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, and mesotrione, will make possible the use of additional herbicide sites of action in soybean than is currently available. When this research was conducted, these soybean traits were still regulated and testing herbicide programs with the appropriate soybean genetics in a single experiment was not feasible. Therefore, the effectiveness of various herbicide programs (PRE herbicides followed by POST herbicides) was evaluated in bare-ground experiments on glyphosate-resistant Palmer amaranth and glyphosate-resistant waterhemp (both tall and common) at locations in Arkansas, Illinois, Indiana, Missouri, Nebraska, and Tennessee. Twenty-five herbicide programs were evaluated; 5 of which were PRE herbicides only, 10 were PRE herbicides followed by POST herbicides 3 to 4 wks after (WA) the PRE application (EPOST), and 10 were PRE herbicides followed by POST herbicides 6 to 7 WA the PRE application (LPOST). Programs with EPOST herbicides provided 94% or greater control of Palmer amaranth and waterhemp at 3 to 4 WA the EPOST. Overall, programs with LPOST herbicides resulted in a period of weed emergence in which weeds would typically compete with a crop. Weeds were not completely controlled with the LPOST herbicides because weed sizes were larger (≥ 15 cm) compared with their sizes at the EPOST application (≤ 7 cm). Most programs with LPOST herbicides provided 80 to 95% control at 3 to 4 WA applied LPOST. Based on an orthogonal contrast, using a synthetic-auxin herbicide LPOST improves control of Palmer amaranth and waterhemp over programs not containing a synthetic-auxin LPOST. These results show herbicides that can be used in soybean and that contain auxinic- or HPPD-resistant traits will provide growers with an opportunity for better control of glyphosate-resistant Palmer amaranth and waterhemp over a wide range of geographies and environments.

Effects of the herbicide dicamba on nontarget plants and pollinator visitation.

Nearly 80% of all pesticides applied to row crops are herbicides, and these applications pose potentially significant ecotoxicological risks to nontarget plants and associated pollinators. In response to the widespread occurrence of weed species resistant to glyphosate, biotechnology companies have developed crops resistant to the synthetic-auxin herbicides dicamba and 2,4-dichlorophenoxyacetic acid (2,4-D); and once commercialized, adoption of these crops is likely to change herbicide-use patterns. Despite current limited use, dicamba and 2,4-D are often responsible for injury to nontarget plants; but effects of these herbicides on insect communities are poorly understood. To understand the influence of dicamba on pollinators, the authors applied several sublethal, drift-level rates of dicamba to alfalfa (Medicago sativa L.) and Eupatorium perfoliatum L. and evaluated plant flowering and floral visitation by pollinators. The authors found that dicamba doses simulating particle drift (≈1% of the field application rate) delayed onset of flowering and reduced the number of flowers of each plant species; however, plants that did flower produced similar-quality pollen in terms of protein concentrations. Further, plants affected by particle drift rates were visited less often by pollinators. Because plants exposed to sublethal levels of dicamba may produce fewer floral resources and be less frequently visited by pollinators, use of dicamba or other synthetic-auxin herbicides with widespread planting of herbicide-resistant crops will need to be carefully stewarded to prevent potential disturbances of plant and beneficial insect communities in agricultural landscapes.

Effectiveness of Herbicides for Control of Hairy Vetch (Vicia villosa) in Winter Wheat

A field experiment was conducted in 2009–2010 at Pennsylvania and Maryland locations, and repeated it in 2010–2011 to test the effectiveness of POST-applied herbicides at fall and spring timings on seeded hairy vetch in winter wheat. A total of 16 herbicide treatment combinations was tested that included synthetic auxins, acetolactate synthase (ALS) inhibitors, and a protoporphyrinogen oxidase inhibitor. Spring applications tended to be more effective than fall applications. Among synthetic auxins, clopyralid (105 g ae ha−1) and treatments containing dicamba (140 g ae ha−1) were effective at both timings, resulting in greater than 90% hairy vetch control at wheat harvest. Pyroxsulam and prosulfuron applied at 18 g ai ha−1provided the most effective hairy vetch control (> 90%) at both application timings among ALS inhibitors. Spring applications of several herbicides provided moderate (> 80%) to high (> 90%) levels of hairy vetch control, including: 2,4-D amine (140 g ae ha−1), mesosulfuron-methyl (15 g ai ha−1), tribenuron-methyl (13 g ai ha−1), and thifensulfuron/tribenuron-methyl treatments (16 and 32 g ai ha−1). Winter wheat injury was evaluated, but symptoms were negligible for most treatments. Winter wheat yields declined with increasing hairy vetch biomass. Fall herbicides may be prioritized to reduce hairy vetch competition during the fall and early spring growing season. Our research has established that several synthetic auxin and ALS-inhibiting herbicides, applied POST in fall or spring, can be safely used in winter wheat to control hairy vetch in an integrated weed management program.

Relationship between Visual Injury from Synthetic Auxin and Glyphosate Herbicides and Snap Bean and Potato Yield

Agronomic crops with resistance to the herbicides dicamba and 2,4-D are currently in the regulatory approval process. The potential increased use of these herbicides has raised concern among vegetable producers about potential off-target movement and implications to crop yield. The overall goal of this research was to describe the relationship between visually estimated crop injury and snap bean and potato yield and quality. In snap bean in 2011, injury from dicamba 7 d after treatment (DAT) ranged from 19% at the 1.2 g ae ha−1application rate to 45% at the 7.0 g ae ha−1application rate. By 28 DAT in 2011, injury from 2,4-D was similar to the nontreated control. However, early-season injury in 2011 delayed snap bean flowering and reduced crop yield compared to the nontreated control for all treatments except where the 1.4 g ae ha−1rate of 2,4-D and glyphosate at 7.0 g ae ha−1were applied. Snap bean injury from dicamba was greater than that from 2,4-D at all rating timings in 2011 and two of three rating timings in 2012, and crop yield was reduced compared to where 2,4-D was applied and the nontreated control in both years. Potato tuber size distribution was variable and total yield did not differ among treatments and the nontreated control in 2011. In 2012, tuber size distribution was again variable, but more nonmarketable cull potatoes were harvested when dicamba was applied to 25-cm potato plants at the 7.0 g ae ha−1rate compared to any other treatment. Snap bean injury observations about 3 wk prior to harvest were strongly correlated with crop yield (r= −0.84 and −0.88 in 2011 and 2012, respectively), allowing time to make informed harvest decisions relative to crop quality. In contrast, the relationship between potato injury and tuber yield was poor and highly variable in both years.

Influence of Application Timings and Sublethal Rates of Synthetic Auxin Herbicides on Soybean

Synthetic auxin herbicides have long been utilized for the selective control of broadleaf weeds in a variety of crop and noncrop environments. Recently, two agrochemical companies have begun to develop soybean with resistance to 2,4-D and dicamba which might lead to an increase in the application of these herbicides in soybean production areas in the near future. Additionally, little research has been published pertaining to the effects of a newly-discovered synthetic auxin herbicide, aminocyclopyrachlor, on soybean phytotoxicity. Two field trials were conducted in 2011 and 2012 to evaluate the effects of sublethal rates of 2,4-D amine, aminocyclopyrachlor, aminopyralid, clopyralid, dicamba, fluroxypyr, picloram, and triclopyr on visible estimates of soybean injury, height reduction, maturity, yield, and yield components. Each of these herbicides was applied to soybean at the V3 and R2 stages of growth at 0.028, 0.28, 2.8, and 28 g ae ha−1. Greater height reductions occurred with all herbicides, except 2,4-D amine and triclopyr when applied at the V3 compared to the R2 stage of growth. Greater soybean yield loss occurred with all herbicides except 2,4-D amine when applied at the R2 compared to the V3 stage of growth. The only herbicide applied that resulted in no yield loss at either stage was 2,4-D amine. When applied at 28 g ae ha−1at the V3 stage of growth, the general order of herbicide-induced yield reductions to soybean from greatest to least was aminopyralid > aminocyclopyrachlor = clopyralid = picloram > fluroxypyr > triclopyr > dicamba > 2,4-D amine. At the R2 stage of growth, the general order of herbicide-induced yield reductions from greatest to least was aminopyralid > aminocyclopyrachlor = picloram > clopyralid > dicamba > fluroxypyr = triclopyr > 2,4-D amine. Yield reductions appeared to be more correlated with seeds per pod than to pods per plant and seed weight. An 18- to 26-d delay in soybean maturity also occurred with R2 applications of all synthetic auxin herbicides at 28 g ae ha−1except 2,4-D. Results from this research indicate that there are vast differences in the relative phytotoxicity of these synthetic auxin herbicides to soybean, and that the timing of the synthetic auxin herbicide exposure will have a significant impact on the severity of soybean height and/or yield reductions.

Persistence and Bioavailability of Aminocyclopyrachlor and Clopyralid in Turfgrass Clippings: Recycling Clippings for Additional Weed Control

The synthetic auxin herbicides, aminocyclopyrachlor and clopyralid, control dicotyledonous weeds in turf. Clippings of turfgrass treated with synthetic auxin herbicides have injured off-target plants exposed to herbicide-laden clippings. Labels of aminocyclopyrachlor and clopyralid recommend that clippings of treated turfgrass remain on the turf following a mowing event. Alternative uses for synthetic auxin-treated turfgrass clippings are needed because large quantities of clippings on the turf surface interfere with the functionality and aesthetics of golf courses, athletic fields, and residential turf. A white clover bioassay was conducted to determine the persistence and bioavailability of aminocyclopyrachlor and clopyralid in turfgrass clippings. Aminocyclopyrachlor and clopyralid were each applied at 79 g ae ha−1 to mature tall fescue at 56, 28, 14, 7, 3.5, and 1.75 d before clipping collection (DBCC). Clippings were collected, and the treated clippings were recycled onto adjacent white clover plots to determine herbicidal persistence and potential for additional weed control. Clippings of tall fescue treated with aminocyclopyrachlor produced a nonlinear regression pattern of response on white clover. Calculated values for 50% response (GR50) for visual control, for normalized difference vegetative index (NDVI), and for reduction in harvested biomass were 20.5, 17.3, and 18.7 DBCC, respectively, 8 wk after clippings were applied. Clippings of tall fescue treated with clopyralid did not demonstrate a significant pattern for white clover control, presumably because clopyralid was applied at a less-than-label rate. The persistence and bioavailability of synthetic auxin herbicides in clippings harvested from previously treated turfgrass creates the opportunity to recycle clippings for additional weed control.

Grain sorghum response and Palmer amaranth control with postemergence application of fluthiacet-methyl

Palmer amaranth is a problematic weed in grain sorghum production in central United States. Due to limited herbicide options available and ever increasing herbicide-resistant weed species, there is a demand for new mode-of-action herbicides for use in grain sorghum. Fluthiacet-methyl is a relatively new active ingredient that inhibits the enzyme protoporphyrinogen oxidase in target plants. Field studies were conducted at three sites in central United States in 2010 and 2011 to evaluate crop response and Palmer amaranth control with postemergence application of fluthiacet-methyl in grain sorghum. Treatments included fluthiacet-methyl at 4.8 and 7.2 g active ingredient (a.i.) ha−1 alone and tank-mixed with 2,4-D amine at 260 g acid equivalent (a.e.) ha−1 or atrazine at 840 g a.i. ha−1. Carfentrazone at 8.8 g a.i. ha−1, atrazine at 840 g ha−1, and a non-treated control were also included. Fluthiacet-methyl treatments caused 9–38% crop injury at 4 ± 1 days after treatment. Tank-mixing atrazine with fluthiacet-methyl seldom affected crop injury, while mixing 2,4-D with fluthiacet-methyl often reduced crop injury. Generally, injury caused by fluthiacet-methyl alone or in combination with atrazine or 2,4-D disappeared within 3 weeks after treatment. Grain yields were reduced in one trial, when 2,4-D mixed with 4.8 or 7.2 g ha−1 of fluthiacet-methyl caused 18% and 13% plant lodging and 24% and 14% grain yield loss, respectively. Across site-years, fluthiacet-methyl alone at 4.8 or 7.2 g ha−1 provided 55–95% control of Palmer amaranth. Greater Palmer amaranth control (≥75%) with fluthiacet-methyl alone was achieved when weeds were small or density was low at the time of spraying. Tank-mixing atrazine with fluthiacet-methyl increased Palmer amaranth control and sorghum yields considerably. Tank-mixing 2,4-D with fluthiacet-methyl also increased Palmer amaranth control, but to lesser extent and less consistently than with atrazine. Results indicated that fluthiacet-methyl has potential for use in grain sorghum to combat weeds resistant to acetolactase synthase-inhibitors, triazines, and synthetic auxin herbicides. Tank-mixing atrazine or 2,4-D with fluthiacet-methyl is desirable for effective Palmer amaranth control.

Synthetic Auxin Herbicides Control Germinating Scotch Broom (Cytisus scoparius)

Scotch broom is a large, nonnative shrub that has invaded forests and grasslands in 27 U.S. states. Without treatment, Scotch broom's persistent seedbank ensures a continuing source of regeneration after soil disturbance. In growth chamber studies, five rates of three synthetic auxin herbicides, aminocyclopyrachlor (AC), aminopyralid (AP), and clopyralid (CP), were compared for PRE control of Scotch broom. Cumulative 90-d emergence, mortality, and biomass of seedlings did not vary among herbicides, averaging 42% of seeds sown, 75% of emerged seedlings, and 9 mg seedling−1 for treated containers, respectively, versus 46%, 17%, and 26 mg seedling−1 for nontreated containers. Low rates of each herbicide (< 50% of the maximum labeled rate [MLR]) provided 60 to 80% control, whereas 100% MLR provided 69 to 89% control. Although the herbicides differed in cost per unit seedling mortality (AP < CP < AC), each demonstrated strong potential as a viable treatment for PRE control of Scotch broom.

Herbicides for Postemergence Control of Mile-a-Minute (Mikania micrantha)

AbstractGreenhouse experiments were conducted to evaluate mile-a-minute response to selected herbicides. In the first experiment, mile-a-minute response was evaluated following the application of aminocyclopyrachlor, aminopyralid, fluroxypyr, glufosinate, glyphosate, imazamox, imazapic, metsulfuron, penoxsulam, and triclopyr. Applications of aminocyclopyrachlor, aminopyralid, fluroxypyr, glufosinate, glyphosate, and triclopyr all resulted in mile-a-minute control 70% or greater 8 wk after treatment (WAT). Mile-a-minute sensitivity to these six herbicides was further evaluated in a dose-response study. Although 90% growth reduction (GR90) values were determined for aminopyralid and glyphosate, statistically significant results were not obtained for aminocyclopyrachlor, fluroxypyr, glufosinate, and triclopyr. The results of these experiments suggest that glyphosate, as well as the synthetic auxin herbicides aminocyclopyrachlor, aminopyralid, fluroxypyr, and triclopyr, should be further evaluated for mile-a-minute control in south Florida.

A Meta-Analysis on the Effects of 2,4-D and Dicamba Drift on Soybean and Cotton

Commercial introduction of cultivars of soybean and cotton genetically modified with resistance to the synthetic auxin herbicides dicamba and 2,4-D will allow these compounds to be used with greater flexibility but may expose susceptible soybean and cotton cultivars to nontarget herbicide drift. From past experience, it is well known that soybean and cotton are both highly sensitive to low-dose exposures of dicamba and 2,4-D. In this study, a meta-analysis approach was used to synthesize data from over seven decades of simulated drift experiments in which investigators treated soybean and cotton with low doses of dicamba and 2,4-D and measured the resulting yields. These data were used to produce global dose–response curves for each crop and herbicide, with crop yield plotted against herbicide dose. The meta-analysis showed that soybean is more susceptible to dicamba in the flowering stage and relatively tolerant to 2,4-D at all growth stages. Conversely, cotton is tolerant to dicamba but extremely sensitive to 2,4-D, especially in the vegetative and preflowering squaring stages. Both crops are highly variable in their responses to synthetic auxin herbicide exposure, with soil moisture and air temperature at the time of exposure identified as key factors. Visual injury symptoms, especially during vegetative stages, are not predictive of final yield loss. Global dose–response curves generated by this meta-analysis can inform guidelines for herbicide applications and provide producers and agricultural professionals with a benchmark of the mean and range of crop yield loss that can be expected from drift or other nontarget exposures to 2,4-D or dicamba.

Volatility of Broadleaf Weed Herbicides When Applied to Turfgrass

Laboratory and field studies were conducted in 2010 and 2011 to evaluate the volatility of commonly used turfgrass broadleaf herbicides when applied to tall fescue (Lolium arundinaceum Schreb.). Herbicides were applied at the highest label rate for dandelion (Taraxacum officinale G.H. Weber ex Wiggers) control. Turfgrass flats treated with herbicides were placed in enclosures containing tomato plants. In laboratory experiments, tomatoes exposed to turfgrass treated with products containing amine formulations of synthetic auxin herbicides exhibited minimal injury, but exposure to products containing low-volatile ester formulations caused significant negative effects on tomato quality, epinasty, and callus formation. Generally, products containing low-volatile ester formulations of synthetic auxin herbicides exhibited less volatility, compared to 2,4-D butyl ester (volatile standard). A subsequent field study corroborated the laboratory results, although injury symptoms were less severe. Turfgrass managers should avoid herbicides containing low-volatile ester formulations when vulnerable non-target plants are nearby, and/or when environmental conditions such as high temperature favor volatility.

Bioavailability of Aminocyclopyrachlor and Triclopyr plus Clopyralid from Turfgrass Clippings in Aquatic and Riparian Plants

Synthetic auxin herbicides are widely utilized in golf course settings for selective broadleaf weed control. Aminocyclopyrachlor (AMCP) is a newly registered pyrimidine carboxylic acid with similar chemical mode-of-action and structure to triclopyr (TRIC) and clopyralid (CLPY). Off-target injury on terrestrial plants has been documented following exposure to turfgrass clippings previously treated with TRIC and CLPY. Management practices on golf courses can distribute turfgrass clippings into water bodies; however, research has not evaluated the bioavailability of synthetic auxin residues from turfgrass clippings to aquatic and riparian plants within these environments. A bioassay study was conducted to determine the response of alligatorweed and parrotfeather to tall fescue clippings previously treated with synthetic auxin herbicides. Previously treated AMCP and TRIC + CLPY clippings were placed into growth containers mimicking a lentic system containing both alligatorweed and parrotfeather. Results indicated all herbicide treated clippings induced significant growth responses to alligatorweed and parrotfeather growth compared to a nontreated mulch and nontreated control. Alligatorweed control was greater from AMCP clippings treated 14, 7, 3, and 1 DBCC (49, 60, 90, and 80%, respectively) than comparative TRIC + CLPY clippings (33, 25, 37, and 64%, respectively) at 10 weeks after treatment (WAT). Parrotfeather control was greater from AMCP clippings (57 to 87%) than TRIC + CLPY clippings (9 to 63%) collected from all days before clipping collection (DBCC) timings when evaluated 6 WAT. At 10 WAT, greater parrotfeather control and shoot reduction was observed from AMCP than TRIC + CLPY clippings when treated 14, 7, and 3 DBCC. Based on these data, synthetic auxin residues can become bioavailable to aquatic and riparian plants within aqueous environments.

Analytical Method and Interlaboratory Study for the Quantitation of Aminocyclopyrachlor Residues in Vegetation by Liquid Chromatography/Tandem Mass Spectrometry

The quick, easy, cheap, effective, rugged, and safe (QuEChERS) and the U.S. Food and Drug Administration pesticide multiresidue methods were systematically tested for the analysis of aminocyclopyrachlor, a new synthetic auxin herbicide. Results revealed that these procedures are not suitable, highlighting the need for a publicly available residue method for this new active ingredient. Such a method has been developed and is reported herein for aminocyclopyrachlor residue analysis in vegetation, including white pine, Norway spruce, grass forage, and grass hay. Aminocyclopyrachlor residues were purified from plant tissue extracts by filtration through SPE cartridges with strong anion exchange stationary phases and analyzed using LC/MS/MS. This method was validated at five laboratories across the United States; their results were compared to evaluate reproducibility and ruggedness. The method LOQ was 0.01 mg/kg (ppm) and the LOD 0.003 mg/kg (ppm) for the matrixes tested. A qualitative procedure to confirm the molecular identity of residues based on retention time measurements and ion transition peak area ratios (m/z 214 --> 681m/z 214 --> 101) is also presented.

제초제 저항성작물에서 잡초관리기술 동향 및 전망

This paper reviews current status of weed control practices in herbicide-resistant crops to examine weed management strategies in cope with cropping herbicide-resistant crops in the near future. Herbicide-resistant crops were rapidly adopted weed management technologies due to broad-spectrum weed control without crop injury. Transgenic glyphosate-resistant cultivars in soybean, corn, canola, and cotton were adopted to manage weeds at lower cost in a simplified weed management system. Dual stack crops with glyphosate and glufosinate resistance were developed to control glyphosate resistant weeds in corn, soybean and cotton. New multiple herbicide-resistant crops with resistance to glyphosate and glufosinate, acetolactate synthase (ALS) inhibitors, synthetic auxin herbicides, 4-hydroxyphenyl pyruvate dioxygenase (HPPD) inhibitors or acetyl Coenzyme A carboxylase (ACCase) inhibitors will expended the utility of existing herbicide technologies to manage the evolution of resistant weeds. However, herbicide resistant crops alone cannot solve weed problems and thus studies on diverse weed managements using an array of alternating herbicides of mode of action, mechanical, and cultural practices are needed for integrated weed management systems in the future.

Absorption, Translocation, and Metabolism of Aminocyclopyrachlor in Tall Fescue (Lolium arundinaceum)

Synthetic auxin herbicides are commonly used in forage, pasture, range, and turfgrass settings for dicotyledonous weed control. Aminocyclopyrachlor (AMCP) is a newly developed pyrimidine carboxylic acid with a chemical structure and mode of action similar to the pyridine carboxylic acids—aminopyralid, clopyralid, and picloram. Injury to sensitive dicotyledonous plants has been observed following exposure to monocotyledonous plant material previously treated with pyridine compounds. The absorption, translocation, and metabolism of AMCP has been documented in susceptible broadleaf weeds; however, no information is available, to our knowledge, regarding AMCP fate in tolerant Poaceae, which may serve as the vector for off-target plant injury. Based on this premise, research was conducted to characterize absorption, translocation, and metabolism of AMCP in tall fescue.14C-AMCP was applied to single tiller tall fescue plant foliage under controlled laboratory conditions at North Carolina State University (Raleigh, NC). Radiation was quantified in leaf wash, treated leaf, foliage, crown, roots, and root exudates at 3, 12, 24, 48, 96, and 192 h after treatment (HAT).14C-AMCP was rapidly absorbed by tall fescue, reaching 38 and 68% at 3 and 48 HAT, respectively. Translocation of14C-AMCP was limited to the foliage, which reached maximum translocation (34%) at 96 HAT. Most of the recovered14C-AMCP remained in the leaf wash, treated leaf, or foliage, whereas minimal radiation was detected in the crown, roots, or root exudates throughout the 192-h period. No AMCP metabolism was observed in tall fescue through the 192 HAT. These data suggest AMCP applied to tall fescue can remain bioavailable, and mishandling treated plant material could result in off-target injury.

Direct and Indirect Effects of the Synthetic-Auxin Herbicide Dicamba on Two Lepidopteran Species

Herbicides are the most commonly applied pesticides in agroecosystems, and therefore pose potentially significant ecotoxicological risks to plants and insects. Glyphosate is the most common herbicide worldwide, and glyphosate-resistant weeds are quickly becoming serious challenges in some agroecosystems. Because of this resistance epidemic and the recent development of crops with resistance to dicamba or 2,4-D, herbicide-use patterns are likely to change. Presently, dicamba and 2,4-D cause most herbicide-drift damage to nontarget plants despite limited agricultural usage, but the effects of these synthetic auxin herbicides on insects have been poorly explored. To understand the influence of dicamba on insects, we applied several sublethal, drift-level rates of dicamba to soybean, Glycine max L., and Carduus thistle, and measured growth and survival of Helicoverpa zea (Boddie) and Vanessa cardui (L.) larvae, respectively. For thistle, we measured percent nitrogen content before and after dicamba application. We also performed direct toxicity bioassays on the two caterpillar species with several rates of dicamba. Dicamba was not directly toxic to larvae of either species, and H. zea showed no negative effects when feeding on soybeans dosed with dicamba. We did, however, detect significant negative, indirect effects of higher rates of dicamba on V. cardui larval and pupal mass, total nitrogen of thistles post application, and thistle biomass in the presence of V. cardui larvae. Notably, thistle biomass was not related to dicamba dose in absence of larvae. Our results indicate that dicamba can indirectly influence the performance of some caterpillar species, possibly by altering plant nutritional content.

Effect of Ambient Moisture on Aminocyclopyrachlor Efficacy

Aminocyclopyrachlor (AMCP) is a newly developed synthetic auxin herbicide for broadleaf weed control in turfgrass systems. AMCP has been observed to undergo rapid photodecomposition in shallow water when exposed to sunlight. Most herbicide applications on golf courses occur during the morning when dew is still present on the turfgrass canopy. These conditions could result in efficacy loss if photolysis occurred while AMCP is suspended in dew droplets. Research was conducted to determine the effect of ambient moisture on AMCP efficacy. AMCP (79 and 105 g ae ha−1), aminopyralid (280 g ae ha−1), and two AMCP granular formulations (84 g ha−1) were applied to dew-covered (WET) and dew-excluded (DRY) ‘Tifway' bermudagrass plots. Herbicide treatments applied to WET plots had greater visually rated bermudagrass injury than respective treatments applied to DRY plots at 7 and 21 d after treatment (DAT), with the exception of aminopyralid at 21 DAT. Normalized difference vegetative index on turfgrass quality complemented visual ratings, indicating greater turfgrass quality reductions when applied to WET vs. DRY plots. These results indicate that AMCP applications made to dew-covered turfgrass can increase herbicidal efficacy, and no significant losses due to photodegradation were observed.

Vapor Movement of Aminocyclopyrachlor, Aminopyralid, and Dicamba in the Field

Vapor movement of synthetic auxin herbicides can injure desirable plants outside the treatment zone. Vapor movement of the synthetic auxin herbicides aminocyclopyrachlor and aminocyclopyrachlor methyl was compared with that of the relatively volatile herbicide dicamba and the low volatile herbicide aminopyralid with a soybean bioassay under greenhouse and field conditions. Soybean is very sensitive to these active ingredients. Under greenhouse conditions, 82 (61 to 104) mg ae ha−1of aminocyclopyrachlor, 26 (18 to 33) mg ae ha−1of aminocyclopyrachlor methyl, 82 (69 to 95) mg ae ha−1of aminopyralid, and 61 (47 to 75) mg ae ha−1of dicamba produced an estimated 25% visual soybean phytotoxicity response when soybean was treated POST at the V3 growth stage (GR25[95% confidence interval]). In field studies, aminocyclopyrachlor, aminocyclopyrachlor methyl, and aminopyralid were applied at 70 g ae ha−1and dicamba was applied at 560 g ae ha−1(labeled application rates) to soybean at the V3 growth stage. All herbicides were applied within an enclosed chamber (3 m by 3 m by 1 m) to mitigate movement of spray droplets. The enclosures were removed shortly after spray application and soybean response immediately surrounding the treated area was recorded in each of eight directions approximately 10 d after treatment. On the basis of bioassay responses, relative amount of vapor movement was dicamba > aminocyclopyrachlor methyl > aminopyralid ≈ aminocyclopyrachlor. Vapor movement of aminocyclopyrachlor was very low indicating that the risk of phytotoxic response of sensitive plants due to volatility of aminocyclopyrachlor is negligible.

Benchmark Study: III. Survey on Changing Herbicide Use Patterns in Glyphosate-Resistant Cropping Systems

Approximately 1,300 growers from 22 states were surveyed during 2010 to determine herbicide use. Cropping systems included continuous glyphosate-resistant corn, cotton, and soybean, and various combinations of these crops and rotations with non–glyphosate-resistant crops. The most commonly used herbicide for both fall and spring applications was glyphosate followed by synthetic auxin herbicides. Herbicide application in spring was favored over application in the fall. The percentage of growers in a glyphosate-only system was as high as 69% for some cropping systems. Excluding glyphosate, the most frequently used herbicides included photosystem II, mitotic, and protoporphyrinogen oxidase inhibitors. A higher percentage of growers integrated herbicides other than glyphosate during 2010 compared with 2005. Extensive educational efforts have promoted resistance management by increasing the diversity of herbicides in glyphosate-resistant cropping systems. However, a considerable percentage of growers continued use of only glyphosate from the period of 2005 to 2010, and this practice most likely will continue to exert a high level of selection for evolved glyphosate-resistant weed species.