Numerous agrochemicals can be applied in peanut to control pests. Field and laboratory experiments were conducted in North Carolina during 2009 and 2010 to determine peanut response to paraquat and tobacco thrips (
Peanut (
Paraquat can be applied within 28 days after peanut emergence to control small annual broadleaf weeds, grasses, and sedges (
Removal of aldicarb from the commercial market required growers to rely on postemergence applications of insecticides to control tobacco thrips. Although phorate controls tobacco thrips adequately in most instances, control is often less consistent than control by aldicarb (
Although interactions of paraquat and acephate have been evaluated in some instances with respect to weed control and peanut injury, compatibility of these pesticides in spray solution prior to application has not been documented. Lack of information on physical compatibility of different agrochemicals in the spray tank can lead to the formation of precipitates or suspended solids which may adversely affect application and consequently affect pest control (
Defining interactions of paraquat and acephate with respect to visible injury caused by paraquat and damage from tobacco thrips feeding will be of value for growers formulating pest management strategies in peanut. These findings will be especially important if greater use of paraquat occurs due to herbicide-resistant weeds and if acephate is applied more frequently in absence of aldicarb availability. Therefore, objectives of this research were to define interactions of acephate and paraquat applied alone or in combination with chloroacetamide herbicides with respect to tobacco thrips damage and peanut injury and to determine changes in spray solution characteristics when these pesticides are co-applied.
Experiments were conducted in North Carolina during 2009 and 2010 at the Peanut Belt Research Station near Lewiston-Woodville on a Norfolk sandy loam soil (fine-loamy, kaolinitic, thermic Typic Kandiudults). The peanut cultivar Phillips (
Treatments included two levels of acephate (Orthene97®, Amvac Chemical Corp., Los Angeles, CA 90023) (0 and 1.1 kg ai/ha); three levels of contact herbicides including no contact herbicide, paraquat (Gramoxone Inteon Herbicide, Syngenta Crop Protection, Inc. Greensboro, NC 27419) at 0.14 kg ai/ha, and paraquat plus bentazon (Basagran Herbicide, BASF Corp., Research Triangle Park, NC 27709) at 0.14 kg/ha + 0.28 kg ai/ha; and four levels of chloroacetamide herbicides including no chloroacetamide herbicide, alachlor (Intrro Preemergence Herbicide, Monsanto Co., St. Louis, MO 63167) at 3.4 kg ai/ha, dimethenamid-
The experimental design was a randomized complete block with four replications. An ordinal scale of 0 to 5 where 0 = no damage, 1 = noticeable feeding but no stunting, 2 = noticeable feeding and 25% stunting, 3 = noticeable feeding with blackened terminals and 50% stunting, 4 = severe feeding and 75% stunting, and 5 = severe feeding and 90% stunting was used to visibly assess damage from tobacco thrips feeding 10 days after postemergence treatment (DAT). Visible estimates of percent injury relative to symptoms associated with paraquat (
Experiments were conducted in North Carolina during 2011 at the Peanut Belt Research Station near Lewiston-Woodville and the Upper Coastal Plain Research Station located near Rocky Mount in two separate fields at each location. Soils were a Norfolk sandy loam described previously at Lewiston-Woodville and an Adcock fine sandy loam soil (fine-loamy, siliceous, subactive, thermic Aquic Paleudults) at Rocky Mount. The peanut cultivar Phillips was planted as described previously in plots with two rows spaced 91 cm apart by 9 m in length. Peanut was not irrigated.
Treatments included no aldicarb or aldicarb (Temik 15G insecticide, Bayer CropScience, Research Triangle Park, NC 27709) at 1.1 kg ai/ha applied alone in the seed furrow at planting following seed drop but before slit closure. These treatments also received acephate postemergence, paraquat postemergence, acephate plus paraquat postemergence, or no additional pesticides to establish a 2 by 4 factorial treatment structure. Acephate and paraquat were applied at the rates described previously. Nonionic surfactant at 0.125% (v/v) was applied with all treatments containing paraquat.
The experimental design was a randomized complete block with four replications. Visible estimates of tobacco thrips damage from feeding were determined 10 and 14 DAT using the ordinal scale described previously. Visible estimates of percent injury relative to symptoms associated with paraquat were recorded 10 and 14 DAT using the injury scale described previously. The number of days from peanut emergence to canopy closure was determined, and peanut was dug and vines inverted when approximately 65% of pods were in the brown and black category based on pod mesocarp color for the treatment of aldicarb with no other postemergence treatment (
Laboratory experiments were conducted to compare physical compatibility of acephate combinations used in the field experiments during 2009 and 2010. Unlike experiments in the field where a municipal water source was used, deionized water at pH 6.27 was used in the laboratory experiments. Pesticides were mixed in the following order: dry flowables (acephate), emulsifiable concentrates (alachlor and
Data for peanut damage, visible peanut injury, and solution pH from field and physical compatibility studies were subjected to ANOVA using the PROC MIXED procedure of SAS (Statistical Analysis Systems®, version 9.2, SAS Institute Inc., Cary, NC 27513) appropriate for the factorial arrangement of treatments using expected mean squares to test fixed and random effects. Pesticide treatments were considered as fixed effects while experiments and replications were considered as random effects. Data for peanut damage from thrips feeding and peanut injury from paraquat were log and arcsine transformed, respectively, using Box-Cox procedure. However, transformation of data did not change data interpretation, hence, data were presented without transformation (
Damage from tobacco thrips feeding 10 DAT was affected by interaction of acephate X chloroacetamide herbicide (p = 0.0028). When pooled over contact herbicides and years, damage from tobacco thrips feeding was no more than 1.3 on a scale of 0 to 5 when acephate was applied irrespective of chloroacetamide herbicide (
Peanut damage 10 d after application when acephate was applied alone or in combination with chloroacetamide herbicides during 2009 and 2010.a,b
Interactions of acephate by contact herbicide by chloroacetamide herbicide (p = 0.0075) and acephate by contact herbicide (p = 0.0011) were significant for peanut injury associated with paraquat symptomology during 2009 and 2010, respectively. In the absence of acephate, co-application of paraquat with alachlor, dimethenamid-
Peanut injury 10 d after application associated with paraquat symptomology when acephate was applied alone or in combination with chloroacetamide and contact herbicides for the control of tobacco thrips in 2009.a,b
Peanut injury 10 d after application associated with paraquat symptomology when acephate was applied alone or in combination with contact herbicides for the control of tobacco thrips at Lewiston-Woodville in 2010.a,b
Results from the experiment with acephate, chloroacetamide herbicides, and paraquat were unclear with respect to whether or not reduction in paraquat injury by acephate was associated with interactions in the spray solution, physiological response on the leaf surface, or the plant's ability to compensate for injury through more expansive growth in absence of tobacco thrips feeding. Therefore, the experiment comparing paraquat injury and tobacco thrips damage was conducted to address this question. In theory, applying combinations of acephate, bentazon, and paraquat in presence and absence of tobacco thrips feeding could allow a greater distinction among peanut response to these interactions.
Peanut damage caused by tobacco thrips at 10 and 14 DAT was affected by the interaction of aldicarb (no aldicarb or aldicarb) X foliar pesticide (no pesticide, acephate, paraquat, or acephate plus paraquat) (p = 0.0379 at 10 DAT; p ≤ 0.0001 at 14 DAT). Damage from tobacco thrips feeding did not exceed a value of 1.0 at 10 DAT regardless of foliar pesticide treatment but ranged from 0 to 4.0 by 14 DAT (
Peanut damage from tobacco thrips feeding and injury from paraquat in presence or absence of tobacco thrips damage due to in-furrow application of aldicarb in 2011.a,b,c
Solution pH determined when acephate was applied alone or in combination with chloroacetamide and contact herbicides.a,b
Main effect of aldicarb and foliar pesticide treatment were significant (p ≤ 0.0001) 10 DAT for peanut injury reflecting paraquat symptoms. The interaction of aldicarb X foliar pesticide treatment was also significant (p ≤ 0.0001) 14 DAT. When pooled over foliar pesticide treatments at 10 DAT, paraquat injury was 16% (data not presented in tables). When pooled over aldicarb treatments and experiments, injury by acephate plus paraquat exceeded that of paraquat alone at 10 DAT (
Main effects aldicarb and foliar pesticide treatment were significant for the number of days from peanut emergence to row closure (p ≤ 0.0001 for both main effects) and pod yield (p = 0.0033 and 0.0169 for these respective main effects). When pooled over foliar pesticide treatments, fewer days from emergence to row closure were required when aldicarb was applied (77 vs. 82 days, data not shown in tables), and pod yield was higher following aldicarb (5760 kg/ha vs. 5430 kg/ha, data not shown in tables). These differences most likely reflect a reduction in tobacco thrips feeding resulting in more rapid and sustained peanut growth when aldicarb was applied. Peanut yield is often higher when aldicarb is applied and tobacco thrips damage is minimized (
The interaction of acephate by contact herbicide by chloroacetamide herbicide was significant for solution pH (p = ≤ 0.0001). When compared to carrier pH (6.27), all combinations reduced solution pH except the combination of alachlor with paraquat plus bentazon. When compared with mixtures without acephate, solution pH decreased when acephate was included irrespective of chloroacetamide or contact herbicides. Regardless of chloroacetamide herbicide in solution, paraquat plus bentazon decreased solution pH to approximately 5.0. When compared with solutions containing paraquat with or without acephate, including bentazon increased solution pH.
In most of instances, solutions either with or without acephate combinations formed temporary precipitates at 0, 6, 24, and 72 h sampling times (data not shown in tables). Combinations of acephate with dimethenamid-
Practical application of these physical observations is currently unknown. However, very little information on changes in pH and observations on precipitate formation are found in the peer-reviewed literature. These results may at some point contribute to a greater understanding of issues related to agrochemical compatibility.
Collectively, results in the field during 2009 and 2010 demonstrated that chloroacetamide herbicides were more injurious when acephate was not applied and damage from tobacco thrips was present compared with co-application with acephate. Acephate and aldicarb contributed to tobacco thrips control independently and protected pod yield, although the benefit from acephate was marginalized when paraquat was included. Co-application of paraquat with alachlor and
This research was supported financially by the North Carolina Peanut Growers Association. The authors would like to appreciate Dewayne Johnson, Gary Little, and staff at the Peanut Belt Research Station and Upper Coastal Plain Research Station for technical assistance.