Evaluation of neonicotinoids as pyrethroid alternatives for rice water weevil management in water-seeded rice

Crop Protection 56 (2014) 37e43

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Evaluation of neonicotinoids as pyrethroid alternatives for rice water weevil management in water-seeded rice Srinivas K. Lanka*, Michael J. Stout, James A. Ottea Department of Entomology, Louisiana State University Agricultural Center, Louisiana State University, 404 Life Sciences Building, Baton Rouge, LA 70802-0001, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 May 2013 Received in revised form 13 September 2013 Accepted 18 September 2013

The rice water weevil (RWW), Lissorhoptrus oryzophilus (Kuschel) (Coleoptera: Curculionidae), is the most destructive insect pest of rice in the United States. Water-seeded rice, which is flooded at an earlier stage of crop development than drill-seeded rice, is at heightened risk of loss from root-feeding RWW larvae. Pyrethroids, the most widely used group of foliar insecticides for RWW control, have inherent limitations such as limited residual activity, narrow window of activity and extreme toxicity to nontarget aquatic organisms. An array of field, lab and greenhouse experiments was conducted to compare the activity of two neonicotinoids with that of l-cyhalothrin, a widely used pyrethroid, against the RWW. Small-plot efficacy trials were conducted during 2009, 2010 and 2011. Foliar clothianidin (Belay 2.13 SC) and a granular formulation (3%) of dinotefuran applied to plots were as effective as, and showed greater residual activity than, foliar applications of l-cyhalothrin. Topical bioassays on adult weevils revealed that clothianidin possessed lower contact toxicity than l-cyhalothrin. Residual assays using weevils placed on foliage of sprayed plots revealed that the toxic and sublethal behavioral effects of clothianidian on adult weevils were more persistent for clothianidin than for l-cyhalothrin. Granular dinotefuran applied to greenhouse-grown plants previously infested with weevil larvae showed excellent larvicidal activity. Overall, these studies showed that neonicotinoids have potential as pyrethroid replacements against the RWW in water-seeded rice culture. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Rice water weevil l-Cyhalothrin Dinotefuran Clothianidin Toxicity Behavioral effects

1. Introduction The rice water weevil (RWW) is the most important insect pest of rice in the United States (Way, 1990). Over the past few decades, this insect has invaded important rice growing regions of the world, including Asia and Europe, and thus has now assumed global importance as a pest of rice (Saito et al., 2005). The interaction of the RWW with rice involves all life stages of the insect. Adult weevils feed on leaves of young rice plants causing characteristic feeding scars parallel to the venation of leaves. Oviposition is triggered by the presence of standing water and eggs are laid in leaf sheaths at or below the water line; thus, the majority of egg-laying occurs after fields are flooded (Everett and Trahan, 1967; Muda et al., 1981; Smith, 1983; Stout et al., 2002). Eggs hatch after an incubation period of 5e9 days (Raksarart and Tugwell, 1975).

Abbreviations: AI, active ingredient; DPF, days post-flooding. * Corresponding author. Tel.: þ1 2255781850, þ1 2253294291 (mobile); fax: þ1 2255781643. E-mail addresses: [email protected], [email protected] (S.K. Lanka). 0261-2194/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cropro.2013.09.007

Neonates mine through leaf sheaths or shoots, then quickly move down to the roots and establish feeding sites on or in rice roots (Zhang et al., 2004). Larvae pass through four instars to undergo pupation in 27e30 days (Zou et al., 2004a). Adult feeding is not considered economically important except under unusually heavy infestations, but root pruning can result in poor crop stand and reduced tillering at the vegetative stage and reduced panicle size and grain weight at the reproductive stage of rice (Zou et al., 2004b). This pest has the potential to cause economic losses in excess of 10% under heavy weevil pressure (Stout et al., 2011). Rice in the United States is direct-seeded rather than transplanted as it is in much of Asia. The majority of rice in the southern United States is cultivated under a drill-seeded system in which a dry seed bed is prepared, seed is sown using a grain drill, and the field is typically flooded when rice starts tillering, three to five weeks after planting (Blanche et al., 2009). Alternatively, rice can be cultured by water-seeding, in which dry or sprouted seeds are broadcast into standing water (Blanche et al., 2009). After seeding, the flood may be maintained continuously until the field is drained for harvest or, more commonly, the field may be drained for a short period of time after seeding to allow plants to establish, and the

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S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

flood re-established a short time later (Blanche et al., 2009). Regardless of whether water-seeded rice is flooded continuously or drained temporarily before applying a permanent flood, waterseeding involves flooding fields at an earlier stage of crop development than does drill-seeding. As a result, water-seeded rice is subject to infestation by RWW at an earlier stage of crop development and is, therefore, at a greater risk of loss from RWW. Water seeding is practiced on approximately 30e40% of rice acreage in southwest Louisiana (J. Saichuk, LSU AgCenter, personal communication). Until recently, RWWs in rice were managed largely through the use of foliar applications of pyrethroids to eliminate adult female weevils before they oviposited. Use of pyrethroids in rice has several limitations. Because pyrethroids have limited residual activities, and because the damaging larval stages are shielded from insecticides, timing of application is critical for pyrethroid-based control of RWWs. Untimely pyrethroid applications may result in inadequate control. In addition, there is widespread concern over the use of pyrethroids because they are extremely toxic to nontarget aquatic invertebrates such as the red swamp crawfish, Procambarus clarkii (Girard) (Decapoda: Cambaridae), a major aquaculture commodity that is co-cultivated with rice in southwest Louisiana (Jarboe and Romaire, 1991; Barbee and Stout, 2010). Finally, heavy use of a single class of insecticide against RWW is an unwise strategy because it has a history of developing resistance to insecticides (Bowling, 1968) which calls for development of alternative insecticides for weevil management in rice. Over the past few years, seed treatment formulations of two neonicotinoid insecticides, Cruiser MaxxÒ Rice (AI: thiamethoxam) and Nipsit INSIDEÒ (AI: clothianidin), and an anthranilic diamide insecticide, DermacorÒ X-100 (AI: chlorantraniliprole), have been labeled for RWW control in drill-seeded rice. These seed treatments provide effective protection of rice from damaging populations of Lissorhoptrus oryzophilus. More recently, the use of Dermacor X-100 has been extended to water-seeded rice in Louisiana under a special “local need” (Section 24c) label. However, because of the high price of Dermacor X-100, and because damaging weevil populations do not always occur in rice fields, some rice growers are unwilling to take this prophylactic approach. Neonicotinoids target nicotinic acetylcholine receptors and are less toxic to vertebrates due weak affinity of neonicotinoids toward mammalian receptors (Tomizawa and Casida, 2003, 2005). The fast growing commercial use of these compounds has been attributed largely to their long residual activities, favorable physico-chemical

properties, and amenability to diverse use patterns in agriculture such as seed treatments, surface applications on stem, foliar applications and soil drenching. Therefore, several formulations of these compounds are available for use. In the present study, the efficacies of two neonicotinoid insecticide formulations, one containing clothianidin (BelayÒ 2.1 SC; Valent Corporation, USA) and another containing dinotefuran, (Dinotefuran 3G; Mitsui Chemicals, Inc. USA), were compared with that of a commercially registered pyrethroid, l-cyhalothrin (Karate ZÔ; Syngenta Corporation, USA) against the RWW in water-seeded rice. Clothianidin and l-cyhalothrin were evaluated as foliar sprays while dinotefuran was evaluated as a granule applied to soil after permanent flooding. Also, the contact toxicities of clothianidin and l-cyhalothrin on RWW adults were determined by conducting topical bioassays during 2008 and 2010. In addition, these insecticides could affect insects through complex modes of activity when populations are exposed to residues (Lanka et al., 2013). Therefore, the residual activities of clothianidin and l-cyhalothrin were compared by conducting feeding assays with adult weevils using foliage from insecticide-treated plots. Effects on both adult survival and behavior were measured. Finally, the larvicidal efficacy of dinotefuran on root-feeding larval stages was evaluated under greenhouse conditions. 2. Material and methods 2.1. Small-plot efficacy trials Small-plot field experiments were conducted during the 2009, 2010, and 2011 growing seasons at the Louisiana State University Agricultural Center Rice Research Station, Crowley, LA. The soil type at this location is a Crowley silt loam (fine, montmorillonitic, thermic typic albaqualf). The rice variety CL-131, a conventional, herbicide-tolerant, long-grain variety was used for all experiments. Plots in all experiments measured 1.5  6.0 m and were surrounded by metal flashing, approximately 25 cm in height, to restrict movement of water and insecticides among plots. Plots were separated by at least 1.5 m on all sides. Plots were flooded and sprouted seed was hand sown into plots at a rate of 135 kg per ha. Fields were drained two to three days after planting to allow plants to peg down. Permanent floods were applied to plots when rice plants possessed 1 or 2 leaves on the main stem (V1 or V2 stage) (Counce et al., 2000). Dates of planting and permanent flooding are shown in Table 1. Fertilization and weed control practices followed

Table 1 Rice planting dates, flooding dates, rates and timings of insecticide treatments used in three small-plot evaluations of the insecticides l-cyhalothrin, clothianidin, and dinotefuran, LSU-Agricultural Center Rice Research Station, Crowley, Louisiana, 2009e2011. Insecticide treatmenta

Date at

Timing (DPF)b

Planting

Flooding

Active ingredient

Rate (g/ha)

Application

Sampling

21-May-09

9-Jun-09

Clothianidin

100.9 & 100.9 121.5 223.2 & 223.2 (split)c 446.4 33.7 & 33.7

1&7 7 1&7 14 1&7

21, 31 & 42

91 372 372 33.7

16 16 32 16

32, 38 & 49

91 91 372 33.7

5 12 21 5

21, 27 & 34

Dinotefuran

l-Cyhalothrin 24-Mar-10

5-Apr-10

Clothianidin Dinotefuran

28-Apr-11

11-May-11

Clothianidin

l-Cyhalothrin

Dinotefuran

l-Cyhalothrin a b c

Formulations: clothianidin (BelayÔ 2.1SC); dinotefuran granular (3G); l-cyhalothrin (KarateÔ Z). Days post-flooding. Split treatment was half of a full dose applied two times.

S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

the recommendations of LSU-Agricultural Center for water-seeded rice (Saichuk, 2009). Treatments consisted of various rates and timings of applications of l-cyhalothrin (KarateÔ Z), clothianidin (Belay 2.1 SC) and granular dinotefuran (Dinotefuran 3G) (Table 1). l-Cyhalothrin and clothianidin were applied as foliar sprays and dinotefuran granules were broadcast in rice plots. Application timing for the foliar insecticides (KarateÔ Z and Belay 2.1S C), were based on current recommendations, which call for applications to be made when adult weevils are found in flooded fields. Thus, initial applications of the foliar insecticides in these experiments were made one to two days after adult weevils were first observed in flooded plots. Dinotefuran granules were applied later to test for potential efficacy as a post-flood larvicide. In 2009, weevils were present in plots at flooding; thus, the first foliar application was made to appropriate plots immediately after flooding and the second application was made seven days later (Table 1). In 2010, weevils appeared in plots much later and initial applications were not made until 16 d after flooding. In 2011, initial treatments were applied five days after permanent flood, coinciding again with first appearance of weevils in flooded plots. Treatments were assigned to plots according to a randomized complete block design with six treatments in 2009 and five treatments in 2010 and 2011. In all years, treatments were replicated in four blocks. Foliar applications were made using a backpack, CO2-powered sprayer calibrated to deliver 141 l per hectare. Broadcast applications of dinotefuran granules were made by mixing granules with sand into a jar with a perforated lid and distributing evenly by shaking by hand into plots. Population densities of immature weevils (larvae and pupae) were estimated by taking three root/soil core samples from each plot at several times after flooding and counting the number of immature weevils associated with roots. Initial core samples were taken 16 to 20e21 days after application of insecticides, and two additional core samples were taken at approximately weekly intervals after the initial sampling. Core samples were taken using a metal cylindrical sampler (9.2 cm in diameter with depth of 7.6 cm), and contained from 1 to 8 rice plants (more plant numbers when plants were younger). Samples were placed in 40-mesh sieve buckets, and soil was washed from roots. Buckets were then placed into basins of salt water, and larvae and pupae were counted as they floated to the surface of the salt solution (N’Guessan and Quisenberry, 1994). Entire plots were harvested using a mechanical small-plot grain harvester and yields of rough rice from plots were adjusted to 12% moisture. A mean insect density (larvae and pupae per core sample) for each plot at each sampling date was calculated by averaging the numbers obtained from the three soil cores. Treatment effects on mean insect densities and yields were analyzed by mixed-model ANOVA using PROC MIXED in SAS (SAS Institute, 2008). Effects of insecticide were treated as fixed and blocks were treated as random. Mean separations were made using Tukey’s honestly significant difference test at a ¼ 0.05. 2.2. Insecticide bioassays Topical bioassays were used to determine acute toxicities of clothianidin and l-cyhalothrin to adult RWWs in 2008 and 2011. Analytical grade clothianidin (99.9%; Sigma Aldrich, St Louis, MO) and l-cyhalothrin (99.1%; Chemservice, West Chester, PA) were serially diluted with acetone, and a repeating dispenser (Hamilton PB-600) equipped with a 25 ml beveled point syringe (Hamilton, Reno, NV) was used to apply 0.5 ml of each concentration to the notum of each weevil. Clothianidin was diluted in acetone to apply doses between 1 and 80 ng/weevil for assays in 2008 and between 0.05 and 100 ng/weevil in 2011. Similarly, l-cyhalothrin was diluted

39

to doses of 0.12e31.6 ng/weevil in 2008 and between 0.06 and 15.8 ng/weevil in 2011. In all assays, weevils in control treatments were treated with acetone only. Unsprayed field plots at the LSU AgCenter Rice Research Station served as a source of adult weevils for these assays. These field plots were bordered by other research plots and were in proximity to commercial rice fields. Fieldcollected adult weevils were starved overnight in plastic containers provided with water at the bottom. Before the beginning of each assay, groups of 10e50 weevils were weighed to obtain a mean weight per weevil in milligrams (2008: 2.48  0.01 mg per weevil; 2011: 2.53  0.06 mg per weevil). For each insecticide dose, 30 or 50 weevils were treated and released in petri dishes (CorningÒ 22.5 cm diameter 2.5 cm deep) lined with moistened cotton batting. Fresh rice foliage from plants grown in the greenhouse was provided daily during the 72-h observation period. Daily mortality in weevils was assessed based on complete immobility of weevils for 5 min after being placed on a flat surface. Bioassay data at 72-hr after treatment were corrected for control mortality (Finney, 1972), which never exceeded 6.0%. The LD50s, fiducial limits, slope  SE, and chi-square for slope estimates were determined using PROC PROBIT in SAS 9.2 (SAS Institute, 2008). 2.3. Comparison of residual activities of foliar insecticide sprays Residual activities of foliar sprays of l-cyhalothrin and clothianidin were investigated by feeding adult weevils leaf material from sprayed plots during 2011. Plots were treated with insecticides (Table 1) and approximately 8 plants at the 3e4 leaf stage were pulled from untreated (control) and treated plots at 2 h and three days after foliar sprays. Paper towels were wrapped around the roots, and the plants were transported to the lab where leaves were separated from roots and inserted into a 1 cm layer of 1.5% agar lining the bottoms of polystyrene petri dishes (22.5 cm diameter and 2.5 cm deep; CorningÒ). Four dishes (replicates) were prepared from four replicate plots of each treatment and ten weevils were released in each of the four dishes. Periodic observations of adult weevil behavior and mortality were made after placing weevils in dishes. Behavioral observations were made by releasing weevils in the center of a 25 cm circle drawn on a sheet of white paper placed on a lab bench. Weevils categorized as “normal” were capable of righting themselves using their appendages and exiting the 25 cm circle. Abnormal (intoxicated) weevils were unable to right themselves from their nota or could right themselves but failed to move beyond the 25 cm circle. Observations of adult weevils were made 24, 72 and 144 h after placement of adults on foliage in the 0-d bioassay. In the 3-d bioassay, observations were recorded 72 and 144 h after release. In these bioassays, leaf material was not replaced for the duration of the assay but leaf quality remained acceptable throughout the assay period. In addition to these observations, feeding activity was measured at 72 h in the 0-day assay by counting the number of adult weevil feeding scars on leaves. Mortality was defined as immobility of weevils for 5 min after being placed on a flat surface. The number of dead weevils and live weevils exhibiting normal or abnormal behavior were converted to percentages and analyzed using analysis of variance in PROC MIXED in SAS version 9.2 (SAS Institute, 2008). Cumulative percent weevil mortality and cumulative feeding scars were used in analyses. All percentage data were subjected to arcsine transformation before analysis. Mean comparisons among treatments were performed using Tukey’s honestly significant difference at alpha ¼ 0.05. 2.4. Efficacy of dinotefuran granules on weevil larvae Efficacy of a granular formulation of dinotefuran against weevil larvae already established on rice roots was examined by applying

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S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

dinotefuran 3G at field rates to potted rice plants infested with RWW larvae in a greenhouse on the campus of Louisiana State University, Baton Rouge. Rice (cv. ‘Jefferson’) was sown in 90, 15 cm diameter (2.0 L) pots in a potting mixture that consisted of autoclaved silt loam soil, sand, and peat moss (4:2:1). Ten days after germination, plants were thinned to two plants per pot and 5 g of complex fertilizer containing 20e14e38N:P:K was applied to each pot. The greenhouse was maintained at 28.0  5  C with ambient lighting. Plants were grown in large wooden basins lined with a heavy pond liner that allowed basins to be flooded. When plants reached the 5e6 leaf stage, they were placed in cages and infested with adult weevils at a density of one male: female pair per plant. Cages were constructed of cylindrical wire frames (46 cm diameter, 61 cm height) covered with a mesh fabric screening. Six pots (2 plants/pot) were placed in each infestation cage and 12 weevil pairs were released in each cage. A total of 10 cages were used for the experiment. Wooden basins were flooded with water up to a depth of 20 cm above the soil line. Adults used for infestation were collected from untreated plots at the Louisiana State University Agricultural Center Rice Research Station and were starved overnight in a plastic tray provided with water. Weevils were allowed to feed, mate and oviposit on plants for six days. Pots were removed from cages and any adult weevils found on plants were removed. Infested plants in pots were maintained in flooded benches to allow egg hatching and larval establishment on roots. Pre-treatment larval densities were determined from a subsample of plants two weeks after termination of adult infestation by washing soil from roots in a sieve bucket as described previously. Initial larval densities were determined from two pots from each cage (20 pots). Roots and associated soil were removed from pots and larvae were counted as described earlier. The remaining four pots in each cage were separated into groups of two and placed in separate greenhouse basins. The cage from which each pot had originated was recorded. All pots in one basin were treated with granular dinotefuran while the second basin did not receive insecticide. Granular dinotefuran was weighed, mixed with sand in plastic cups, and applied to each pot at a rate equivalent to 150 g ai/ acre (0.68 mg ai per pot). To restrict movement of insecticide into surrounding water in basins, depth of flood was adjusted to the rim of pots. This water level was maintained for the remainder of the experiment. Post-treatment larval densities in both dinotefurantreated and untreated pots were determined 4 and 7 days after application of dinotefuran. The effects of treatment and sampling time were analyzed using a repeated measures analysis of variance. The infestation cage from which the pot originated was treated as a random effect. Data were square root transformed (sqrt (0.5 þ larvae)) before analysis and analysis of variance was performed using PROC MIXED in SAS 9.2 version (SAS Institute, 2008).

3. Results 3.1. Field efficacy trials In 2009, insecticidal efficacies depended on timing and rate of insecticide application. The overall effects of insecticide treatment on larval densities were significant at 21 (F ¼ 10.5; df ¼ 5, 15; P ¼ 0.0002), 31 (F ¼ 9.8; df ¼ 5, 15; P ¼ 0.0001) and 42 DPF (F ¼ 8.6; df ¼ 5, 15; P ¼ 0.0005) (Table 2). At 21 DPF, all insecticide treatments reduced larval densities by more than 65%. For clothianidin, control provided by a single (7 DPF) or double application (1 and 7 DPF) was >90%. Similarly, the full and split applications of dinotefuran provided >90% control. At 31 DPF, two full applications of lcyhalothrin and a split application of dinotefuran did not differ significantly from the control (15 and 23% suppression, respectively). Although larval densities in both clothianidin treatments were significantly lower (53%) than in the control, the suppression of larval densities was lower than observed at 21 DPF. Notably, the full application of dinotefuran provided persistent control of weevil larvae (w94%) at 31 DPF. At the last core sampling (42 DPF), the full application of dinotefuran and split applications of clothianidin suppressed larval populations by 91 and 50%, respectively. Insecticidal treatments significantly increased rice yields (F ¼ 4.9; df ¼ 5, 15; P ¼ 0.008). However, yields were low, probably because of the extremely late planting date. In 2010, the effect of insecticide treatment on larval densities was significant on the first (32 DPF) (F ¼ 5.25; df ¼ 3, 12; P ¼ 0.008) and the second sampling dates (38 DPF) (F ¼ 6.6; df ¼ 4, 15; P ¼ 0.003) but not on the third sampling date (49 DPF) (F ¼ 0.8; df ¼ 4, 15; P ¼ 0.5) (Table 3). Infestations in untreated plots were low at both 32 and 38 DPF and application of l-cyhalothrin (16 DPF) generally resulted in greater reductions in larval densities than did the other treatments, although densities in the l-cyhalothrin treatment were never significantly lower than in the clothianidin treatment. Although treatment effects were not significant at 49 DPF, larval densities in the late dinotefuran treatment were as low as at 38 DPF. In contrast to 2009, rice yields were not significantly impacted by treatments (F ¼ 0.1; df ¼ 4, 15; P ¼ 0.9). Weevil population densities in 2011 were also low. No statistical analysis was performed on core sampling data collected at 14 DPF since densities in untreated plots were less than 1.0 larvae per core (data not shown). At 21 DPF, the effect of treatment was significant (F ¼ 3.8; df ¼ 4, 15; P ¼ 0.02) (Table 4). The late clothianidin (12 DPF) and granular dinotefuran (14 DPF) treatments provided better control of larvae than foliar applications of l-cyhalothrin and clothianidin at 5 DPF. On the third core sampling date (27 DPF), larval densities in all treatments were significantly lower than densities in untreated plots (F ¼ 5.8; df ¼ 4, 15; P ¼ 0.005), with reduction in larval densities ranging from 64 to 83%. Significant effects on larval densities due to treatments were not detected at the fourth core

Table 2 Population densities of rice water weevil immatures (larvae and pupae per core sample  s.e.) in treated and untreated small plots sampled over a three-week period in a water-seeded experiment in 2009. Treatment and application timinga

No. immature weevils/core sample 21 DPFb

Untreated

l-Cyhalothrin 1 & 7 DPF Clothianidin 1 & 7 DPF Clothianidin 7 DPF Dinotefuran split 1 & 7 DPF Dinotefuran full 14 DPF

7.7 2.4 0.1 0.9 1.8 0.7

     

1.1 a 1.6 b 0.1b 0.5 b 1.0 b 0.2 b

31 DPF 16.0 12.3 7.4 7.8 13.7 1.1

     

Adjusted yield (kg/ha) 42 DPF

1.7 2.5 1.8 0.8 1.9 0.5

a ab bc bc ab c

23.0 16.3 11.5 14.3 20.0 2.1

     

2.1 3.2 2.1 2.7 4.3 0.8

a ab bc ab a c

Values accompanied by same letter indicate means not significantly different from one another as per Tukey pair-wise comparisons. a Insecticides were applied twice at 1 & 7 DPF or once at 7 or 14 DPF. b Days post-flood.

619 1225 1282 1168 1296 1325

     

122 b 115 a 154 a 80 a 87 a 154 a

S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

41

Table 3 Population densities of rice water weevil immatures (larvae and pupae per core sample  s.e.) in treated and untreated small plots sampled over a three-week period in a water-seeded experiment in 2010. Treatment and application timing

No. immature weevils/core sample 32 DPF

Untreated l-Cyhalothrin 16 DPF Clothianidin 16 DPF Dinotefuran 16 DPF Dinotefuran 32 DPF

2.8 0.5 0.8 0.8 4.1

    

0.6 0.2 0.3 0.3 1.7

38 DPF ab b ab ab

4.8 0.5 2.8 3.4 3.8

    

Adjusted yield (kg/ha) 49 DPF

0.4 0.3 0.5 1.1 0.3

a b ab a a

10.2 5.0 10.2 10.4 3.8

    

3.4 1.6 4.2 5.4 2.4

4243 4364 4515 4310 4362

    

355 408 154 279 197

Values accompanied by same letter indicate means not significantly different from one another as per Tukey pair-wise comparisons.

sampling (34 DPF), although numerical reductions in the clothianidin 12 DPF and dinotefuran 14 DPF treatments were more than 50%. Rice yields were not significantly impacted by treatments (F ¼ 0.4; df ¼ 4, 12; P ¼ 0.8). 3.2. Topical bioassays on RWWs

l-cyhalothrin was more toxic to adult RWWs than clothianidin (Table 5); LD50 values for clothianidin were 2.3e30 times higher than LD50 values for l cyhalothrin. In addition, there was both within-season and between-season variation in LD50 values for insecticides. Susceptibility to l-cyhalothrin decreased over the 2008 season, and the LD50 values for l-cyhalothrin in 2011 were more than threefold higher than those determined in 2008. 3.3. Residual bioassays using treated foliage In assays initiated 2 h after spray, mortalities of weevils released on insecticide-treated foliage were significantly higher at 72 h (F2, 6 ¼ 9.5; df ¼ 2, 6; P ¼ 0.0006) and 144 h after release (F ¼ 20.0; df ¼ 2, 6; P ¼ 0.002) than on untreated foliage (Fig 1A). The percentage of weevils exhibiting abnormal behavior was substantially higher when fed on leaf material from treated plots than untreated plots (F ¼ 29.4; df ¼ 2, 6; P ¼ 0.0001) (Fig. 1A). In addition, cumulative number of feeding scars on insecticide-treated foliage was substantially lower than on control leaves (F ¼ 71.1; df ¼ 2, 6; P < 0.0001) at 72 h (data not shown). Number of feeding scars did not differ between the two insecticide treatments. Effects of insecticide residues also were evident in weevils released on foliage removed from plots 72 h after treatment (Fig. 1B). A significantly higher percentage of weevils showed symptoms of abnormality on treated than on untreated foliage at 72 h (F ¼ 12.7; df ¼ 2, 6; P ¼ 0.002) and 144 h (F ¼ 4.2; df ¼ 2, 6; P ¼ 0.04) (Fig. 1B). The percentage of weevils exhibiting abnormal behavior was higher in the clothianidin treatment than in the lcyhalothrin treatment at 144 h. The percentage of weevils exhibiting abnormal behavior on foliage from l-cyhalothrin plots did not differ significantly from foliage from untreated plots at 144 h after release.

3.4. Efficacy of granular dinotefuran against larvae Application of dinotefuran to potted plants infested with larvae significantly reduced larval densities (F ¼ 22.1; df ¼ 1, 9; P ¼ 0.001) compared to plants not treated with dinotefuran (Fig. 2). Application of granular dinotefuran reduced larval abundance 46 and 75% relative to untreated plants at four and seven days after application, respectively. The effect of time was not significant (F ¼ 1.9; df ¼ 18; P ¼ 0.06) (Fig. 2). 4. Discussion In three water-seeded small-plot field experiments, applications of formulated clothianidin suppressed populations of weevil larvae to an extent equal to or greater than did applications of l-cyhalothrin, a widely used pyrethroid, despite the fact that the LD50s for lcyhalothrin against adult weevils were at least 10-fold lower than for clothianidin. Several factors possibly contributed to the effectiveness of clothianidin in field settings despite its lower acute toxicity. First, the application rates of clothianidin were three times higher than those of l-cyhalothrin (91.0e121.5 and 33.7 g active ingredient per hectare, respectively), and the higher application rates of clothianidin undoubtedly helped compensate for its lower acute toxicity. Second, clothianidin exhibited greater residual activity against adult weevils than did l-cyhalothrin at 3 d after insecticide application. The greater residual activity of clothianidin relative to l-cyhalothrin was probably due in part to the systemic activity of clothianidin and in part to the higher rates of active ingredient used. Finally, neonicotinoids like clothianidin may have complex behavioral and sub-lethal effects on rice water weevils that may have contributed to suppression of larval weevil populations; greenhouse experiments with adult rice water weevils using foliage from rice plants treated as seeds with thiamethoxam, another neonicotinoid insecticide, have revealed effects on egglaying and early instar survival in addition to effects on adult survival (Lanka et al., 2013). Applications of a granular formulation of another neonicotinoid, dinotefuran, to small field plots 14e32 days after flooding were also generally as effective as foliar applications of l-cyhalothrin. Unlike

Table 4 Population densities of rice water weevil immatures (larvae and pupae per core sample  s.e.) in treated and untreated small plots sampled over a three-week period in a water-seeded trial in 2011. Treatment and application timing

No. immature weevils/core sample 21 DPF

Untreated l-Cyhalothrin 5DPF Clothianidin-5DPF Clothianidin-12 DPF Dinotefuran 14 DPF

3.8 2.3 1.4 0.8 0.8

    

1.0 0.7 0.7 0.3 0.3

27 DPF a a a b b

10.8 3.5 2.5 1.8 2.6

    

Adjusted yield (kg/ha) 34 DPF

2.9 0.7 1.4 0.9 0.7

a b b b b

8.9 5.6 5.3 2.8 4

    

2.0 1.0 1.4 0.7 1.0

Values accompanied by same letter indicate means not significantly different from one another as per Tukey pair-wise comparisons.

4901 5157 5174 4846 4875

    

80 197 281 223 388

42

S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

Table 5 Effects of insecticides at 72 h after topical application to rice water weevil adults in 2008 and 2011.

16-May-08 16-Jun-08 26-May-11 16-Jun-08 26-May-11 a

LD50a (95% fiducial limits)

l-Cyhalothrin 0.17 (0.13e0.21) l-Cyhalothrin 0.33 (0.22e0.5) l-Cyhalothrin 1.15 (0.87e1.5) Clothianidin Clothianidin

5.15 (2.89e7.78) 2.7 (1.8e4.0)

Slope 2.17 2.8 1.37 0.89 1.0

    

0.5 0.5 0.12 0.12 0.09

Chi-square <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

ng AI/weevil.

the other insecticides used in this study, dinotefuran granules were applied to flooded soils with the intent of directly controlling rice water weevil larvae, the damaging stage of this pest. The acute larvicidal potential of this granular insecticide was supported by the results of the greenhouse experiment, in which application of dinotefuran granules to plants infested with weevil larvae significantly reduced larval numbers relative to controls within four days of application. The LD50s obtained in this study are the first topical LD50s reported for any insecticides against the RWW since Rahim et al. (1981) determined LD50s for two older pyrethroids, fenvalerate and permethrin, and a number of organophosphate and carbamate insecticides. The LD50 values for l-cyhalothrin in the current study were lower by an order of magnitude than the LD50s reported by Rahim et al. (1981) for fenvalerate and permethrin. In addition, the topical LD50s for clothianidin are the first topical LD50s reported for a neonicotinoid against the RWW. In addition to the differences in acute toxicity of l-cyhalothrin and clothianidin, which are consistent with the former compound’s greater lipophilicity, the 72-h

Fig. 1. Effects of RWW feeding on foliage from treated and untreated plots. Bars represent percent incidence of weevils showing each category of symptoms at different time points in assays initiated 2 h (A) and 72 h (B) after foliar sprays. Asterisk or double dagger symbol in each weevil category indicates statistically significant difference in incidence of symptoms.

Number of larvae per plant

Assay date Insecticide

20

larval density on control plants larval density on dinotefuran-treated plants

18 16 14 12 10 8 6 4 2 0 DAT

4 DAT

7 DAT

Days After Treatment (DAT) Fig. 2. Reduction in densities of rice water weevil larvae on roots of plants treated with a granular dinotefuran formulation under greenhouse conditions. Larval numbers at 0 DAT represent larval densities before imposition of the dinotefuran treatment.

LD50 values for these two insecticides varied both within and between growing seasons. Within-season variation in susceptibility of the RWW to insecticides was also observed by Rahim et al. (1981). Variation in acute toxicity among experiments in this study could be due to several factors such as prior exposure of weevils to insecticides, host-plant quality, temperature, and even circadian rhythms (Godfrey and Fuson, 2001; Hamby et al., 2013). Variation in the efficacy of insecticides among the three smallplot field trials most likely resulted from differences in timing of insecticide applications relative to the abundance and population dynamics of weevils. In the 2010 experiment, which was planted early in the season, applications of insecticides were not triggered until adult weevils were observed in plots 16 d after flooding. Under these conditions, the performance of clothianidin and l-cyhalothrin were statistically indistinguishable, although there was a trend toward lower larval densities in l-cyhalothrin-treated plots. Not surprisingly, the early application of dinotefuran (16 d after flooding) was more effective at controlling weevil larvae at 32 and 38 d after flooding, while the late application of dinotefuran was more effective at controlling weevil larvae 49 d after flooding. In 2011, weevil populations were again low, and applications of insecticides were not triggered by presence of adults in plots until 5 d after flooding. In this experiment, applications of clothianidin at 12 d post-flood out-performed applications of both clothianidin and l-cyhalothrin made at 5 d after flooding, perhaps because insecticide application at this time better coincided with peak populations of adult weevils. Post-flood applications of dinotefuran at 21 d post-flood were also more effective than foliar insecticide applications made at 5 d after flooding in this experiment. In the 2009 experiment, which was planted later in the season, adult weevils were present in plots at the time of flooding and insecticide applications were made immediately after flooding. Under these conditions, applications of both clothianidian and dinotefuran were as effective as applications of l-cyhalothrin. Interestingly, the effectiveness of a single application of a high rate of clothianidin at 7 d was equivalent to those of split applications of l-cyhalothrin and clothianidin at 1 and 7 d post-flood, suggesting perhaps that peak populations of adults occurred about a week after flooding. The greatest control of weevil larvae in the 2009 experiment was given by application of a high rate of dinotefuran at 14 d post-flood. Although more research is clearly needed to refine current recommendations for application of foliar insecticides in rice against

S.K. Lanka et al. / Crop Protection 56 (2014) 37e43

the RWW to optimize control, it is clear from these data that the efficacies of clothianidin and dinotefuran are comparable to that of the most widely used pyrethroid. There are a number of serious issues associated with the use of pyrethroids to manage RWWs in water-seeded rice in the southern United States, including inadequate residual activity and toxicity to non-target organisms. Recently, a foliar formulation of clothianidin (Belay 2.13 SC with a label rate of 84.4 g AI/ha) was approved for use in rice against L. oryzophilus. The results of the current study as well as another published study (Gore et al., 2011) suggest that Belay will provide control of the RWW similar to that currently provided by pyrethroids. In fact, the longer residual activity of clothianidin may allow longer flexibility in timing of applications. Gore et al. (2011) showed that the efficacy of l-cyhalothrin against RWW was more dependent on timing of application than was the efficacy of clothianidin. Furthermore, neonicotinoids such as clothianidin are less toxic to crawfish than are pyrethroids as revealed by lab and simulated paddy studies (Barbee and Stout, 2009; Stout and Lanka, unpublished data). Acknowledgments The authors acknowledge the assistance of Marty Frey with field trials and student workers for collection of weevils. This manuscript was approved by the Director, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center (manuscript number: 2013-234-9510). References Barbee, G.C., McClain, W.R., Lanka, S.K., Stout, M.J., 2010. Acute toxicity of chlorantraniliprole to non-target crayfish (Procambarus clarkii) associated with ricee crayfish cropping systems. Pest Manag. Sci. 66, 996e1001. Barbee, G.C., Stout, M.J., 2009. Comparative acute toxicity of neonicotinoid and pyrethroid insecticides to non-target crayfish (Procambarus clarkii) associated with riceecrayfish crop rotations. Pest Manag. Sci. 65, 1250e1256. Bowling, C.C., 1968. Rice water weevil resistance to aldrin in Texas. J. Econ. Entomol. 6, 1027e1030. Counce, P.A., Keisling, T.C., Mitchell, A.J., 2000. A uniform, objective, and adaptive system for expressing rice development. Crop Sci. 40, 436e443. Everett, T.R., Trahan, G.B., 1967. Oviposition by rice water weevils in Louisiana. J. Econ. Entomol. 60, 305e307. Finney, D.J., 1972. Probit Analysis, third ed. Cambridge University, London. Godfrey, L.D., Fuson, K.J., 2001. Environmental and host plant effects on insecticide susceptibility of the cotton aphid. J. Cotton Sci. 5, 22e29. Gore, J., Cook, D., Awuni, G., 2011. Evaluation of Application Timings with Karate and Belay for Rice Water Weevil Control (last accessed 08.05.13) http://www.

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