NEW CHEMICALS FOR THE CONTROL OF CANE GRUBS R.M. Bull

Transcription

1 Entomology NEW CHEMICALS FOR THE CONTROL OF CANE GRUBS R.M. Bull BSES, Bundaberg, Queensland, Australia ABSTRACT A summary of field and laboratory trials is presented in which insecticides are tested for activity against four white grub species of the Bundaberg and southern districts of Queensland. Relative efficacies of ethoprop, fensulfothion, carbofuran and chlorpyrifos insecticides are discussed and the potential commercial use of proprietary and new formulations of these compared. INTRODUCTION The introduction in 1947 of benzene hexachloride (BHC) to the Queensland sugar industry marked the commencement of a thirty year period during which the control of white grubs was neither difficult nor expensive. Relief from their devastating effects was equally successful with various crude (mixed isomer) BHC formulations and because the commercial recommendations enabled three successive crops to be protected, little pressure existed to find alternative insecticides. During the 1970s the organochlorine and cyclodiene insecticides fell into increasing disfavour and BSES investigated the feasibility of replacing BHC. An initial field trial at Bundaberg served only to endorse the superiority of BHC as none of the alternative candidates was as cheap, nor capable of protecting following ratoons because of limited soil persistence. In 1981 the turning point occurred at Bundaberg when numerous commercial treatments of BHC dust failed to control Antitrogus mussoni (Blackburn) the major white grub pest of sandy soils. The situation was further compounded by the rapid escalation to major pest status of Lepidiotapicticollis Lea, a previously rare species with known natural tolerance to BHC (Bull1). The distribution of soil types in the Bundaberg and adjacent sugar districts is highly complex and it is common for radical changes to occur within relatively small areas on the volcanic margins. Thus specific host soil requirements of the seven white grub species of the district result in equally complex population distributions. Efficient control under such conditions strongly favours a single method for all species and one that is capable of extended activity through a normal crop cycle (plant and three ratoons). Controlled release (CR) formulations of organo-phosphate or carbamate insecticides theoretically possessed physical properties conforming to those re- Keywords: Cane grub control, white grub control, controlled release insecticides, chlorpyrifos, terbufos, BHC, ethoprop, fensulfothion, Lepidiota negatoria, L. picticollis, L. crinita, An titrogus mussoni. 626

2 R.M. BULL 627 quirements and in 1980 BSES commenced research on experimental CR formulations of chlorpyrifos. Meanwhile, attention to the uncontrolled outbreak of A. mussoni in at Bundaberg was urgently required and experiments with a number of short lived insecticides were commenced to identify those suitable for knockdown treatment. This paper describes laboratory and field experiments conducted in the Bundaberg district of Queensland aimed at developing commercially acceptable alternatives to BHC for the control of four economically important white grub species. METHODS AND MATERIALS Laboratory bioassay8 Candidate insecticides were finely ground (granules or dust) or diluted (liquids) in ethanol and accurately measured quantities mixed with sieved (6 mm mesh) soil in a concrete mixer. Field collected larvae were individually weighed and those of extreme weight were discarded to provide the least variance around the median. The larvae were placed individually in 150 g screw-top glass jars each with a small (25 x 10 mm) quarter segment of cane stalk for food and filled completely with insecticide treated soil. The soil was unsterilised and was collected at field moisture level from canefields infested with the species to be bioassayed. There were 20 replicates per treatment. Each larva was inspected at five- to seven-day intervals, mortality was recorded and fresh food provided for the survivors. Field trials All field trials were conducted in commercial canefields on randomised block or Latin square layouts using plots of ha. Knockdown treatments were applied as granular formulations by row treatment. In some a shallow slit ( mm depth) was made on either side of the ratoon row by single cut-away discs set 250 mm apart, the insecticide was applied immediately as a mm wide band then covered with 50 mm of soil; in others, the treatments were simply applied to the undisturbed row and then covered with mm of soil. If reasonable rainfall (15-20 mm) did not occur within 7 to 10 i days spray irrigation (25 mm) was applied. CR-granules were applied either in a mm wide band across the bottom of the plant drill at tillering and covered with a minimum of 100 mm of soil, or in mm deep slits made with discs on either side of the young first ratoon row to cut the row to 250 mm width, then covered immediately with soil to 50 mm above original ground level. Populations were assessed 28 to 70 days after knockdown treatment or, in CR trials when larvae were large enough (late second or early third instar) to find readily, by field inspection. In each plot, five 300 mm cubes of soil were dug by small garden spade from the cane row and included at least portion of a cane stool. Samples were not taken from outside rows or from within 1.5 m of the end of a row. Crop yields were determined by the standard BSES method (Hogarth2) and c.c.s. was based on sample mill juice. The statistical treatment of har

3 TABLE I. Bioassay 85-1 : Insecticides vs. A. mussoniyoung third instar larvae* Treatments Rate a.i. Progressive % mortalities per days after commencement x20 replicates in soil PPm chlorpyrifos (Lorsban 15G ) ethoprop r) (Mocap log ) fensulfothion (Dasanit 1 OG ) terbufos (Counter 1 OG ) isazafos (Miral5G ) Y BHC (Crop King l5 m BHC, dust) zl 3 Control nil O P * Mean weight at commencement = 1.46 g, n = 500 OI h, CO P 0-2

4 R.M. BULL 629 tion data was by analysis of variance (randomised complete block or Latin square design). The insecticides tested in bioassays were commercial or experimental products as specified in the Tables. RESULTS Bioassays Bioassays demonstrated a sufficiently wide range of larval mortalities to permit segregation of potentially useful commercial insecticides. These were advanced to field trials if suitable formulations were available. Larval body mass was found to have a significant effect on mortality (Tables I and 11); the implication is that better commercial results should be obtained if treatments contacted larvae before the third instar was reached. Ethoprop, fensulfothion and terbufos consistently produced high mortality in bioassays of all species except Lepidiota negatoria Blackburn (Table 111). In the latter instance ethoprop and fensulfothion had low activity at only 0.5 and 2 ppm rates yet good control and strong crop responses were obtained in field trials (Table VI). L. picticollis larvae were unaffected when exposed to BHC (Table IV) confirming the inability of growers to obtain any degree of control of the pest with commercial treatments of BHC dust. A. mussoni third instar larvae showed tolerance to 0.5 and 2 ppm BHC (Table I) but were moderately susceptible at the same rates when in the second instar (Table 11). Other species (L. negatoria, and L, crinita) showed sufficient susceptibility to BHC to indicate that the long-term contact achieved in commercial treatments of TABLE ll. Bioassay 83-2: BHC and ethoprop vs. A. mussoni second instar larvae* Treatments Rate a.i. Progressive % mortalities x20 replicates in soil per days after commencement PPm *9* Y BHC (Crop King BHC," dust) ethoprop (Mocap 10G " ) Control nil * Mean weight at commencement = 0.55 g, n = 140 ** Mortalities corrected (Abbott's formula) against control

5 TABLE 111. Bioassay 84-16: Insecticides vs. L. negatoria third instar larvae* o\ w 0 Treatments Rate a.i. Progressive % mortalities per days after commencement x19 replicates chlorpyrifos (Lorsban 15G ) in soil P P ~ fensulfothion (Dasanit 1 OG ) ethoprop (MocaplOG ) carbosulfan (Marshall 25E ) terbufos (Counter 1 OG ) Y BHC (Crop Kinq BHC,, dust) Control nil * Mean weight at commencement = 2.98, n =

6 R.M. BULL 63 1 TABLE IV. Bioassay 83-12: Insecticides vs. Lepidiotapicticollis third instar larvae Treatments Progressive % mortalities 2.5 ppm a.1. ~n soil per days after commencement x20 replicates terbufos (Counter 1 OG ) fensulfothion (Dasanit 10G ) carbofuran (Furadan 10G ) chlorpyrifos (Lorsban 15G ) ethoprop (Mocap IOG ) isofenphos (Oftanol5G ) phorate (Thimet 10G ) Y BHC (Crop King BHC,, dust) Control this insecticide should provide satisfactory control. The fact that numerous well documented failures of BHC to control these two species occurred from 1980 onwards, suggested that accelerated loss of BHC from the soil was probably involved. Field trials with knockdown insecticide formulations A. mussoni was shown to be adequately controlled by BHC in the 1977 trial (77-16 in Table V) and larval mortality was not significantly different (p < 0.05) from chlorpyrifos, terbufos or ethoprop. In trial 81-1 (Table V) ethoprop and fensulfothion produced the highest larva mortalities while those chemicals with nematicidal properties (ethoprop, terbufos, fenamiphos and carbofuran) improved their relative ranking over non-nematicides in final crop yield with ethoprop gaining 25 tonnes cane per hectare over fensulfothion. Field trials using knockdown treatments of various insecticides against L. negatoria (Table VI) showed similar superior activity of ethoprop and fensulfothion, but significantly poorer (p < 0.05) mortalities f ~om carbofuran, chlorpyrifos and BHC. The optimum rate of application of etho$rop and fensulfothion in commer-

7 TABLE V. Replicated field trials: Insecticides vs. Antitrogus mussoni Treatments Xgrubsl xgrubsl tonnes cane 5 samples 5 samples per hectare 4 kg ailha 2 kg ailha terbufos 10G 5.75 b bc bc fensulfothion 10G d abc carbofuran 5G a bc ab chlorpyrifos 15G 4.75 b cd ab ethoprop 10G 8.75 ab d c fenamiphos l0g b ab isofenphos 5G cd bc phoxim 50 EC d ab BHC,, dust) 5.50 *b Untreated a a a f value LSD 5% % Means not followed by a common letter differ significantly (p < 0.05) * application rate = 3.4 kg Y isomerlha cia1 control of L. negatoria was shown to be 1.5 kg ai/ha, this rate being not significantly different (p < 0.05) from 2, 2.5 or 3 kg ai/ha for both grub mortality and cane yield. Field trials with CR-insecticide formulations CR-chlorpyrifos formulations applied to the plant drill in trial produced significant (p < 0.05) reductions of L. crinita larva populations for five successive crops (Table VII) and in other trials for at least two crops at 2 or 3 kg ai/ha. Where rate of application was tested in trials, 2 kg ai/ha chlorpyrifos was found to be optimum for two year control of A. rnussoni and L. picticollis (Table VIII). In trial (Table VII) CFL 1104 (subsequently marketed as Suscon 140G) showed significantly (p < 0.01) greater mortality of L. crinita larvae at 6 kg than at 3 kg ai/ha in the second ratoon, but by the fourth ratoon this difference, although still evident, was not significant (p < 0.05). For the control of L. picticollis in trial (Table VIII) the CR-chlorpyrifos formulation (CFL-G01001) was superior to CR-terbufos (CFL-G18001) as larva mortalities for 3, 4 and 6 kg chlorpyrifos were significantly greater (p < O<y,)than terbufos at 2, 3 and 4 kg ai/ha. Although cane yields for all treatments were significantly greater than the control (p < 0.01) those with a mean larval population over 4.6 per five samples were recorded at harvest to be badly lodged as a result

8 TABLE VI. Replicated field trials: Insecticides vs. Lepidiota negatoria _- - icl Treatment kg a.i./ha - m Control nil a Kgrubst tonnes cane Tigrubst tonnes cane Tigrubst tonnes cane F t- 5 samples per hectare 5 samples per hectare 5 samples per hectare ethoprop b b b b b b c c 8.80 b b c b carbofuran ab ab ab b abc b bc b V BHC (dust) b chlorpyrifos (ec) b b f value (treatment) LSD 5% LSD 1 % m W W Means not followed by a common letter differ significantly (p < 0.05)

9 634 ENTOMOLOGY TABLE VII.;Replicated field trial :.CR-chlorpyrifos formulations applied in the plant drill vs. Lepidiota crinita. Treatments Second ratoon Third ratoon Fourth ratoon xgrubsl tonnes cane Xgrubsl xgrubsl 5 samples per hectare 5 samples 5 samples GFL % bcd 94.92bc 1.OO c 6.50 b cd c 2.25 bc 5.00 b CFL\ % b 88.97b 4.75b i : d bc 0.51 c 4.50 b 18' I dontroi 10.25a 70.37a 8.00 a a f value (treatments) LSD 5% LSD 1 % Means not followed by a common letter differ significantly (p<0.05) of feeding damage and the accompanying loss of stools caused severe reduction in the second ratoon crop. Despite indications of high activity by terbufos against L. picticollis in bioassay (Table IV) the comparatively poor (relative to chlorpyrifos) field performance in trial was ascertained by soil persistence studies (Stickley3) to be due to exhaustion of most of the active ingredient from the CR granule after 12 months Formulation GI8001 would thus not be suitable for a normal cropping cycle of a plant and two or three ratoons. DISCUSSION The use of bioassays for identifying the most efficacious candidate insecticides was of value where extremes of reaction by the test insects occurred. Thus high levels of tolerance or susceptibility were repeatable in the field, but often those chemicals producing a range of low through high mortality with increasing concentration of insecticide gave contradictory field performance. In the case of ethoprop and fensulfothion against L. negatoria (Table VI) the unexpectedly high field mortality was probably related to the physical conditions associated with the method of application which produced a rapid release of relatively concentrated chemical into a localised zone where the grubs were active. On the other hand, long exposure to very low concentrations of chlorpyrifos produced by CR-formulations enabled excellent control of larvae for up to five crops. One weakness of CR-chlorpyrifos was found to be its lack of nematicidal proper-

10 TABLE. VIII. Replicated field trials CR-insecticides vs. Antitrogus mussoni and Lepidiota picticollis Treatment Rate First ratoon Second ratoon First ratoon (kg) A. mussoni A. mussoni L. picticollis L. picticollis - xgrubsl tonnes cane Rgrubsl5 samples tmnes cane Xgrubst tonnes cane C.C.S. 5 samples per hectare per hectare 5 samples per hectare Method of application in plant drill side-row bands in first ratoon in plant drill Untreated nil 12.00a 28.40a a 5.17a a a 65.85a ab CR-chlorpyrifos b bc 2.67 b 1.17b b (DOW 1 OCR) 3 1.OO b bc 2.33 b 2.00 b b 4 0 b 50.70~ 2.50 b 0.67 b b 6 0 b 48.85~ 3.17b 0.67 b b CR-chlorpyrifos 2 (1) 1.00b bc (2) 6.20 bcd bc bc (CFL % (11, 3 (1) 0.75 b (2) 4.60 cd cd bc CLF-GO % (2), 4 (1) 1.50b 46.49bc (2) 0.60d bcd 14.55bc 6 (1) c (2) 2.50 b 0.67b b (2) 2.20d 94.32d 14.60bc CR-terbufos b bcd c (CFL-GI %) bc b a b bc ab bcd bcd c f value (treatments) _LSD 5% LSD 1 % , Means not followed by a common letter differ significantly (p < 0.05)

11 ENTOMOLOGY / ties as where A. mussoni was a serious problem in sandy soils, crop yield responses to CR-chlorpyrifos treatments were reduced. Under those conditions ethoprop was found to maintain larger crops. (A strip trial of ethoprop 10 G at 2.5 kg ailha contiguous to trial (Table VIII) yielded 19.2 tonnes cane per hectare more than, the mean of all CR-chlorpyrifos treatments). With two alternative methods available for control of white grubs growers were faced with several factors to consider when making the decision which to use. CR-chlorpyrifos originally available as Suscon 140G@ then replaced by Suscon Blue@ cost approximately $215 (Aust.) for 4 kg ai/ha in 1985 but gave control for three (and probably up to five) years. Where nematodes were an economic problem Suscon Blue@ still required additional application of nematicide at least once in a crop cycle which added considerably to costs depending on the type and frequency of use. Knockdown treatment (Mocap 10G@ or Dasanit 10G@ ) costing $75 to $85 per hectare is used only when and if grub populations develop. Thus low grub density areas may require only infrequent use of these insecticides but high density areas could require annual treatment. (Biennial treatments were found to be the reality). Ethoprop (Mocap 10G@ ) appeared the best single treatment for A. mussoni on sandy soil because of the nematode problem. The current registered rate of application for Mocap 10G@, 3 kg ai/ha, was unnecessarily high as trials 81-1 (Table V) and (Table VI) showed 1.5 to 2 kg was equally effective. ACKNOWLEDGEMENTS 1, My thanks go to all canegrowlers who made land available for the numerous trials in this project. Also to BSES and Cane Pest and Disease Control Board personnel for assistance with data collection to which Mr N.G. McGill made the ma-, jor contribution. REFERENCES 1. Bull, R.M. (1986), Leprdiotapicticollis Lea (Scarabaeidae: Melolonthinae). An expanding pest problem in the Bundaberg district. Proc. Aust. Soc. Sugar Cane Technol., (in print). 2. Hogarth, D.M. and Skinner, J.C. (1967). A sampling method for measuring yields of sugarcane in replicated trials. BSES Tech. Comm. No Stickley, B.D.A. (1985). BSES internal correspondence. 4. Thomson, W.T. ( ). Agricultural Chemicals, Book 1 Insecticides, Thomson Publications, California. I