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© 2007 Plant Management Network.
Accepted for publication 24 August 2006. Published 19 February 2007.


Efficacy of Insecticides for Control of Insect Pests of Pearl Millet for Grain Production


G. David Buntin, Department of Entomology, University of Georgia, Griffin 30223; Wayne A. Hanna, Department of Crop & Soil Sciences, University of Georgia, Tifton 31793; and Jeffrey P. Wilson and Xinzhi Ni, Crop Genetics and Breeding Research Unit, USDA-ARS, Tifton, GA 31793


Corresponding author: David Buntin. gbuntin@griffin.uga.edu


Buntin, G. D., Hanna, W. A., Wilson, J. P., and Ni, X. 2007. Efficacy of insecticides for control of insect pests of pearl millet for grain production. Online. Plant Health Progress doi:10.1094/PHP-2007-0219-01-RS.


Abstract

Pearl millet [Pennisetum glaucum (L.) R. Br.] is an alternative drought-tolerant grain crop for dryland summer production. Few insecticides are registered for use and insect management has not been extensively studied in pearl millet for grain production. Eleven trails were conducted during 2002-2004 in central and southern Georgia to understand the relative importance of insect pests and to evaluate the efficacy of currently registered insecticides against key pests. The main defoliator species were the fall armyworm [Spodoptera frugiperda (J.E. Smith)] and striped grassworm (Mocis latipes Guenιe). Main insects feeding on grain heads were the corn earworm (Helicoverpa zea Boddie), sorghum webworm, (Nola sorghiella Riley), and stink bugs — brown stink bug (Euschistus spp.) and southern green stink bug [Nezara viridula (L.)]. All insects were effectively controlled by cypermethrin at 0.025 lb a.i./acre. The 0.015-lb a.i. rate was effective against sorghum webworm, striped grassworm, and stink bugs, but efficacy against the corn earworm was variable. Spinosad was effective against corn earworm, striped grassworm, and sorghum webworm but not against stink bugs. Azadirachtin was not effective against any insect tested. Grain yield was not significantly affected by treatments in any trial indicating that pearl millet is tolerant of insect injury. Further work will be needed to define economic thresholds for key insect pests of pearl millet for grain production.


Introduction

Pearl millet [Pennisetum glaucum (L.) R. Br.] has been grown for many years in the southeastern US as a temporary summer forage crop and new varieties are being developed as an alternative drought-tolerant grain crop for dryland summer production. Pearl millet is tolerant of poor soils, low fertility, and drought conditions (1,8). A short-stature hybrid with high grain production has been released as 'TifGrain 102' (5,6,8). It matures early (about 85 days) and is suitable for double cropping (11). TifGrain 102 pearl millet produces a small-seeded grain with about 12% protein content. The crop has been commercially produced in Georgia on limited acreage since 2004. It is currently finding greatest use in commercial bobwhite quail production and supplemental feeding for the recreational wildlife industry, but is well suited as a feed for poultry (3).

Hudson (7) summarized key pests of pearl millet for grain production in the southeastern US. The most serious pest of pearl millet for both forage and grain production is the chinch bug [Blissus leucopterous leucopterous (Say)]. Chinch bugs feed mostly on the leaf sheath by sucking plant sap causing stunting and severe discoloration and necrosis (9). Infestation of seedlings can cause stand loss. In addition, Hudson (7) listed lesser cornstalk borer [Elasmopalpus lignosellus (Zeller)] as a pest of seedling plants, and fall armyworm [Spodoptera frugiperda (J. E. Smith)] was noted to cause defoliation of whorl-stage plants. Insects attacking grain were corn earworm (Helicoverpa zea Boddie), stink bugs (Pentatomidae), and leaf-footed bugs (Coreidae).

Insect management in pearl millet for grain production has not been extensively studied. The objectives of this study were to understand the relative importance of key insect pests and evaluate the efficacy of the currently labeled insecticides, azadirachtin, cypermethrin, and spinosad for control of these key insect pests.


Insecticide Efficacy Trials

A total of 11 trials were conducted in 2002-2004 in 2- to 5-acre pearl millet fields located at the Coastal Plain Experiment Station (31°29’38”N, 83°33’47”W; elevation 120 m), Southeast Branch Experiment Station (32°52’34”N, 83°12’58”W; elevation 79 m), and the Dempsey Research Farm (31°15’04”N, 84°17’56”W; elevation 279 m) near Tifton, Midville, and Griffin, GA, respectively. In all trials, pearl millet was planted at 3 lb of seed per acre in 21-inch rows using a grain drill. Tillage was chisel plowing followed by disking. Fertility conformed to Georgia Cooperative Extension Service recommendations (8). Atrazine was applied at planting to control weeds.

Plots were inspected periodically throughout the season for insects. A series of trials were established at grain head emergence, 41 to 61 days after planting. Trials treated at 40 to 45 days after planting (DAP) occurred at 20 to 50% pollen shed stage. Trials treated about 10 to 15 days later occurred during milk stage. In each trial, plots measured six rows by 25 or 30 ft long and were arranged in a randomized complete block design with 3 to 5 replicates. Treatments were an untreated check; azadirachtin (Azatin XL) at 0.0325 lb a.i./acre; spinosad (Tracer 4SC, Dow AgroSciences) at 0.047, 0.062, or 0.094 lb a.i./acre (1.5, 2.0, or 3.0 fl oz per acre, respectively); and cypermethrin (Fury 1.5EC in 2002 and Mustang MAX 0.8EC in 2003 and 2004, FMC Corp.) at 0.011 to 0.025 lb a.i./acre (1.76 to 4.0 fl oz/acre for Mustang MAX, respectively). Trials in 2002 used the highest labeled rate of cypermethrin and spinosad but did not include azadirachtin. Insecticides were applied using a CO2-powered backpack sprayer with TXVK-8 hollow-cone nozzles which delivered 20 gal/acre.

Insects were sampled by two techniques. Head samples consisted of striking 20 grain heads individually using a PVC rod (1-inch diameter) over a sweep net (15-inch diameter). Whole-plant shake samples consisted of a sweep net being held below the foliage and the entire plant shaken using a PVC rod. Ten whole-plant samples were collected per plot. Samples were bagged and returned to the laboratory for sorting and counting insects. Trials were sampled pretreatment and two times up to 16 days after application.

To estimate grain yield the number of grain heads in one row of each plot was counted. Small heads without grain were excluded. About two weeks before maturity, paper bags were placed individually over 20 to 30 grain heads per plot to prevent bird feeding. Bagged heads were cut and threshed using a small-bundle grain thresher. Grain was dried and grain weight, moisture content, and test weight (bulk density) were measured. Plot yield was estimated from head number and grain weight per head.

Results of insect counts were analyzed by sample date and trial using ANOVA (Proc GLM, SAS Institute Inc., Cary, NC). Means were separated using protected LSD (α = 0.05). Grain yield also was analyzed using a 2-way ANOVA for a randomized complete block design and protected LSD.


Chinch Bugs and Seedling Insect Pests

Over 10 years of research plots and field trials have revealed that the chinch bug is the main insect pest of pearl millet (7,8). Chinch bugs can kill seedling stands if large populations are not treated. They also can reach enormous levels in pearl millet during heading and grain fill where they prematurely kill the plant. Despite its importance, chinch bug numbers were low in all trials.

Lesser cornstalk borer reduced stand of one trial at Tifton, GA in 2002 when planted in mid-June under hot, dry conditions. A foliar application of Mustang Max at 0.025 lb a.i./acre at 14 days after planting did not prevent lesser cornstalk borer damage and stand loss (data not shown).


Foliage Insects Pests

Key defoliators were the fall armyworm and striped grassworm (Mocis latipes Guenιe). Fall armyworms were observed infesting whorl-stage pearl millet in several trials but were not present in large enough numbers for meaningful treatment comparisons in any trial. Striped grassworm was prevalent in 2003 and caused noticeable defoliation of pearl millet. In the first trial at Tifton, striped grassworm was effectively controlled by two rates of cypermethrin and spinosad at 0.062 lb a.i./acre (Table 1). Striped grassworm numbers were lower in the second than the first trial and were significantly lower in the cypermethrin and spinosad treatments than one of the two untreated checks. Azadirachtin did not control striped grassworm in either trial (Table 1).


Table 1. Striped grassworm control by registered insecticides on grain millet heads in two trials at Tifton, GA in 2003.

Treatment Rate (lb a.i.
/acre)
Trial 1x (No. per 10
whole-plant samples)
Trial 2y (No. per 10
whole-plant samples)
4-day 14-day 7-day 15-day
Untreated-1 — 8.33 a 2.33 ab 1.33 b 1.33 ab
Untreated-2 — 9.00 a 2.67 a 3.33 a 1.00 bc
Azadirachtin 0.0325 8.00 a 1.00 ab 1.00 b 2.33 a
Cypermethrin 0.015 0.67 b 0 b 0 b 0 c
Cypermethrin 0.025 0.67 b 0 b 0.33 b 0.33 bc
Spinosad 0.062 0.33 b 0.33 b 0 b 0 c
Spinosad 0.094 — — 0 b 0 c
LSD(0.05) 3.42 1.44 2.00 1.16
F
P
15.95*
0.0002
3.73*
0.036
3.96*
0.020
5.56*
0.006

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05).

 x Trial 1 sprayed at 47 days after planting at 50% pollen shed.

 y Trial 2 sprayed at 54 days after planting at milk-soft dough stage.


Lepidopterans Feeding on Grain Heads

Lepidopterans feeding on developing seed were corn earworm and sorghum webworm (Nola sorghiella Riley). Corn earworms fed in grain spikelets causing blank areas or gaps on the grain spike. The amount of grain removed varied, but was at least equal to the length and width of a mature larva. In two trials at Griffin in 2002, cypermethrin and spinosad at the highest labeled rate (0.025 lb and 0.094 lb a.i./acre, respectively) reduced corn earworm numbers by 81 to 100% at 3 and 7 days after application at 50% pollen-shed stage (Trial 1: F = 4.68; df = 3, 9; P = 0.017; and Trial 2: F = 18.24; df = 2, 8; P < 0.001; data not shown).

In trials at Midville and Tifton in 2003 and Tifton in 2004, corn earworm was effectively controlled by cypermethrin at 0.025 lb a.i. per acre on the sample dates from 4 to 9 days after treatment, but corn earworm numbers in this treatment were not significantly less than the untreated check on sample dates from 12 to 16 days after treatment in all trials (Tables 2, 3, and 4). Cypermethrin at 0.015 lb a.i./acre also reduced earworm numbers from 4 to 9 days after treatment but reductions were not always significant such as in Trial 2 at Midville (Table 2). Spinosad at 0.062 lb a.i./acre significantly reduced corn earworms numbers on the first sample date in all trials, and this control also was evident on the second sample date when significant differences occurred (Tables 2, 3, and 4). Azadirachtin did not control corn earworm in any trial.


Table 2. Corn earworm control by registered insecticides on grain millet heads in two trials at Midville, GA in 2003.

Treatment Rate
(lb a.i.
/acre)
Trial 1x
(No. per 20 heads)
Trial 2y
(No. per 20 heads)
9-day 16-day 6-day 12-day
Untreated — 13.75 a 21.50 a 4.25 ab 0.25 a
Azadirachtin 0.0325 — — 5.25 a 0.25 a
Cypermethrin 0.015 7.00 b 20.00 a 3.25 abc 0.50 a
Cypermethrin 0.025 1.75 b 20.25 a 0.75 c 0 a
Spinosad 0.062 1.75 b 12.50 b 1.50 bc 0.25 a
LSD(0.05) 5.93 4.95 2.77 NS
F
P
9.44*
0.004
7.01*
0.010
4.33*
0.021
0.38
0.816

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05); NS = not significant.

 x Trial 1 sprayed at 45 days after planting at 50% pollen shed.

 y Trial 2 sprayed at 61 days after planting at milk-soft dough stage.


Table 3. Corn earworm numbers using two sampling techniques after treatmentx of grain heads by registered insecticides at Tifton, GA in 2003.

Treatment Rate (lb a.i.
/acre)
Head samples
(No. per 20 heads)
Whole-plant samples
(No. per 10 samples)
4-day 14-day 4-day 14-day
Untreated-1 — 12.00 ab 0.67 a 11.33 a 2.67 b
Untreated-2 — 16.33 a 0.33 a 10.33 a 3.00 ab
Azadirachtin 0.0325 8.00 abc 0.33 a 9.67 a 5.00 a
Cypermethrin 0.015 3.67 bc 0 a 1.00 b 2.33 bc
Cypermethrin 0.025 1.33 c 0 a 0.67 b 1.00 bc
Spinosad 0.062 0.33 c 0 a 1.67 b 0.33 c
LSD(0.05) 9.66 NS 6.74 2.14
F
P
2.79*
0.0383
0.60
0.7647
5.80*
0.0091
5.78*
0.089

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05); NS = not significant.

 x Trial sprayed at 47 days after planting at 50% pollen shed.


Table 4. Corn earworm numbers using two sampling techniques after treatmentx of grain heads by registered insecticides at Tifton, GA in 2004.

Treatment Rate
(lb a.i.
/acre)
Head samples
(No. per 20 heads)
Whole-plant samples
(No. per 10 samples)
4-day 8-day 4-day 8-day
Untreated-1 — 2.0 a 2.0 a 6.6 a 5.4 a
Untreated-2 — 2.4 a 2.0 a 5.2 a 5.4 a
Azadirachtin 0.0325 1.8 a 1.2 ab 5.6 a 5.2 a
Cypermethrin 0.011 0.2 b 0.6 b 1.6 b 1.4 b
Cypermethrin 0.015 0.4 b 1.0 ab 0.8 b 1.6 b
Cypermethrin 0.020 0 b 0.4 b 2.0 b 2.4 b
Spinosad 0.047 0.2 b 0.4 b 1.2 b 2.6 b
Spinosad 0.062 0.4 b 0.2 b 2.2 b 0.6 b
LSD(0.05) 1.21 1.20 2.7 1.8
F
P
2.79*
0.0383
3.46
0.0132
5.76*
0.0008
9.86*
0.0001

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05).

 x Trial sprayed at 45 days after planting at 50% pollen shed.


Sorghum webworm usually fed on the surface of grain kernels causing less injury than corn earworm. Cypermethrin and spinosad treatments effectively reduced webworm numbers on millet heads on the first sample date after treatment in three trials (Table 5). Webworm numbers in two of the three trials were not significantly different among treatments on the second sample date, because numbers in the untreated checks declined to low levels. In the Tifton trial in 2003, azadirachtin did not control sorghum webworm.


Table 5. Effect of insecticide treatments applied at pollen shed stage on sorghum webworm numbers on grain heads of pearl millet in three trials.

Treatment Rate
(lb a.i.
/acre)
2002 Tifton 2002 Griffin 2003 Tifton
7-day 14-day 3-day 7-day 4-day 14-day
Untreated-1 — 36.0 a 21.0 a 2.80 a 1.50 a 26.00 a 0.67 a
Untreated-2 — 20.0 a 23.8 a 4.33 a 1.50 a 17.67 ab 1.67 a
Azadirachtin 0.0325 — — — — 11.33 bc 1.67 a
Cypermethrin 0.015 — — — — 1.00 c 1.00 a
Cypermethrin 0.025 0.6 b 4.8 b 0.50 b 0 a 0.33 c 0 a
Spinosad 0.062 — — — — 0.33 c 0 a
Spinosad 0.094 0 b 0.2 b 0.33 b 0 a 0 c 0 a
LSD(0.05) 17.0 8.3 6.74 NS 11.35 NS
F
P
13.35*
0.0001
4.46*
0.013
14.31*
0.001
2.43
0.111
6.50*
0.0008
1.67
0.1810

Means followed by the same letter are not significantly different (protected LSD, α = 0.05), NS = not significant.


Stink Bugs during Grain Fill

Nymphs and adults of brown stink bug (Euschistus spp.) and southern green stink bug [Nezara viridula (L.)] fed on developing seed causing deformed and shriveled seed. Numbers were too low for treatment evaluations in 2002. Stink bug numbers were greater in 2003 and were greatest on heads of millet planted in early July. Stink bugs were sampled using both head sample and whole-plant techniques in all 2003 trials. Generally the whole-plant samples collected more bugs and provided better separation of treatment effects. Both rates of cypermethrin effectively reduced stink bug numbers in Trial 1 and at the first sample date in Trial 2 at Tifton (Table 6). Stink bug numbers were not different among treatments on the second sample date in Trial 2. Spinosad treatments did not reduce stink bugs in either trial. Leaf-footed bugs [Leptoglossus phyllopus (L.)] also were present feeding on millet grain, but numbers were too low for meaningful treatment comparisons.


Table 6. Stink bug control by registered insecticides on pearl millet in two trials at Tifton, GA in 2003.

Treatment Rate
(lb a.i.
/acre)
Trial 1x (No. per 10
whole-plant samples)
Trial 2y (No. per 10
whole-plant samples)
4-day 14-day 7-day 15-day
Untreated-1 —   3.33 ab 14.33 a     6.33 a     1.33 a
Untreated-2 — 7.33 a 7.00 ab     7.00 a     2.67 a
Azadirachtin 0.0325 7.00 a 13.00 a     6.33 a     2.33 a
Cypermethrin 0.015 2.00 b  1.33 c     1.00 b     5.67 a
Cypermethrin 0.025 1.33 b  1.00 c     0.33 b     2.67 a
Spinosad 0.062   3.67 ab 10.00 ab     3.67 ab     3.33 a
Spinosad 0.094 — —     8.33 a     4.00 a
LSD(0.05) 6.74  5.08     4.96 NS
F
P
3.40*
0.047
12.12*
0.0006
    3.71*
    0.026
    0.49
    0.801

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05); NS = not significant.

 x Trial 1 sprayed at 47 days after planting at 50% pollen shed.

 y Trial 2 sprayed at 54 days after planting at milk-soft dough stage.


Effective Rate of Cypermethrin

One trial in September 2003 at Tifton compared four rates of cypermethrin for control of corn earworm, sorghum webworm, and stink bugs. Numbers of all three insects at 5 days after application were significantly lower in the treated than the untreated plots, and numbers were not different among the four rates of cypermethrin (Table 7). Counts of striped grassworm were inconclusive.


Table 7. Effect of cypermethrin rate on insect numbers on pearl millet at Tifton, GA in 2003x.

Rate
(lb a.i.
/acre)
Corn earworm Sorghum webworm Stink bugs Striped grassworm
No. per 20 grain spikes No. per 20 grain spikes No. per 10 whole-plant samples No. per 10 whole-plant samples
0 1.00 a 16.00 a 9.25 a 0.75 a
0.011 0.25 b 2.75 b 0 b 0 a
0.015 0.25 b 2.25 b 0 b 0 a
0.020 0 b 0.50 b 0.50 b 0 a
0.025 0 b 1.50 b 0 b 0 a
LSD(0.05) 0.70 5.89 5.87 NS
P
F
3.24*
0.050
11.33*
0.0005
4.60*
0.018
2.45
0.102

Means within columns followed by the same letter are not significantly different (LSD, α = 0.05); NS = not significant.

 x Sprayed on September 20, 2003 at 45 days after planting at 20% pollen shed; sampled 5 days after application.


Insecticide Effects on Nature Enemies

Populations of natural enemies were examined in two trials at Griffin in 2002, where insecticides were applied at 50% pollen shed and milk stage in the trials, respectively. Trial 1 also had a cypermethrin treatment at 14 days after planting. Natural enemies present were lady beetle larvae mainly convergent lady beetle [Hippodamia convergens Guιrin-Mιneville], and big eyed bug [Geocoris punctipes (Say)]. Cypermethrin virtually eliminated both taxa at 3 and 7 days after application in both trials, but reductions were significant only in the second trail. Spinosad had no significant effect on numbers of either taxon in both trials (Table 8).


Table 8. Effect of insecticide treatments on lady beetle larvae and big eyed bugs on grain heads of pearl millet in two trials at Griffin, GA in 2002.

Treatmentx Lady beetle larvae
(No. per 20 grain heads)
Big eyed bugs
(No. per 20 grain heads)
Trial 1 Trial 2 Trial 1 Trial 2
3 day 7 day 3 day 7 day 3 day 7 day 3 day 7 day
Untreated-1 0.5 a 2.3 a 1.2 a 3.0 a 2.5 a 0.5 b 4.6 a 2.0 a
Untreated-2 0.8 a 3.5 a — — 1.0 a 2.8 a — —
Cypermethrin at 0.025 lb a.i./acre 0 a 0 a 0 b 0 b 0 a 0.3 b 0.4 b 0 b
Spinosad at 0.094 lb a.i./acre 1.8 a 1.3 a 1.6 a 3.0 a 1.0 a 0.8 b 3.2 ab 1.6 a
LSD(0.05) NS NS 0.9 2.3 NS 1.8 2.9 0.5
F
P
1.63
0.230
2.81
0.074
5.39*
0.041
5.81*
0.028
2.15
0.163
3.77*
0.033
5.67*
0.029
42.67*
0.001

Means followed by the same letter are not significantly different (protected LSD, α = 0.05); NS = not significant.

 x Treatments applied at 50% pollen shed; Untreated-2 was treated with cypermethrin at 14 days after planting but was not treated during heading/grain fill.


Grain Yield Response to Insecticide Treatments

Although grain yield usually was greater in millet treated with spinosad or the high rate of cypermethrin, yield was not significant different among treatments in the six trials where yield was taken (Table 9). Yield in the two trials at Tifton in 2003 was lost because birds removed seed before heads could be bagged for harvest. Grain yield also was not significantly different between treatments in the cypermethrin rate trial at Tifton in 2003 (F = 3.17; df = 4, 12; P = 0.054; data not shown).


Table 9. Effect of insecticide treatments during grain fill on grain yield (lb / acre) of pearl millet in six trials at three locations in GA.

Treatment Rate
(lb a.i.
/acre)
2002 Tifton Trial 1 2002 Griffin Trial 1 2002 Griffin Trial 2 2003 Midville Trial 1 2003 Midville Trial 2 2004 Tifton
Untreated-1 — 2450 2549 4376 2508 2398 1859
Untreated-2x — 2393 2638 — — — 1916
Azadirachtin 0.0325 — — — — 2329 1952
Cypermethrin 0.011 — — — — — 2112
Cypermethrin 0.015 — — — 2445 2321 2231
Cypermethrin 0.025 (0.020)y 2566 3032 4320 2223 2850 2292
Spinosad 0.047 — — — — — 2005
Spinosad 0.062 — — — 2393 2675 2020
Spinosad 0.094 2567 3015 5058 — — -
LSD(0.05) NS NS NS NS NS NS
F
P
0.42
0.70
0.31
0.86
0.57
0.59
0.10
0.96
2.38
0.11
1.70
0.13

Means followed by the same letter are not significantly different (protected LSD, α = 0.05); NS = not significant.

 x Treated with cypermethrin at 0.025 lb a.i./acre at 14 days after planting but not treated during heading/grain fill in 2002; untreated in 2003 and 2004.

 y Rate in parentheses is for the 2004 Tifton trial.


Implications for Insect Management

Insects attacking pearl millet are similar to the insects attacking grain sorghum in Georgia (2), with the exception of sorghum midge [Contarinia sorghicola (Coquillett)], which does not attack millet. The propensity of pearl millet to produce multiple tillers tends to reduce the impact of whorl-stage defoliation in pearl millet as compared with corn and sorghum. Pearl millet can be planted from May to early August in Georgia (8), but insect populations tended to be largest when millet was planted in early July and grain head emergence and grain filling occurred in August. The short maturity of TifGrain 102 also tends to limit the time for attack by insects. Rapid grain development plus high levels of natural mortality from natural enemies and rain events accounted for the decline in insect numbers in untreated plots over the sample period of most trials.

Insect sampling techniques are not well developed for peal millet for grain production. Although this study did not rigorously compare the two techniques, in general stink bugs and striped grassworm were best sampled using the whole-plant technique, whereas the grain head sampling technique was best for sorghum webworm. Both techniques were useful for sampling corn earworm but usually the grain head technique provided greater counts than the whole-plant technique.

Corn earworm, sorghum webworm, and stink bugs were effectively controlled by cypermethrin when applied during pollen shed and grain filling. Cypermethrin also controlled striped grassworm feeding on foliage during this stage. The 0.025-lb a.i. rate was effective up to 9 days after application in all trials. The 0.015-lb a.i. rate was variable in efficacy against all insects studied. This suggests that the 0.02- to 0.025-lb a.i. rates should be used especially when corn earworm is present.

Spinosad was very effective against corn earworm, sorghum webworm, and striped grassworm at all rates tested. Usually control persisted to the second sampling date (8 to 16 days after treatment). Spinosad did not control stink bugs. Azadirachtin was not effective against any insect pest sampled on millet in this study. Lepidopterans and stink bugs on grain heads were best controlled by a single application at about 50% pollen-shed stage. Cypermethrin also reduced populations of lady beetle larvae and big eyed bugs. However, spinosad did not reduce populations of these natural enemies which is consistent with previous studies (4,10).

Grain yield was not significantly affected by treatments in any trials although insecticide-treated plants usually yielded more than the untreated check in most trials where insects attacking grain were controlled by the high rates of cypermethrin and spinosad. Nevertheless, lack of significant yield response indicates that pearl millet is tolerant to injury by insects. Peak populations of insects sampled in these trials were about 1 striped grassworm or 1 stink bug per whole-plant sample and 1 corn earworm or 1.5 sorghum webworms per grain head. Presumably population levels greater than these are needed to cause significant yield loss. Further research will be needed to define economic injury levels for of these pests on pearl millet for grain production.


Acknowledgments

We thank W. R. Slaughter, M. Esary, and T. Crawford for technical assistance with data collection and entry. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture.


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