An Evaluation of the Residual Activity of Traditional, Safe, and Biological Insecticides Against the gypsy Moth

Materials and Methods

Eighty groups of 15 accessible oak branch tips (primarily pin oak [Quercus palustris]) were marked along the edge of a forest clearing in the Cedar Swamp Wildlife Area northeast of Smyrna, Delaware, in the spring of 1996. There was no evident natural gypsy moth population in the woodlot. Each group of tips was separated by at least 10 m (33 ft). The groups of tips were divided into 8 blocks of 10, and each block was randomly assigned 10 treatments, 1 per group of 15 tips, for a randomized block experimental design with 8 replications. All insecticides (Table 1) were applied in distilled water on May 13, 1996, following label directions and in consultation with manufacturer agents. Differences in dosage rates and the presence or absence of stickers reflect label or manufacturer requirements or suggestions. The first 4 treatments were combinations of Gypchek (gypsy moth nuclear polyhedrosis virus, LD226 strain, USDA Forest Service, Hamden, Connecticut) and Blankophor BBH (disodium salt of 2,2’-(1,2-ethenediyl) bis (5(4-(4-morpholinyl)-6-(phenylamino)-1,3,5-triazin-2-yl) amino-benzenesulfonic acid [CAS No. 16090-02-1], Burlington Chemical, Burlington, North Carolina), and were 1) Gypchek at 1 x 10¹¹ occlusion bodies (OB) per 378 L (100 gal) final solution; 2) Gypchek at 1 x 10¹² OB per 378 L final solution; 3) Gypchek at 1 x 10¹¹ OB plus 0.1% (wt:vol) Blankophor BBH per 378 L final solution; and 4) Gypchek at 1 x 10¹¹ OB plus 0.5% (wt:vol) Blankophor BBH per 378 L final solution. All 4 of the above treatments were applied with 2% (v/v) Bond sticker (Loveland Industries, Greeley, Colorado).

Treatment 5 consisted of Bt (Foray 48B, Abbott Laboratories, North Chicago, Illinois) at 2.84 L (0.7 gal, 36 BIU) per 378 L final solution without sticker. Treatment 6 consisted of azadirachtin (an active compound from the neem plant) (Azatin 4.5 WP, biosys, Columbia, Maryland) at 267 g formulation per 378 L final solution (= 12 g a.i.) applied twice at 7 days apart, with 0.125% Dragon (Dragon Corp., Roanoke, Virginia) spreader-sticker (containing alkyl aryl polyoxyethelene glycols). Treatment 7 consisted of tebufenozide (as Experimental Insecticide RH-5992 2F, Rohm and Haas, Philadelphia, Pennsylvania) at 237 mL per 189 L final solution (label recommends 8 oz per 50 gal solution per acre), with 0.25% (v/v) Bond sticker. Treatment 8 consisted of diflubenzuron (as Dimilin 25 W, Uniroyal Chemical Co., Naugatuck, Connecticut) at 237 mL per 378 L final solution, without added sticker. Treatment 9 consisted of cyfluthrin (as Tempo 2, Bayer Corporation, Kansas City, Missouri) at 30 mL per 378 L final solution, with 0.25% (v/v) Bond sticker. Treatment 10 consisted of unsprayed branch tips (experimental controls). Sprays were applied to runoff using a backpack sprayer at 20 psi.

Laboratory-reared gypsy moth larvae from a colony maintained at Beneficial Insects Introduction Research, Newark, Delaware, were placed in bags fastened over treated branch tips and then tied off. Four tips from each of the 80 groups of branch tips were bagged for a 1-hour evaluation, with a bag from each group receiving 10 first-instar, a second bag receiving 10 second-instar, a third bag receiving 10 third-instar, and the fourth bag receiving 10 fourth-instar larvae. Subsequent bags were placed over a tip from each of the 80 groups of tips 1, 2, 7, 14, 21, 28, and 35 days after treatment, with 10 larvae (second-instar, days 1, 2, 7; third-instar, days 14, 21; fourth-instar, days 28, 35) added per bag. Bags consisted of 60 x 60 cm (23.6 x 23.6 in.) squares of organza cloth seamed to make a bag. Larvae were left in the bags for 7 days and then were removed and placed, 1 per cup, in 30-cc plastic diet cups half filled with gypsy moth diet (Webb et al. 1993; Webb et al. 1994a; Webb et al. 1994c; Webb et al. 1996) and held under natural conditions in a field insectary (Webb et al. 1993; Webb et al. 1994a; Webb et al. 1994c; Webb et al. 1996). Mortality was assessed after 7,15, and 21 days.

For the 1 -hour study with 4 instars, data were analyzed by analysis of variance (ANOVA) using the General Linear Models (GLM) procedure of the SAS statistics package (SAS 1996). The data were analyzed as a randomized block design, with treatment by block interaction as the error term to test the significance of the treatment effect, while instar by block interaction was the error term to test the significance of the instar effect, and treatment by block by instar interaction was used as the error term for the treatment by instar effect. For the 1- to 35-day residue study, a repeated measures ANOVA was conducted by fitting a mixed linear model (Proc Mixed) to the data (SAS 1996). An unstructured covariance matrix was specified. Where the treatment effect was significant, means were separated at a comparison-wise error rate of 0.05 using the least significant difference (LSD) procedure (SAS 1996).

Results

Highly significant treatment effects were seen for insecticides for both the insecticide versus larval instar (F = 44.78; df = 9; P< 0.01 and the insecticide versus residual date portions of the study (F = 33.41; df = 9; P < 0.01). As expected, there were highly significant instar effects (F = 20.76; df = 33; P < 0.01) (generally, younger larvae were more easily controlled than older larvae) and residual date effects (F = 11.66; df = 6; P < 0.01) (generally, insecticide effectiveness eroded over time). More interestingly, treatment x instar effects (some insecticides were controlling older larvae better than other insecticides) and treatment x date effects (some insecticides were retaining residual effectiveness better than other insecticides) were highly significant (F= 1.96; df = 27; P< 0.01; and F= 2.90; df= 54; P< 0.01). The specific treatment effectiveness by instar of larvae fed on 1-hour residues is given in Table 2. Gypsy moth larval mortality was significantly higher for all treatments against all larval instars when compared with untreated controls. All treatments except for the unenhanced NPV treatments (1 and 2) gave a level of control that was statistically equivalent to that of the standard insecticides (diflubenzuron and cyfluthrin, treatments 8 and 9) for instars 1 and 2. NPV enhanced by 0.5% Blankophor BBH, Bt, and tebufenozide gave a level of control that was statistically equivalent to that of the standard insecticides for third instar larvae, but only tebufenozide gave equivalent control for fourth instar larvae. Speed of kill by the various treatments of second and fourth instar larvae is given in Figure 1. Cyfluthrin gave complete control of both instars within 1 week, while tebufenozide and diflubenzuron both took 2 weeks to achieve such control. Bt took 2 weeks to achieve excellent control of second instars and partial control of fourth instar larvae, while the remaining treatments took 3 weeks to reach such control levels.

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Figure 1

Mortality of (A) second instar and (B) fourth instar gypsy moths confined for 1, 2, and 3 weeks on foliage bearing 1-hour residues of the indicated treatments: (1) NPV low dose, (2) NPV high dose, (3) NPV low dose + 0.1% Blankophor BBH, (4) NPV low dose + 0.5% Blankophor BBH, (5) Bt, (6) azadirachtin, (7) tebufenozide, (8) diflubenzuron, (9) cyfluthrin, (10) untreated controls.

The residual effectiveness of the treatments (“active” defined as significantly different from untreated controls, “highly active” as not significantly different from standard insecticides) is given in Table 3. Cyfluthrin, tebufenozide, and diflubenzuron all showed highly active residual activity for the 35 days of the study, while azadirachtin residues were highly active after 21 days and Bt residues highly active after 7 days. Unenhanced NPV was largely inactive 1 day after treatment, but active to highly active NPV activity was seen even after 35 days when enhanced with Blankophor BBH, especially at the 0.5% concentration. The source of the control mortality was not determined. Three weeks in a cloth cage will result in a certain amount of death through trauma, predation by spiders, etc.

Discussion

The results permit the arborist to develop a decision matrix to address the varied concerns of customers that include speed of kill, residual activity, and to a public increasingly concerned about the environment, the speciesspecificity of control materials. As expected from previous studies (Webb et al. 1989; Webb et al. 1991; Thorpe et al. 1995), both of the standard insecticides, cyfluthrin and diflubenzuron, demonstrated excellent activity against all instars in the 1 -hour residue study and for the 35 days of the extended residue study. Because cyfluthrin (a broad-spectrum synthetic pyrethroid) also gave a quick kill, which impresses customers and prevents additional foliar feeding, it would seem to be the material of choice. However, the use of broadspectrum insecticides can lead to outbreaks of secondary organisms (Raupp et al. 1992) such as scales, aphids, and mites. Ideally, an insecticide should be less active against the natural enemies of insect pests than it is against the pest itself (Bartlett 1963). The toxicity of cyfluthrin for secondary organisms compared to the predators and parasites of such organisms is unknown but is likely in some cases to be unfavorable. Diflubenzuron, an insect molting inhibitor, was nearly as efficacious as cyfluthrin against all 4 gypsy moth instars as a 1-hour residue, and to provide an equivalent 35 day residual activity, and while slower acting, diflubenzuron is known to be relatively safe for adult predators and parasites (Wilkinson et al. 1978). Alone among the alternative insecticides tested, the ecdysone agonist tebufenozide gave high activity against all 4 larval instars and remained highly active for the 35 days of the residue study. Tebufenozide, known to be sparing of natural enemies (Brown 1996), and for that reason considered by the Environmental Protection Agency to be a “safer” insecticide, was equal to the standard insecticides in efficacy, although its speed of kill was similar to diflubenzuron.

Interest is increasing in the use of integrated pest management (IPM) in woody landscapes (Raupp et al. 1992) and in the evaluation of more natural control materials for use in these programs (Nielsen 1990). The natural insecticides evaluated here were Bt, azadirachtin, NPV, and NPV with an enhancer. A properly timed application of Bt, a bacterial insecticide, has been shown to provide satisfactory foliage protection from gypsy moth feeding, although levels of kill were less than that seen for cyfluthrin (Thorpe 1996). In the present study, Bt was clearly more toxic for second instar larvae than against fourth instar larvae, demonstrating the desirability of timing sprays against young larvae. The speed of kill for Bt was similar to that of diflubenzuron. Bt lost significant activity against the gypsy moth after 7 days. Extracts of seeds of the neem tree, Azadirachta indica, are known to be active against the gypsy moth (Skatulla and Meisner 1975). Although several compounds found in neem seed are insecticidal, the product used in this study, Azatin 4.5 WP, contains only 1 compound, azadirachtin. At the rate used in this study, azadirachtin efficacy against gypsy moth was similar to that of Bt in that it controlled younger instars better than older instars. Although its speed of action was slower than that of Bf, it remained highly active against gypsy moth 21 days after treatment.

Gypsy moth NPV is a natural disease organism that is specific to the gypsy moth and is thus the material of choice for those who seek gypsy moth control with no impact on other “nontarget” organisms. Being a disease, it has the potential to do things that the standard insecticides cannot do, such as to grow, to persist, and to spread to nontreated areas. Thus treatment of the lower foliage of shade trees can immediately lead to significant levels of control in the treated area while leading to a late-season epizootic that controls the gypsy moth population over the entire tree, and neighboring trees (Webb et al. 1990). Gypsy moth NPV, while registered with the EPA for use against gypsy moth, is not commercially available. However, our group and others are currently developing protocols for using NPV with ground equipment when it does become available (Webb et al. 1990; Podgwaite et al. 1991; Webb et al. 1993). USDA-Forest Service research has featured a tank mix of NPV plus a lignosulfonate sunscreen and a sticker (Webb et al. 1990); other work failed to confirm that a sunscreen helped the NPV when applied from the ground (Webb et al. 1993). However, NPV can be combined with other active compounds, such as neem (Shapiro et al. 1994; Cook et al. 1996), or with a stilbene disulfonic acid optical brightener such as Blankophor BBH (Webb et al. 1994a; Webb et al. 1994c; Webb et al. 1996). In laboratory bioassays, optical brighteners increase the potency of NPV by a thousand fold by a mechanism still under investigation, and also act as sunscreens (Dougherty et al. 1996). Field studies have demonstrated that the addition of Blankophor BBH to NPV tank mixes results in increased speed of kill and percent of kill, especially against older larvae (Webb et al. 1994a; Webb et al. 1994c; Webb et al. 1996), and with the suggestion of increased residual effectiveness. In the present study, both doses of NPV without Blankophor BBH gave significantly increased (compared with controls) levels of mortality against all 4 larval instars fed on 1 -hour residues, although mortality of younger instars was higher than that of older instars. A tenfold increase in dose did not lead to an obvious increase in effectiveness, except that the higher dose did retain significant activity for 1 day following treatment, while the lower dose lacked such activity. The addition of Blankophor BBH led to improved kill in the 1-hour study and to improved residual activity, with the 0.5% rate giving substantially better control than the 0.1 % rate. The activity seen against larvae fed on 35-day residues is consistent with our previous report of extended activity (Webb et al. 1996).