Efficacy of Insecticides to Citrus Thrips on Lemons in Yuma Arizona 1997

Abstract

Three small plot efficacy trials were conducted evaluating different insecticide rotation regimes using commercially available insecticides and the effectiveness of new insecticide chemistries to control citrus thrips. Because of its long residual activity, and ability to control post-application egg hatches, Carzol appears to be the product that best fit the petal fall application window. Agri-Mek, Baythroid, Dimethoate or Vydate are probably good follow-up insecticides. However, Agri-Mek and Baythroid will probably provide greater control, especially under hotter conditions. If temperatures are cool, Agri-Mek looks good at reduced rates. The best insecticide for subsequent applications depends on temperatures and what was previously applied. Avoid making back-to-back applications of the same materials, and Dimethoate or Vydate applications should probably be followed by Carzol to catch post-application egg hatches. Overall, Vydate appear very similar to Dimethoate in efficacy and residual activity, while Baythroid appear slightly better. Although the addition of Lannate to Dimethoate does slightly enhance thrips control, the additional cost probably does not justify the tank mix. Of the new chemistries (Alert, Success, Ni-25, and M-96-015) evaluated, Success and M-96-015 appeared to offer the best fruit protection. However, M-96-015 does not appear to be very effective in killing the thrips, but very effective in repelling them. Also, M-96-015 will need to be applied at a high gallonage, i.e. 500 gal/A. None of the new products tested appear to fit the petal fall application window very well. Ni-25, Alert and Success appear to lack the residual activity of Carzol, and M-96-015 should not be used as a clean-up material but preventively following Carzol at petal fall.

Introduction

Citrus thrips, Scirtothrips citri (Moulton), is the most severe insect pest attacking citrus in the low desert areas of Arizona, and are far have the more severe than elsewhere in the United States. Presently, pest control advisors (PCA's) and growers rely primarily on Carzol, Agri-Mek, Baythroid or Dimethoate, often tank-mixed with Lannate or other insecticides for control of this pest. Although these products have proved efficacious under cool conditions, except for Carzol, they will often provide only knock-down control and suppression under hot conditions. Recent investigations suggest that insecticide resistance may be a contributing factor in poor insecticidal control. Additionally, there is concern that highly toxic and non-specific products such as Lannate and other carbamate and organophosphorus insecticides, are being targeted by the Environmental Protection Agency for discontinuation. Identifying alternative control measures for control is critical.

Within recent years, several new insecticides have begun development for citrus thrips control in citrus. Unlike many of the products developed in the past, these insecticides tend to be more target-specific and more mundane to the environment. Alert, chlorfenapyr (American Cyanamid), is in a novel class of insecticide and has a unique mode of action. It acts as a mitochondrial poison, and disrupts the electron transport chain. Basically, it shuts down the insect's power generating capabilities. Although Alert has contact activity and a rapid knockdown, it is most toxic after consumption. Once ingested, the insect converts the molecule into a more toxic structure. Alert is translaminar and will move into the leaf tissue where contacted, but is not systemic. Alert appears to be toxic to a broad range of Lepidopterous pests, thrips and mites. Ni-25, acetamiprid, is a new product being developed by Rhone Poulenc. It belongs to the chloronicotynl class of insecticides. Although, Ni-25 is noted for its activity towards whiteflies and aphids it has demonstrated some activity to thrips. Success, spinosad (Dow), is a fermentation by-product produced by the bacterium Saccharopolyspora spinosa. Success has demonstrated good activity against a broad range of Lepidopterous pests, leafminers and thrips. Like Alert, it has translaminar activity and will move into the leaf tissue, and is most active after ingestion. It is thought to act at the GABA site and also at the nicotinic receptor on the nerve synapse. Success is very safe to beneficial insects and is low in toxicity to mammals. M-96-015 is a new product being developed by Engelhard. Unlike most products currently being developed for insect control in citrus, M-96-015 is thought to act primarily as a repellent.

The purpose of this research was to evaluate the efficacy new insecticides and insecticide currently available for citrus thrips control, for their potential for use in the unique growing conditions of the low desert citrus production areas of Arizona. The following report represents citrus thrips control data collected during 1997 from three separate small plot efficacy trials.

Materials and Methods

Nine year old lemon trees managed by Glen Curtis Inc. in Yuma, AZ were treated with insecticides for control of in three separate efficacy trials during 1997. All small plot tests were randomized complete block designs consisting of four replicates. Each plot (30 ft by 90 ft) consisted of three trees in a row spaced 30 ft apart. Applications were made using a backpack air-blast sprayer calibrated to deliver 100 gal/acre.

Temperature data was obtained from the AZMET weather database system, from the weather station located at the University of Arizona Yuma Mesa Station, ca. 300 yd from the test site. Temperature data are presented as an average maximum daily temperature in °F for each 24 hr period following the most recent insecticide application. Percent infested fruit were estimated by sampling ten fruit per tree for the presence or absence of immature citrus thrips. In addition to thrips, products were evaluated for their tendency to flare or control twospotted spider mite, Tetranychus urticae Koch. Fruit sampled for thrips infestation were also evaluated for mite infestation. Fruit damage was estimated on by rating the degree of scarring to the rind. Scarring was rated as 1=no scarring, 2=slight scarring around the calyx, 3=significant scarring around the calyx, 4=slight scarring on the side of the fruit and 5=major scarring on the side of the fruit. Fruit with a damage rating of 2, are not considered to be scarred heavy enough to cause a downgrade in quality. Fruit with 3 or higher damage ratings, are considered significantly scarred and subject to downgrading. Percentage fruit infested with immature citrus thrips or twospotted mites were transformed using a square-root transformation for analysis. Percentage of fruit with no scarring and significant scarring were transformed using an arc-sine transformation, while the percentage of fruit with slight scarring was transformed using a square-root transformation for analysis. Percentage values are presented in the tables. Differences among treatments were separated using ANOVA and an F protected LSD.

Agri-Mek Test

Agri-Mek was evaluated for its efficacy at normal (10 oz. /A) and reduced rates (5 oz. / A) in a rotation with Dimethoate and Carzol. These treatments were compared to standard rotations of Carzol and Dimethoate. Applications were initiated at petal fall. Agri-Mek applications included NR-415 spray oil at 1.0 gal / A, and Carzol and Dimethoate treatments included Kinetic spreader-sticker at 0.1% v/v and Neutralizer buffer at 0.125% v/v. Treatments were applied on an as needed basis, when the number of fruit infested with immature citrus thrips was greater than or equal to 10%. Application dates and rates are displayed in Table 1.

Commercial Insecticides Test

Various rotation regimes including: Dimethoate and Carzol; Danitol, Dimethoate and Carzol; Baythroid, Dimethoate and Carzol; Dimethoate, Carzol and Dimethoate + Lannate; Agri-Mek, Dimethoate and Carzol; and Dimethoate, Vydate and Carzol were evaluated for efficacy towards citrus thrips and twospotted spider mites. Applications were initiated at petal fall. Agri-Mek applications included NR-415 spray oil at 1.0 gal / A. Other treatments included Kinetic spreader-sticker at 0.1% v/v. Applications including Carzol or Dimethoate included Neutralizer buffer at 0.125% v/v. Treatments were applied on an as needed basis, when the number of fruit infested with immature citrus thrips was greater than or equal to10%. Application dates and rates are displayed in Table 5.

Experimentals Test

Experimental insecticides were evaluated for their potential for controlling citrus thrips and twospotted spider mites on lemons. Treatments included two rates of Ni-25, Alert, Success, M-96-015 and a standard rotation regime of Dimethoate and Carzol. All treatment applications included Kinetic spreader-sticker at 0.1% v/v. Applications including Carzol or Dimethoate included Neutralizer buffer at 0.125% v/v. Treatments were applied on an as needed basis, when the number of fruit infested with immature citrus thrips was greater than or equal to10% or if thrips numbers were moderate and substantial time had elapsed since the last application . Application dates and rates are displayed in Table 9.

Results and Discussion

Agri-Mek Test (Tables 1, 2, 3, 4)

Following the first application at petal fall on Mar 21, of the treatments applied, only Carzol provided more than 1 week of control. (Table 2). Citrus thrips populations at this time were no doubt unsynchronized, meaning there were many life stages present and eggs were continually hatching. The long residual activity of Carzol allowed this product to control thrips that had hatched after the application, Agri-Mek and Dimethoate are fairly short-lived materials. Because of the long residual activity of Carzol, it has a good fit for the first application, especially if it is warm, because it can pick up post-application hatches. The second application was applied on Mar 26, treatment 6 which was Carzol didn't require re-treatment. Following application 2, temperatures cooled and populations remained below 10% infestation in all treatments until April 22. On April 22, only the plots spray with Dimethoate or skipped during application 2 required re-treatment. If a period of cool weather is expected, Carzol and Agri-Mek, even at reduced rates, appear to offer the best control. During the period of applications 3 and 4, temperatures began to warm, with average maximum daily temperatures beginning in the lower 90s and ending in the upper 90s (Table 2 and 3). During this period, Carzol provided about 3 weeks control, while Dimethoate provided only 1 week. For the most part, temperatures for the remainder of the season were hot, in excess of 100°F (Tables 3 and 4). Under these extreme conditions, Agri-Mek at 5 oz/A did not provide sufficient control, while the 10-oz rate did. For the most part, Carzol was still performing well, and Dimethoate was performing marginally, especially if it followed a weak material the previous week, i.e. Agri-Mek at 5 oz/A.

Differences among treatments in the number of twospotted spider mites were detected only on May 3. None of the rotation regimes seemed to flare mites, relative to the untreated, and it was evident that those rotations containing Agri-Mek had fewer mites.

All of the treatment regimes evaluated contained a lower percentage of significantly scarred fruit (fruit that would be downgraded). The only treatment regime that did not differ from the untreated in the percentage of fruit with no scars, was when Dimethoate was sprayed at petal fall, followed by a reduced rate of Agri-Mek. Treatment regimes that included Agri-Mek at 10 oz/A required 4 applications to get through the season, relative to 5 or 6 applications for the other regimes. This reflects the increased efficacy over Agri-Mek at the low rate and Dimethoate.

This trial demonstrated the importance of achieving good thrips control at petal fall. Carzol appears to fill this niche nicely. During periods of cooler temperatures, Agri-Mek, Dimethoate and Carzol all performed well, however Carzol and Agri-Mek were the better products. During this time it is feasible to use Agri-Mek at reduced rates without jeopardizing control. Under hot conditions, it is important to avoid using weaker materials so the thrips don't spread out their egg lays. Once egg lays are spread out in time, even the more efficacious insecticides will appear weak. Avoid using Dimethoate or reduced rates of Agri-Mek is temperatures are 95°F or greater. Based on these data and considering only those products used in this trial, although we didn't test this specific regime, a good treatment regime might be Carzol, followed by Agri-Mek at 5 oz/A, followed by Carzol, then followed by Carzol, or Agri-Mek at 10oz/A or Dimethoate if temperature aren't too hot and most the fruit is near 2.0 inches in diameter.

Commercial Insecticides Test (Tables 5-8)

Application 1 was made at petal fall on 19 March. Neither Dimethoate, Baythroid nor Danitol offered extend control at this period, probably because of post-application egg hatches (Table 6). Neither Carzol nor Agri-Mek required re-treatment within a week, however Carzol appeared to offer better residual control. Following application 2, temperatures cooled and all treatments provided good thrips control and numbers remained low for about 4 weeks. All treatment regimes required an insecticide application by April 24. Because of the long period since the second application, the thrips egg hatch was probably unsynchronized. Thus similar to the first application, following application 3 Carzol was the only product to offer good residual activity. All treatment regimes except the no. 8, which had been treated with Carzol during application 3 and didn't require re-treatment, were treated with Carzol at application 4 (Table 7). Following application 4, there was no clear cut distinction as to why two of the Carzol treatments (regimes no. 2 and 6) provided only 1 week of control and required an application on May 8. Otherwise Carzol provided 2 to 3 weeks control. Although I haven't been able to sort out the intricacy of the rotation relationships, it appears that weaker insecticides may reduce the apparent efficacy of insecticide applications 2 to 3 weeks latter, especially under hot conditions. The few thrips that survive the applications of the weaker materials probably spread out the egg hatch, resulting in overlapping generations, and control difficulty. When comparing efficacy of Dimethoate alone to Dimethoate + Lannate at application 5, statistically the addition of Lannate does not significantly enhance the activity under hot conditions. Adding Lannate is probably not cost effective. Additionally, under hot conditions, Carzol appeared to offer the best control relative to Dimethoate, Vydate or Agri-Mek (Tables 7 and 8). None of the treatment regimes evaluated appeared to flare mite over the untreated check.

Based on the amount of significant fruit scarring, all of the treatment regimes offered fruit protection over the untreated control. Treatment regime no. 5 contained the smallest amount of significantly scarred fruit. This regime contained Baythroid rotated with Dimethoate and Carzol. The efficacy of the pyrethroids appears to fall between Dimethoate and Carzol (see Table 6, April 30) which explains the lower amount of scarring compared to regimes with more frequent applications of weaker materials. Regimes that consisted of Carzol rotated with Dimethoate and non-pyrethroid products, similar in efficacy to Dimethoate, i.e. Vydate or Dimethoate + Lannate, appeared to be the weaker regimes. Overall, Vydate appears very similar to Dimethoate in its activity, and should be treated as an alternative to Dimethoate.

Experimentals Test (Tables 9-11)

All treatments were applied at petal fall on March 19 (Table 9). At 2 days after treatment M-94-015 was the only treatment that did not differ from the untreated. By 6 days after treatment there were no differences among any of the treatments and the untreated (Table 10), suggesting that none of the products evaluated provided enough residual activity to control the post-application eggs hatches. All treatments were again treated on March 26. However, Carzol was substituted for Dimethoate, and M-96-015 was applied using a hand-gun sprayer instead of the air-blast sprayer. M-96-015 requires full coverage to be effective. The air-blast sprayer was not providing sufficient coverage (100 gal/A) of this material, the hand-gun was calibrated to deliver 250 gal/A. Additionally, M-96-015 is not thought to have strong insecticidal properties but acts primarily as a repellent. In retrospect, this treatment should have probably had the thrips killed with Carzol before the M-96-015 was applied. Following the second application the temperatures cooled and another application was not necessary until late April. The third application was made on April 24, none of the treatments changed except Dimethoate was rotated with Carzol. All of the products evaluated appeared equally efficacious. The only products that required spraying at application 4 were Carzol, which followed Dimethoate at application 3, Alert, and Ni-25 (Table 11). Success and M-96-015, appeared to offer greater residual activity, than Alert, Ni-25 or Dimethoate, especially when temperatures >90^°F. Following application 4, thrips populations became stagnant throughout May, the hottest portion of the season, which is usual. No further applications were necessary. It is possible that the products evaluated were so efficacious that overlapping generations were eliminated and the populations never fully recovered. Thrips populations in the Dimethoate/Carzol and untreated plots did not build probably because of inter-plot interactions (the effect of highly efficacious materials used nearby). Although mites were present, none of the products evaluated flared them in this study.

Damage ratings indicated that all of the treatments were equally effective at protecting the fruit. However, the commercial standard, Alert, and Ni-25 required an additional application to achieve this protection. Overall, Success and M-96-015 appeared to be the most efficacious materials evaluated. However, Carzol, or a similarly efficacious material should precede applications of both products, especially M-96-015 at petal fall. Alert and Ni-25 also appeared to be very effective materials, but similarly to Success and M-96-015 probably do not fit the petal fall application window as effectively as Carzol.