Commercial Evaluation of M-96-015 for Control of Citrus Mealybug, Woolly Whitefly and Citrus Thrips in Lemons

David L. Kerns, Assistant Specialist, Entomology, Yuma Agricultural Center
Tony Tellez, Research Specialist, Yuma Agricultural Center

Abstract

M-96-015 did not appear effective towards woolly whitefly but does appear to kill citrus mealybug. However, like with other insecticides coverage is a problem. The real benefit of M-96-015 towards citrus mealybug would occur if it prevented their spread. However, we were not able to measure this in this study. As with previous trials, M-96-015 is an effective citrus thrips material.

Introduction

Citrus thrips, CT, Scirtothrips citri (Moulton), is the most severe insect pest attacking citrus in the low desert areas of Arizona, and are far more severe than elsewhere in the United States. Presently, pest control advisors (PCA's) and growers rely primarily on Carzol, Agri-Mek 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 knockdown control and suppression under hot conditions. Recent investigations suggest that insecticide resistance may be a contributing factor in poor insecticidal control. Additionally, these insecticides are notorious for inducing outbreaks of secondary pests. In lemons grown in the low desert region of Arizona, the most severe secondary pests include various species of mites and the citrus mealybug, Planococcus citri (Risso). Under conditions undisturbed by insecticides, citrus mealybug is controlled primarily by native parasitoids. Once insecticides targeting thrips kill these parasitoids, groves containing low numbers of citrus mealybugs quickly become heavily infested. Citrus mealybugs cause direct and indirect damage. Heavy infestation on the fruit can cause the fruit to drop from the tree. Indirect damage is caused by unsightly contamination of the fruit from the mealybugs themselves and from honeydew excretions and sooty mold. Heavy infestations of citrus mealybug are very difficult to chemically control. Mealybugs prefer to infest the fruit and cover themselves with a dense waxy material. It is difficult for insecticides to penetrate the waxy covering and mealybugs on the backside of a piece of fruit or in the inner tree canopy are difficult to cover.

Another secondary pest that has recently been detected infesting lemons in Arizona is the woolly whitefly, Aleurothixus floccocus (Maskell). The woolly whitefly causes damage similar to the citrus mealybug except it does not infest the fruit. Not much is known about the woolly whitefly in Arizona. It was found in the State in 1996, and currently infests only one commercial lemon grove. Like the citrus mealybug, it produces a protective waxy covering and is difficult to control with insecticides.

An ideal control strategy for secondary pests in citrus is to avoid destroying the natural enemies that keep these populations in check. An insecticide that controls thrips, and suppresses or controls citrus mealybug and woolly whitefly without destroying their natural enemies would fit this niche perfectly. The purpose of this study was to evaluate M-96-015 for its ability to control citrus thrips, and to control or prevent further spread of citrus mealybug and woolly whitefly.

Materials and Methods

A 20-year-old, 60 acre lemon grove located in Yuma, AZ was selected for this study. A 10-acre block of trees containing the heaviest infestations of citrus mealybug and woolly whitefly was selected to receive the M-96-015 treatment. An adjacent 10-acre block severed as the untreated check. Each treatment block was split into 4 replicates, to create statistically valid experimental design with pseudo-replication. M-96-015 was applied at 75 lbs + 12 gallons of methanol per acre on 9 July at a volume of 300 gallons per acre using a standard orchard speed sprayer.

Two methods were used to evaluate the control and spread of citrus mealybugs, a tree evaluation method and a fruit evaluation method. For the tree evaluation method, 3 trees infested with mealybugs were selected within each replicate. These trees were rated for mealybug infestation using a 1 to 10 rating scale, where 1 = no live mealybugs, 2 = few on the trunk or branches, 3 = 1-25% fruit lightly infested, 0% heavily infested, 4 = 25-50% fruit lightly infested, < 10% heavily infested, 5 = 50-75% fruit lightly infested, < 10% heavily infested, 6 => 75% fruit lightly infested, < 10% heavily infested, 7 = 10-25% fruit heavily infested, 8 = 25-50% heavily infested, 9 = 50-75% fruit heavily infested, and 10 = > 75% fruit heavily infested. The fruit infestation method was used to better determine the prevention of the spread of mealybugs from infested to non-infested fruit. Five pieces of lightly infested fruit and non-infested fruit from five trees within each replicate in each treatment area were tagged. The non-infested fruit were located on the same trees, and in close proximity to the infested fruit. Each fruit was rated for relative infestation using a 1 to 5 scale where 1 = no mealybugs, 2 = < 10% of surface covered, 3 = 11-20% covered, 4 = 21-50% covered, and 5 = > 50% covered.

Woolly whitefly infestations were rated within each replicate of each treatment area using a technique similar to the mealybug fruit-rating scheme. In stead of fruit, the degree of infestation on flush growth was evaluated where 1 = no woolly whiteflies, 2 = < 10% of leaf area covered, 3 = 11-20% covered, 4 = 21-50% covered, and 5 = > 50% covered. Citrus thrips were evaluated using a beat-pan sampling procedure. One piece of flush growth from 10 trees within each treatment-replicate area was sampled by tapping the flush onto a black pan covered with hardware cloth. The numbers of dislodged thrips were counted.

A pre-spray evaluation was made on 8 July. Post-spray evaluations were made on 24 and 30 July, and 8, 13 and 20 August. Data were analyzed using analysis of variance and a F-protected LSD (P <0.05) where appropriate

Results and Discussion

Unfortunately neither the citrus mealybug nor the woolly whitefly infestations appeared to be greatly increasing during the test period (Figures 1, 2 and 3). Based on the citrus mealybug tree ratings, M-96-015 did not reduce population relative to the untreated (Figure 1). Based on the mealybug fruit infestation ratings mealybug inflections were not spreading from infested fruit to non-infested fruit in either the M-96-015 or the untreated areas (Figure 2). Thus, the ability of M-96-015 to prevent the spread of mealybug could not be evaluated. However, based on the fruit evaluations, we were able to gain insight into the ability of M-96-015 for killing citrus mealybug. Following the application, we noticed that where the M-96-015 contacted the mealybug's colonies, most of them died. This observation is expressed in the large amount of variability of citrus mealybugs in the fruit evaluations (Figure 2). As expected the efficacy of M-96-015 to existing mealybugs is limited to the quality of the spray coverage. Where contacted, the mealybugs were killed, but mealybug infestations on the backside of fruit or on fruit deep in the tree canopy were not controlled.

Although we did not detect a large movement and spread of woolly whitefly populations from infested to non-infested flush, there was statistically significant spread of the population (Figure 3). M-96-015 did not appear to control these populations either by direct efficacy or prevention of their ability to spread (Figure 3).

M-96-015 was effective in control citrus thrips populations (Figure 4). Following the application, thrips populations in the M-96-015 area dropped while they increased in the untreated area. On 26 July, the grove received 0.11 inches of rain. Although it did not seem to significantly wash the M-96-015 from the tree, it did appear to negatively impact the thrips population as evident from the drop in the population on 30 July. Following the rain the thrips populations greatly increased in early August in the untreated, while they remained relatively low and constant in the M-96-015 area.

Overall, M-96-015 did not appear effective towards woolly whitefly but does appear to kill citrus mealybug. However, like with other insecticides coverage is a problem. The real benefit of M-96-015 towards citrus mealybug would occur if it prevented their spread. However, we were not able to measure this in this study. As with previous trials, M-96-015 is an effective citrus thrips material.

 
This is a part of publication AZ1051: "1998 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1051/az10512.html
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