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.
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