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BY Lucia G. Varela, Rhonda J. Smith,
Mark Battany, and Walt Bentley
University of California Cooperative Extension
Three mealybugs species now occur in wine grape regions of California:
grape mealybug (Pseudococcus maritimus), obscure mealybug
(P. viburni) and vine mealybug (Planococcus ficus).
It is important to distinguish among them because their life cycles
are different, and thus chemical control measures (if necessary),
must be tailored to the specific pest.
The recently introduced vine mealybug is spreading throughout the
grape growing regions in California. It can reach damaging levels
very quickly by contaminating grape clusters with mealybugs, honeydew,
and sooty mold. North and Central Coast growers have been battling
vine mealybug for three to six years respectively and it has been
a problem in San Joaquin Valley vineyards since the mid 1990s.
Movement of vine mealybug to new regions has necessitated aggressive
chemical management and modified farming practices to curtail its
spread and the damage it can cause. As a result, vine mealybug has
grabbed headlines and become a focus of attention for growers and
wineries.
A strong effort is underway to detect new vine mealybug infestations
early in order to control the insect and prevent further spread,
both within the block where it is first discovered and to adjacent
vineyards.
This extra scrutiny has also increased concerns regarding two other
mealybugs that are more commonly found in vineyards. While searching
for signs of vine mealybugs, growers and Pest Control Advisers (PCAs)
frequently find grape mealybug or obscure mealybug. They now have
to recognize the similarities and differences in three species of
mealybugs to correctly identify, monitor and control them.
Origins and need
for chemical control
Grape mealybug is native to North America and exists in many parts
of California. It has probably gone unnoticed in many vineyards
and has not caused economic damage due to low populations. Because
it is native, a large complex of natural enemies exists and thus
it is often under good biological control.
Obscure mealybug is an introduced species that has been present
in California since the late 1800s. Because it is an exotic pest,
it has few natural enemies. Obscure mealybug has a narrower tolerance
to cold temperatures than the grape mealybug and is found primarily
in coastal regions. Currently, the largest populations of obscure
mealybug are found in the Central Coast.
Vine mealybug is an economic pest of vineyards in the Mediterranean
regions of Europe, Africa, and the Middle East. It has also been
introduced into South Africa, South America, and the southeastern
U.S. Vine mealybug has a few natural enemies in California that
were introduced in the 1940s for control of a related species, the
citrus mealybug.
Occasionally, control of grape or obscure mealybug requires chemical
application(s). Fruit can become infested with these mealybugs when
populations are high, especially when clusters touch older wood
on the vine such as the cordon. In contrast, vine mealybug infestations
can reach much higher population levels than grape or obscure and
cause more damage to clusters and the vine. At this time, chemical
applications are required to control this pest.
Honeydew and ant tending
Mealybugs feed on the phloem and excrete a sugary solution called
honeydew. Ants are often found in association with mealybugs because
they will feed on the sugary excrement. In fact, the ants will tend
the mealybugs and keep away natural enemies in order to maximize
the production of honeydew.
The degree of honeydew production varies by species and increases
from small to significant amounts with grape, obscure, and vine
mealybugs respectively.
Honeydew can be seen as shiny and sticky areas on leaves, clusters,
and trunks. A wet trunk is an indication that a colony of mealybugs
is feeding under the bark and honeydew is seeping through the bark.
Argentine and gray ants are the most common ant species that tend
mealybugs to obtain honeydew.
The amount of honeydew visible on a vine depends not only on the
mealybug species but also on the number of ants tending the colonies.
When ants are not present, honeydew produced by all mealybugs can
result in wet trunks because the ants are not consuming it. Although
it is more common to see wet trunks with vine mealybug infestations,
a large population of grape mealybugs in the absence of ants will
also result in wet trunks.
Morphological differences
The mealybug covers its body with a waxy secretion that creates
a white, mealy appearance in all three species. Wax also accumulates
on hairs that extend around the circumference of the body, forming
white fringe-like filaments. The appearance of the white filaments
distinguish the vine mealybug from those in the genus Pseudococcus
(grape and obscure mealybug).
GRAPE AND OBSCURE MEALYBUGS: The body shape
of the immature and female grape and obscure mealybug is rectangular
with rounded anterior and posterior ends (Figure I, right). The
hairs surrounding the body are longer than those of the vine mealybug.
Since the hairs are longer, the wax accumulates more thinly along
the length (the filaments may appear crooked, bent or otherwise
irregular).
The waxy filaments in grape and obscure mealybug are longer, thinner,
slightly less parallel and thus have a more untidy appearance than
those of the vine mealybug. More noticeable are the hairs at the
posterior end of the abdomen that are considerably longer than the
ones surrounding the rest of the body. The result is two to four
long, caudal filaments that appear as tails.
If the insect is not disturbed, tails are easily seen in large immatures
or in adult females and are the strongest characteristic with which
to distinguish Pseudococcus mealybugs from vine mealybug.
It is more difficult to see the long tails in younger stages, however
Pseudococcus immatures can still be distinguished by the
rectangular body shape and by the thin, irregularly formed filaments
surrounding the body.
Grape and obscure mealybugs can also be distinguished from each
other by the color of a defensive fluid they secrete when threatened.
The fluid is exuded as a droplet from pores at the anterior and/or
posterior end of the body. Gently poke the insect with a sharp object
without puncturing the body until the fluid is excreted. If the
color of the fluid is reddish orange, the insect is grape mealybug
(Figure II); if it is clear, then it is obscure mealybug (Figure
III).
VINE MEALYBUG: The body shape of the immature
and adult female vine mealybug is oblong (Figure I, left); wider
at the center of the body as compared to the anterior and posterior
ends. The hairs surrounding the body are short in comparison to
those of Pseudococcus mealybugs. Wax accumulation on the
hairs result in short, uniform, and thick filaments. The posterior
end of the adult female has slightly longer filaments than
those that surround the body, but they do not appear as long tails.
Vine mealybug, like obscure mealybug, exudes a clear defensive fluid
when threatened.
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Growers
work together in vine mealybug workgroup
BY Nick Frey, Executive Director
Sonoma County Grape Growers Association
When vine mealybug (VMB) was first discovered in the Carneros
region, the growers and wineries organized a workgroup led
by Katey Taylor (viticulturist for Domaine Chandon). The workgroup
provided trap placement and reading services for growers.
The program expanded in 2005 to include growers outside of
Carneros. Clearly, these growers recognize the value of working
together to combat this exotic pest.
Carneros growers were able to manage VMB infestations in
2005 and harvest fruit that met quality expectations. This
was achieved with the use of insecticides. Several new infestations
were found by vineyard workers when leafing or cluster thinning.
In addition to field workers, the group suggested that fruit
samplers also be trained to identify VMB. When they remove
berries or clusters, mealybugs inside the clusters are revealed.
It is also clear that VMB is spreading naturally now. No longer
are infestations only associated with new vine plantings.
If you have a vineyard near an infested vineyard, you are
at higher risk of becoming infested with VMB. However, new
infestations are not necessarily along the edge of the infested
site.
Both Katey Taylor and Lucia Varela (University of California
Cooperative Extension [UCCE]), emphasize the importance of
growers working together to slow the spread of VMB. If you
are interested in joining the VMB Workgroup in 2006, contact
Katey Taylor (katey_taylor@chandon.com). Participation is
not limited to growers in the Carneros area.
The 2005 VMB workgroup program had 339 trap sites. Of those,
90 sites had VMB males. They do not, as yet, know the number
of female infestations associated with the 90 sites that trapped
males. As learned previously, trappings in September/ October
detected the largest number of VMB males, nevertheless early
trapping is still worthwhile because early detection is critical
to reduce fruit damage and loss and to have any chance of
eradicating an early stage infestation.
Trapping should continue after an infestation is found to
track the effectiveness of insecticide treatments in reducing
VMB numbers. The consensus of the group is that everyone needs
to trap for VMB and everyone needs to talk to their neighbors
to learn if or where VMB exists relative to their property.
The 2005 VMB workgroup results suggest that trap densities
of greater than one per 30 acres may be needed. In windy locations
such as Carneros, traps may be close to infested plants and
yet not catch any males. To be successful, traps need to be
placed downwind of the infestation which may be a challenge
in a windy region such as Carneros. It is also suggested that
more traps be added to the trapping grid during September
and October when male numbers are highest.
If growers fail to identify or treat a VMB infestation, fruit
quality will be affected and ultimately vine health will decline.
Experience to date indicates VMB monitoring, pesticide treatment
and sanitation can cost as much as $1,000/acre under certain
circumstances if the grower’s objective is eradication.
It is clear that new sustainable controls for VMB are needed.
Potential new strategies include registration of a new systemic
insecticide with greater efficacy than Admire, especially
on heavy soils, new ant-bait stations to reduce ant populations
(likely available in 2007), mating disruption and new biological
control agents now being evaluated.
Among the biological control agents, it is hoped that parasites
can be released that will attack different VMB life stages.
Two species of parasites under evaluation by Kent Daane (UC
Cooperative Extension Specialist) are very aggressive in laboratory
tests.
There is a growing consensus that VMB is here to stay. Thus,
we must make efforts to limit or slow its spread. Trapping
and talking with neighbors are critical actions every grower
should take. We also need to develop cost-effective IPM programs
that rely less on heavy insecticide use. There are promising
options, but more work is needed to create a successful IPM
strategy.
The Sonoma County Grape Growers Association encourages the
formation of additional VMB workgroups. Cooperation among
growers is critical. Winery support is important, including
training berry samplers to identify VMB. UCCE north coast
staff will continue to provide training for vineyard employees
and berry samplers in 2006.
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Life cycles
Grape mealybug has two generations per year: overwintering and summer
generation. This mealybug evolved to live in temperate climates
where it spends the winter in an arrested state called diapause.
Diapause is a prolonged dormancy regulated by hormones triggered
by environmental conditions. The ability to diapause, and which
life stage diapauses, is genetically determined.
In grape mealybug, the egg is the most resistant stage to cold weather,
thus it is this diapausing stage that overwinters. The eggs are
laid in a white, cottony egg sac under the bark on the trunk, in
the head of the vine or on cordons. Egg sacs are formed when the
female secretes long waxy threads during oviposition. The cottony
mass, called an ovisac, conceals the eggs.
In late winter the youngest nymphs (called crawlers), emerge and,
shortly before bud break, they move from old wood to the spurs where
they gather under the thin bark on one-year-old wood. After bud
break, a portion of the population moves to the basal leaves and
shoots while the rest remain under the bark primarily on the upper
portion of the trunk, head, and cordons. When high grape mealybug
populations exist in older vines with thick bark, they may also
be found under the bark much lower on the trunk.
The overwintering generation develops through the spring, reaching
maturity in late May and June. At that time, the adult female returns
to the old wood to oviposit. Each female lays 100 to 300 yellow
to orange eggs, then she dies. The eggs hatch in seven to 14 days.
Summer generation nymphs move to the shoots, foliage, and clusters
that are touching old wood. Only eggs or crawlers are found in late
June or early July; mostly immatures are seen through July. Adult
females will appear in late summer and early fall. Some females
will oviposit in the fruit clusters but the majority of females
return to old wood to lay overwintering eggs.
Because grape mealybug has a diapausing stage, the life cycle is
synchronized and the two generations do not overlap. Thus, at certain
times during the growing season, specific stages are not present
while others predominate. For example, the lack of females and eggs
in mid to late July is a strong indication that the immature mealybugs
seen on vines at that time are grape mealybug. Similarly, at bud
break, only crawlers along with remaining ovisacs are found. When
only one or two stages of mealybug are present at a given time,
the insect is most likely grape mealybug.
OBSCURE MEALYBUG does not have a diapausing
stage, thus it has all life stages present throughout the year;
this distinguishes it from the grape mealybug. It has two to three
overlapping generations per year and overwinters as eggs inside
ovisacs and as nymphs under the bark, mostly on the upper portion
of the trunk or on cordons.
In some central coast vineyards, obscure mealybug has been observed
on weeds such as malva during the winter when the vines are dormant.
After bud break, a portion of the nymphal population moves onto
new shoots and basal leaves; however, a large number remain hidden
under the bark.
In the summer, populations may increase dramatically and all stages
are found on above-ground portions of the vine, specifically under
the bark on the trunk, cordons, and spurs; basal portions of the
shoots and leaves; and on clusters that touch older wood. In fall,
nymphs migrate from the canopy to the trunk and cordons and remain
on old wood under the bark.
VINE MEALYBUG, like obscure mealybug, does
not diapause and all life stages might be present year-round on
a vine. It has several generations per year, depending on temperature.
In the San Joaquin Valley, it can have up to seven generations.
In the cooler North Coast, it has approximately four.
During the winter months, eggs, crawlers, nymphs, and mated females
are found under the bark. The majority of the population is found
on the lower trunk at the graft union or near the soil line. They
are also found in old pruning wounds on the trunk, at the crotch
in a bilateral cordon-trained vine, or at the base of the spur.
In light soils present in the Coachella and San Joaquin Valleys
and some areas in the Central Coast, they are found on vine roots.
This has not been seen in the heavier soils of the North Coast.
Reproduction slows considerably in the winter months, and by late
February and March, the majority of stages found are older nymphs
or fully developed females that have not started laying eggs. In
late March and early April, overwintering nymphs develop into adult
females which begin to lay eggs in ovisacs.
When the crawlers of the spring generation emerge, they move up
the trunk, find a spot to feed and continue to develop. The spring
generation develops under the bark on trunks and cordons through
May and by late spring, nymphs can be found on basal leaves.
In summer, populations can increase dramatically and all stages
of overlapping generations are found on canes, clusters, leaves,
and petioles well above the fruit zone and under the bark on the
trunk, cordons and spurs.
In the canopy, vine mealybug forms colonies in the axils of the
petioles and the cane (Figure IV). Females lay eggs on all above-ground
parts of the vine. Only vine mealybug lays eggs on leaves and they
may be found above the fruit zone. Starting in about November, population
densities decline and nymphs migrate to the lower trunk.
Timing of control measures
It is very important to time insecticide applications to coincide
with periods of maximum vulnerability of each pest. Due to the different
life cycle for the three species, control timing differs. For information
on which chemicals to use, please consult the University of California
Grape Pest Management Guidelines at http://www.ipm. ucdavis.edu.
There are two guidelines that address mealybugs. The titles of the
two guidelines are 1) Mealybugs (Pseudococcus) for grape
and obscure and 2) Vine mealybug.
GRAPE MEALYBUG: This pest is best controlled
at the crawler stage. As described previously, grape mealybug is
seldom a pest; control is needed only when populations are high
and a high percent of the fruit touching the old wood is infested.
Grape mealybug prefers vigorous vines. Keep records of infested
fruit near harvest and map the infestation at that time.
The most effective treatment is to control crawlers from the overwintering
generation during delayed dormancy, just prior to bud break. In
vineyards with a history of infestation at harvest, monitor the
spurs before bud break. Gently peel the thin bark from the tip of
the spur and look for the orange-to-yellow crawlers on one spur
per vine in at least 30 vines. If 20% of the spurs are infested,
a treatment may be warranted.
In May, monitor the beginning of the summer generation by examining
the base of spurs for mature females and for ant movement on the
vine. A treatment may be needed if more than 20% of the spurs are
infested in late spring.
A post-harvest application should never be used to control
grape mealybug. Depending on the harvest date, at this time the
majority of the population is in the egg stage inside the ovisac
under the bark. No chemical kills the eggs, thus applying a treatment
at this time will disrupt natural enemies and not control this pest.
OBSCURE MEALYBUG: The most effective treatment
timing for this species is also the delayed dormant period; spurs
can be monitored with a protocol similar to that listed above for
grape mealybug. If additional treatments are required, systemic
insecticides applied with irrigation water can be very effective
in late spring, more so in vineyards with light soils.
If spring or summer foliar treatments are necessary, attempts should
be made to minimize adverse effects on the resident population of
beneficial insects commonly present in vineyards. This can be achieved
by leaving areas of the vineyard untreated, and by utilizing insect
growth regulators that primarily target mealybug pests.
VINE MEALYBUG: If vine mealybug is found
in a vineyard, treatment is recommended. The objectives for chemical
control are to obtain clean fruit and to avoid development of populations
in the canopy. Large populations in the canopy may hasten spread
during leaf fall (which may occur prematurely due to vine mealybug
feeding). Reducing the numbers of vine mealybug in the canopy prior
to harvest is essential to prevent spread of the pest during harvest.
At the delayed dormant period, just prior to bud break, the majority
of the population is under the bark in the lower part of the trunk.
Effectiveness of a treatment at this time may be improved if bark
is removed on the most infested vines prior to the application.
In areas with light-textured soils, an effective spring and early
summer control is a soil-applied systemic insecticide at bloom as
a single or a split application.
Summer foliar applications are warranted to insure clean fruit and
to avoid having a large population in the canopy at harvest. When
making summer applications you may be limited by pre-harvest intervals.
The earlier you find an infestation the more chemical control options
you have to control this insect before harvest. It is strongly recommended
that all vineyards be monitored with vine mealybug pheromone traps
for early detection. For information on trapping, please consult
the references cited below.
The most effective timing to reduce vine mealybug populations is
immediately after harvest before the nymphs begin to move to lower
parts of the trunk. This is when the highest percentage of the population
is exposed in the canopy and on outside of the bark. Good control
at post harvest will minimize the overwintering population and may
reduce the need for a delayed dormant spray, when control is less
effective.
References
Daane, K., E. Weber and W. Bentley. “Formidable Pest
Spreading Through California Vineyards.” Practical Winery
& Vineyard, May/June 2004. http://www.practicalwinery.com.
Smith, R.J. and L.G. Varela. Using Vine Mealybug Traps in
the North Coast. CAPCA Adviser. January/February 2005.
UC IPM Pest Management Guidelines: Grape. University of California
Division of Agriculture and Natural Resources. Publ. 3448. http://www.ipm.ucdavis.edu/PMG/selectnewpest.grapes.html |
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