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FILENAME:  MARAPIS.94
 
            Florida Extension Beekeeping Newsletter
    Apis--Apicultural Information and Issues (ISSN 0889-3764)
                 Volume 12, Number 3, March 1994
 
          CITRUS LEAFMINER--WHAT IT MEANS TO BEEKEEPERS
 
     University of Florida Cooperative Extension Service
publication SP 156, "The Citrus Leafminer, Phyllocnistis citrella,
A New Pest of Citrus in Florida," will be distributed to a large
number of citrus growers at regional meetings in March.  This is in
response to invasion by the citrus leafminer (CLM), native to
eastern and southern Asia.  CLM larvae "mine" tender foliage and
stems, causing leaf drop.  Heavy infestations can retard growth of
nursery and newly-planted trees or may reduce yield of bearing
trees.  According to the publication, "The combination of these
factors makes this species one of the most feared pests in citrus
production."
 
     Like most introduced species, the citrus leafminer has quickly
made its mark.  By time of detection, May 1993, over 90% of Dade
County's newly-replanted lime groves and much of the local dooryard
citrus was already infested.  All citrus growing counties in
Florida are now affected.
 
     Here are some other details about this insect and its control.
According to Dr. Phil Stansly, one of the above publication's
authors, writing in Pest Alert on the IFAS Computer System in
August, 1993:
 
Leafminer Biology:  Unlike the vegetable leafminer, which is a
small fly, the citrus leafminer is a tiny moth, about 1.5 times the
size of a citrus whitefly.  The silvery and brown adults are active
at dusk and at night, hiding in foliage during the day.  Egg laying
generally occurs on the underside of young flush, although in heavy
infestations, eggs may be laid on the top of the leaf or on stems.
The tiny, dome-shaped egg is almost transparent.  The hatching
larva bores directly into the leaf and begins "mining" (eating)
tissue just below the epidermis, leaving a trail of yellow-liquid
frass which dries to white, then brown.  These "mines" zigzag
around the leaf but generally do not cross the vein.  The mature
larva migrates toward the leaf margin in which it rolls up to
pupate.  The total generation time can be between 13 and 52 days
depending on temperature.
 
Damage: In many groves there are 3 or more mines per leaf on young
flush, often this number on the stem.  Mined tissue eventually
becomes necrotic and falls off, so that severely damaged flush is
lost.  Grapefruit seems to be most affected, possibly because it
flushes most, but oranges may be severely damaged as well.  Young
trees are more affected than older trees for the same reason: more
flush.  Naturally, there is no shortage of summer flush in
southwest Florida with so many young trees being pushed so hard.
Its nothing short of leafminer paradise, especially for the first
wave of miners with no natural enemies nor grower controls to
contend with.  As with other introduced pests, the worst is at
first.
 
Chemical Control Options:  Unfortunately, the grower's arsenal of
effective chemical controls is severely limited.  Agrimek + oil
seems to give the best results, though probably not more than two
weeks control under high pressure.  Growers have been using it at
half or less the labeled rate for rust mite with good
success.  However, even at reduced rates the costs would soon
become prohibitive.  An additional concern is the real danger of
losing a powerful tool against rust mite through overuse and the
inevitable resistance that would result.  Other materials such as
Dimethoate (Cygon), or methidathion (Supracide) are less effective.
Here the danger is not only cost and resistance, but the inevitable
damage to biological control of scale insects and other pests.  The
result could be a real nightmare, or at least a return to the bad
old days of constant battles with scales and the like.  Some
selectivity could be achieved by spraying superficially to cover
flush but avoiding penetration of the canopy, thereby allowing for
some degree of refuge survival of beneficials.  Even so, it is
obvious that chemical control alone is not a viable solution.
 
Other Experiences With CLM:  The experiences of China and Australia
in CLM control can be drawn on to some extent, according to
Publication SP 156.  Nine to 15 generations are produced per year
in China, depending on geographic location.  Spring flushes are the
least damaged; summer and fall flushes suffer the most serious
infestations due to buildup of leafminer populations.  Yield
reduction can reach up to 50 percent and fruit weight can be
reduced from 120 g to 70 g.  Cultural control methods include
manual removal of early and late-growing flushes and pre-flush
pruning to create uniform flushing to desynchronized flushing and
CLM population peaks.  The insecticides Carbaryl, Mipcin and
Phosmet were used to control CLM from 1970 to 1980.  However, due
to the development of resistance by CLM and harmful effects on the
natural enemies, these pesticides are no longer used.  Several
predators and parasitic wasps attacking CLM have been found in
China.
 
     CLM was first recorded in Australia in 1940, but did not
become problematic in citrus until 1965.  Naturally occurring
biological control has not proven effective, so parasitic wasps are
being introduced from Thailand and China.  In the meantime,
cultural control of flushing is one means being employed to reduce
losses.  Again, infestations are lowest in the spring, so rapid
growth of spring flush is promoted by application of fertilizer
during winter.  Populations in summer an autumn are generally high,
so summer and autumn growth is discouraged by reducing fertilizer
and irrigation to maintenance levels.  Chemical control of mature
trees is generally not warranted; however, young resets within the
mature grove are at risk.  Young trees and nursery stocks may
require two or more annual sprays.  Unfortunately, effective,
economical chemical control has not been achieved with available
products.  Petroleum oil applied to flush leaves appears to inhibit
egg deposition by the female moth.  The rapidly expanding leaf
requires frequent applications for protection.
 
Long-Range Control Strategy:  According to SP 156, a long-term
approach to solving the CLM problem in Florida should include:  1)
an economic assessment of the damage; 2) a combination of cultural
and chemical controls for the short range; and 3) biological
control as the most economical and environmentally sound solution
for the long range.
 
     An adequate damage assessment will require controlled studies.
The leafminer is an indirect pest, affecting mostly photosynthesis
through reduction of leaf surface area.  Leafminer population, tree
size, variety and market destination must all be considered.
 
Protecting the Flush:  The goal of cultural and chemical controls
should be to protect the main flushes, when the young, tender
vegetation that is critical for tree growth, setting bloom and
preventing excessive June drop is most at risk.  The strategy would
depend on creating "flush free periods" prior to the major spring
and summer flushes.  The spring and summer flushes would have to be
consolidated to separate them from each other and from other minor
flushes during the year.  Winter, post-spring and post-summer
flushing would be suppressed in an effort to reduce leafminer
numbers.  New growth could be discouraged by irrigating at
maintenance levels and without fertilizer until the summer rains.
All of these may not be practical under Florida conditions.
Chemical control should be seen as an adjunct to cultural control;
a way of protecting the main flushes, especially in spring and
early summer.
 
     Unfortunately, the flush and bloom time may overlap and
therein lies a potential problem.  Will efforts to protect the
spring flush affect honey bees?  The answer is maybe.  Publication
SP 165 reads, "When 50 percent of the trees show new flush, begin
a scouting program.  When 30 percent of the new flush leaves show
the typical zig-zag (serpentine) mine, initiate pesticide
application with an effective product.  Continue to scout and if 30
percent of the new flush leaves since the pesticide application
show active zig-zag mines, reapply."
 
     Although there may be reasons to apply insecticides, there are
also strong disincentives.  Beyond possible harm to honey bees,
many other beneficial insects may be affected.  The spray pattern
must also "...provide good distribution of the material to both
sides of flush leaves only.  Canopy penetration is unnecessary and
would increase unwanted contact with non-target organisms."  It
concludes, "Chemical management of this pest should be undertaken
with extreme caution and thought..."  The publication provides
application rates for two chemicals:  Agri-Mek + FC-435-66 oil and
Malathion + FC-435-66 oil.
 
     All this indicates that it will not be business as usual this
year in the citrus groves.  In existing groves of non-bearing or
young bearing trees, the spring flush may account for up to 50
percent of the annual leaf production and efforts to protect it
from CLM must be maximized.  Because this is a new pest, there may
be times when all options or consequences are not considered.
Growers may be quick on the pesticide application trigger.  And
spraying in nearby groves could affect colonies in areas where no
pesticide is being used.   Thus, beekeepers must be ever alert,
actively communicating with growers to avoid possible problems and
in possession of as much detailed information as possible.  A copy
of SP 156 would be a good start.  It is four pages long, complete
with color photographs of the life cycle and damage caused by this
insect.  This is a for-sale publication.  Send a check for $2.00
plus 6 percent sales tax ($.12), if you are a Florida resident to
University of Florida, Publications, P.O. Box 110011, Gainesville,
FL 32611-0011.
 
                       MORE ON FLUVALINATE
 
     Dr. Yaacov Lensky's comments reported in the January issue of
APIS provoked a good deal of response.  The first had to do with
the fact that it looked as if he was encouraging the use of Maverik
(R)-soaked wooden inserts (strips).  He does this in Israel.
However, Maverik (R)-soaked wooden inserts are not a legal option
in the United States!  Only Apistan (R) plastic strips have a label
for Varroa control in this country.  I will have more to say about
Dr. Lensky's remarks and responses to them in future issues of this
newsletter.
 
                     HONEY COOKBOOK RELEASED
 
     I have in my possession the National Honey Board's new
cookbook entitled: Sweetened With Honey -- the Natural Way.  This
is a delightful volume with 100 delectable recipes featuring honey.
It is jammed with color photographs of breads, spreads and meats.
And it's inexpensive.  The book will be available a supermarket
chekout stands throughout the country at a cost of $2.95, but can
be had for $2.50 through the mail.  It is also a great sales tool
at the local level and price breaks are available for bulk orders.
Purchasing ten through 99 copies reduces the cost to $1.75 each and
lots over a hundred go for $1.25 each.  For a single copy, send a
check or money order to National Honey Board -- Dept. BK, P.O. Box
7760, Marshfield, WI 54449.  If you want to buy in bulk, contact
the Longmont office using the Board's new 800 number, 553-7162.
 
                       NEW VIDEO AVAILABLE
 
      The Pennsylvania State University has released "Why Honey
Bees?"  This 29-minute video discusses the value of bees from
different perspectives.  It costs $35.00, including postage and
handling.  Send checks payable to Penn. State University and mail
to Ag. Information Services, Penn. State University, 119 Ag.
Administration Bldg., University Park, PA 16802.
 
 
Malcolm T. Sanford
Bldg 970, Box 110620
University of Florida
Gainesville, FL 32611-0620
Phone (904) 392-1801, Ext. 143
FAX: 904-392-0190
BITNET Address: MTS@IFASGNV
INTERNET Address: [log in to unmask]