APIS Volume 12, Number 3, March 1994
In this issue
- Citrus Leafminer Introduced--What it Means to Beekeepers
- More About Fluvalinate and Dr. Lensky
- National Honey Board Cookbook
- New Video From Penn State
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 AND DR. LENSKY
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 at the present time. I will have more to say about Dr. Lensky's remarks and responses to them in future issues of this newsletter.
HONEY COOKBOOK RELEASED FROM NATIONAL HONEY BOARD
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.
PENN STATE VIDEO AVAILABLE--WHY HONEY BEES?
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
INTERNET Address: MTS@GNV.IFAS.UFL.EDU
