APIS Volume 13, Number 8, August 1995
In this issue
- Sampling for Varroa
- Varroa Elsewhere
- Panhandle Bee Seminar
SAMPLING FOR VARROA
In the July 1995 issue of Bee Culture, Dr. Mark Winston writes that the possibility of creating Varroa mites that are resistant to the one product now labeled for their control is very real (also see November 1994 and February 1995 issues ofAPIS). One way to hold off the inevitable as long as possible is not to treat with Apistan(R) until absolutely necessary. Unfortunately, the number of mites it takes to do irreparable damage to a colony is not known. And sampling techniques that determine the actual number of mites in a beehive are extremely variable in their effectiveness.
In Florida, the "ether roll" sampling technique continues to be used almost exclusively by bee inspectors in the Department of Agriculture and others. This technique is well spelled out in the video production, Varroa Mite Detection," VT 249, available from this office by mailing a blank VHS tape to my attention. It is used so much because it requires only a single visit to a colony and the results are immediately available. However, it only samples a portion of a colony and destroys valuable bees in the process.
Many Florida bee inspectors now urge beekeepers to think about treating soon (within 10 days) when one or two mites are found in an ether roll. More than that number is considered a signal to put Apistan(R) strips in immediately. This recommendation, however, will vary depending on location. Beekeepers should always check with their local inspectors for the latest information on treatment protocol.
Several other methods for sampling Varroa mites besides the ether roll continue to be used. These include counting mites that are either killed/dislodged from bees using the product labeled for control itself [Apistan(R)] or alternative fumigants like formic acid and tobacco smoke. In addition, one can count Varroa present in uncapped drone brood or those on the bottomboard that drop from natural causes or may be groomed/knocked off by the bees.
In one of few studies to determine Varroa levels in honey bees, M.D. Ellis and F.P. Baxdale reported on the efficiency of five sampling techniques from 44 overwintered two-story colonies in BeeScience, "Comparison of Formic Acid Sampling With Other Methods Used to Detect Varroa Mites (Varroa Jacobsoni Oud.) and Mite Distribution Within Colonies in Nebraska," (Vol. 3) 1994: pp. 139-144, 1994. As expected, Apistan(R) recovered many more mites than any of the other methods. Formic acid was less effective than Apistan(R), but more so than ether roll, tobacco smoke fumigation, debris sampling (natural knockdown) or decapping drone brood, none of them being statistically significantly different from each other. The authors saw evidence that formic acid might rival the results of Apistan(R), but this would take more than the 24-hour exposure allotted in that particular study.
To determine the efficiency of the ether roll, the authors compared the number of mites found in two rolls (10-second shakes for each) to those recovered by washing the sample in alcohol for 20 minutes. The alcohol wash detected about 1.7 mites for every one found by ether roll. Earlier studies cited by the authors, however, resulted in a two- to almost three-mite difference. The authors concluded that because ether rolls are inherently variable, beekeepers should develop a baseline standardized figure for their own particular technique by comparing ether roll with alcohol wash.
The authors also described the mite distribution found in colonies during the study period (July and August 1992). Drone brood had over seven times as many mites as adult worker bees. And there were four times the number of mites in drone brood compared to worker brood. The authors stated that while only 10 percent of worker bees in their study had mites, other investigators have estimated as high as 20 percent. The time of year is also an important issue. This is a period characterized by a reduction in drone brood and adult drones will soon be ejected from colonies. It is the time when the switch from drone brood to worker brood is probably occurring (see October 1994 APIS). Thus, percentages found in the study may radically change as more mites begin to parasitize worker brood and adults.
Two other factors also may influence density of mite populations in bee colonies. One is the effect mite population density has on other mites' reproduction (see June 1995 APIS). Another, discussed elsewhere in this newsletter, is the infertility of female mites, which may depend on the host (race of bee) they are found on.
A final factor, according to the authors, is the length of time adult Varroa stay outside brood cells; there is evidence that this is shorter in colder areas. Once sealed inside cells, mites are less susceptible to most of the sampling methods described in the study. The authors, therefore, recommended that regional and seasonal studies are needed to effectively use mite population estimates.
The authors concluded that although beekeepers will not want to treat colonies every time they find a mite, they need a sound basis to decide when or if such treatment can be delayed. Unfortunately, the above study, as most do, only raises more questions about the effectiveness of sampling for mites. We must wait for many more before the complex interactions between host and parasite can be better understood. Only then will it be possible to estimate a mite population and determine a definitive treatment level. Even this, however, may not be enough and studies of this nature may go for naught.
Varroa has been implicated in transmitting viral infections. This may result in what is now being called parasitic mite syndrome (see December 1994 APIS). In situations where this possibility exists, the number of mites feeding on bees becomes less relevant. Unfortunately, less is known about the viral connection than the damage done by physical parasitism. In the absence of this kind of information, beekeepers are left to their own devices. This inevitably results in one philosophy: treating sooner rather than later.
VARROA ELSEWHERE
Evidence corroborating that the Varroa situation can be quite different based on climate and geography continues to accumulate. These differences are evident within a single country, or as the mite population shifts over time, they might even be observed in a single colony (see June 1995 APIS).
For those who have had enough of Varroa, perhaps Brazil is the best place to go. G. Moretto and colleagues published "Reduction of Varroa Infestations in the State of Santa Catarina, in Southern Brazil," American Bee Journal, Vol. 135, No. 7, pp. 498-500 in July 1995. The two-year study sampled bees in 19 locations using the alcohol shake method. The authors found that over the course of about 10 years, Varroa infestation rates have dropped in the state of Santa Catarina. An average of about 17 mites per 100 adult bees was found right after Varroa's introduction. It is now at two per 100 adults, the predominant level in the rest of the country.
The authors stated that this relatively low mite level is not damaging to colonies in their country. Thus, honey bees in Brazil do not require any Varroa treatment, despite the mite being present since 1972. A major reason for this, the authors believe, is the ability of Africanized honey bees to remove the mites from their bodies. Climate may also be responsible. But this fails to explain why honey bees in Santa Catarina state can survive without treatment, whereas those in Florida with a similar temperature and rainfall regime cannot. The authors concluded that the decreasing infestation rates in Santa Catarina state demonstrate an adaptive process has taken place in the host-parasite relationship.
Another study by P. Rosenkranz and W. Engels, "Infertility of Varroa jacobsoni Females in Apis mellifera Worker Brood as a Tolerance Factor Against Varroatosis," Apidologie, Vol. 25, pp 402-411, 1994, compared European bees in Germany (Carniolan) with Africanized bees in Brazil. In Brazil, over 40 percent of the female mites found in worker brood were infertile. In Carniolan bees, 10 to 20 percent were infertile.
These infertility differences are not dependent on either broodnest or ambient (air) temperature, according to the authors. And in both European and Africanized bees, if fertile, female mites produced the same number of eggs. They also said this low fertility rate in Brazil compares with that found in the original host of Varroa, the Asian honey bee (Apis cerana). They concluded: "Since host-dependent fertility evidently has a strong influence on the population dynamics of this parasite (Varroa), it should be properly considered in future selection and breeding efforts in order to produce European bee strains tolerant against varroatosis."
And then there is the New Guinea enigma. A study by D. Anderson, "Non-reproduction of Varroa jacobsoni in Apis mellifera Colonies in Papua New Guinea and Indonesia," Apidologie, Vol. 25, pp: 412-421, 1994, shows some surprising results. While comparing Varroa populations on both Apis cerana and Apis mellifera, the author found that mites were not reproducing on European bees. They were only present by virtue of being spread from nearby Apis cerana colonies.
Extremely low infertility, as observed in the above Brazilian study, is ruled out by the author as a reason for no reproduction. He concluded there should have been some successfully reproducing mites seen in the large number of Varroa-infested cells examined (about 1,500 out of 68,000 total cells examined). In addition, there was no evidence that competition between Varroa and another mite, Tropilaelaps clareae was responsible. The latter mite was only observed in 35 percent of colonies examined. Environmental factors were also ruled out because reproduction was not impaired in Apis cerana colonies, either in the tropical lowlands or more temperate uplands.
The author suggested the answer may come from two areas of study. The first is mite taxonomy or classification. It could be that the mites are different than those seen elsewhere in Asia or Europe and the Americas. Studies using DNA could determine this.
The other area is mite reproductive biology. The fact that female mites infested both worker and drone brood, and were proceeding toward normal reproduction, but then were unable to lay eggs, the author suggested, may be because some crucial chemical factor was lacking. Given the results of this study, the most important experiment to run, he said, is to introduce colonies already found susceptible to Varroa in Europe into Papua New Guinea and see what happens to them. On a broader note, he concluded: "...the results of this study have shown the uncertainty that may be associated with predicting likely impacts of pest species in particular localities based on what is known about these pests in other localities."
And what about the viral connection associated with parasitic mite syndrome as noted elsewhere? This possibility is not mentioned in any of these studies, but until more is known about the honey bee-Varroa connection, it should not be ignored.
PANHANDLE BEE SEMINAR
The 1995 Florida Panhandle Bee Seminar will be held September 8 and 9, 1995 at the Southport Mens Club, in Southport, FL. The meeting runs all day Friday and Saturday. Dr. Clarence Collison, Professor and Head of Entomology and Plant Pathology, Mississippi State University will be the featured speaker. The cost is $35.00 per person for both days if registering early. After August 25, the fee will be $40.00 per person. For more information, contact Sylvia Bullard, Escarosa Beekeeping Association, 904/478-7690 or Vernon Gwaltney, Tupelo Beekeepers Assocation, 904/476-4278. Sincerely,
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:MTS@GNV.IFAS.UFL.EDU
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Copyright (c) M.T. Sanford 1995 "All Rights Reserved"
