Mites such as Varroa became famous not only because of the relative size ratio to the host bee, but for being an example of the rapid infestation due to global traffic. Other pests such as Wax Moth and the Small Hive Beetle also take considerable amount of attention as they became more prevalent and meaningful as general colony health declines.
Mites (varroa, tracheal)
The mites (Acari) that parasitize honey bees, and the viruses they vector,
have become a global problem. They are threatening the survival of managed
and feral honey bees, the beekeeping industry and, due to the role of
bees in pollination, the future of many agricultural crops. Acarapis woodi
and Varroa jacobsoni are the main pests. Bee mites have great dispersal
potential through their hosts and then by humans who move bees primarily
for pollination. These two species of parasitic mites, introduced into
the United States and throughout the world in the early 1980s, changed
beekeeping profoundly by causing epidemic losses, ranging from 25% to
80% of managed colonies. Also, feral bee colonies are virtually gone in
many regions where both mites occur. Further consequences of these introduced
mites and the diseases they cause are:
• Beekeepers and bee breeders going out of business
• There are fewer managed colonies
• There is increased incidence of viral pathogens in colonies
The honey bee tracheal mites, Acarapis woodi, live inside the tracheae and air sacs of adult bees. Mites in tracheal systems are relatively rare in arthropods and not well studied. Tracheal mites were first observed earlier this century, when bees on the Isle of Wight were dying from an unknown disease. The die-off reached epidemic levels between 1904 and 1919. The real cause of the loss of colonies during this time is still unknown and there may have been several diseases or other factors causing the symptoms. In 1922, the identification of A. woodi in Europe led the United States Congress to prohibit importation of all bees and to examine bees for evidence of mites from apiaries in many states.
A heavy load of tracheal mites causes diminished brood area, smaller bee populations, looser winter clusters and subsequent increased honey consumption. Ultimately, tracheal mites alone, or acting in concert with other pathogens, may lead to colony demise. In temperate regions, mite populations increase during winter, when bees are confined to the hive, and decrease in summer when bee populations are highest. In subtropical climates, the cycle is similar, even though bees are not so confined.
Why then was the damage attributable to A. woodi not noticed earlier? Nowadays, beekeeping is much more productive than in the past, and factors that reduce colony yield are more critically observed. Minor diseases and pests, which cause only marginal reductions in bee longevity and yield, were probably masked in the past by more severe mortality factors. The great advances in bee health and management, however, raised the awareness of beekeepers toward pests such as A. woodi, whose ravages did not decline through modern control methods. The pest might have been overlooked, even in England, had not the outbreak of the Isle of Wight disease necessitated the intensive examinations of bees.
Controlling Tracheal Mites
The overriding constraints for chemical control of honey bee pests are
that the chemicals must be effective against the target and harmless to
bees, and they must not accumulate in hive products. Because bees and
mites are both arthropods, many of their basic physiological processes
are similar, narrowing the possibilities for finding suitable toxicants.
Bees are notoriously sensitive to accidental poisoning by many of the
common agrochemicals. To control tracheal mites, the material must reach
the bee tracheae via a volatile compound, be inhaled by the bee, and be
lethal only to the parasite. Bees that are apparently resistant, or at
least tolerant, to tracheal mites seem to accomplish this by increased
grooming behavior.
Varroa Mites
Vector-borne transmission is an indirect route of horizontal transmission
and involves an intermediate biological host, a vector, which acquires
and transmits viruses from one host to another. The varroa mite is an
obligate parasite of the honey bee attacking different developmental stages
and castes of bees and is considered to be the most important pest of
honey bees around the world. The entire life cycle of the varroa mite
is spent with their honey bee hosts. Female mites feed on the bee larvae
and lay eggs of both sexes in the brood cells. Developing mites feed on
immature bees. After the mites mature, male and female mites mate inside
of the capped brood cell. The male dies after copulation and females emerge
from the brood cell along with their bee host and seek another host to
repeat the life cycle. The feeding of varroa mites can result in a decline
in host vigor, immunity, weight, shorter bee life span, and the eventual
destruction of the colonies within a few years.
The varroa mite, Varroa jacobsoni (Oudemans), is currently considered the major pest of honey bees in most parts of the world. Only Australia and New Zealand remain free of this pest. The pathology it causes is commonly called varroasis (also known as varroatosis or varrosis). Initially discovered in Java, varroa was confined to Southeast Asia where it parasitizes the Asian honey bee, Apis cerana. This bee has probably coevolved with the parasite, which adapted to keep the mite under control.
Globalization and the concomitant increase in international travel and commerce have facilitated the worldwide dispersal of varroa. Once established, the mite spreads on drifting, robbing, and feral bees or swarms. Female varroa are often found on adult bees, which provide for dispersal band and serve as short-term hosts. When in an actively reproducing bee colony, the mite disembarks and seeks brood cells containing third-stage bee larvae. Varroa enters the prepupal cells one to two days prior to capping and hides from the nurse bees by submerging in the remaining liquid brood food, lying upside down. The female remains concealed until the brood cell is capped, and then begins to lay its eggs.
Symptoms
Varroasis symptoms can be confused with other disorders, and even with
pesticide poisoning. Here are the most notable symptoms:
|
•
|
Pale or dark reddish-brown mites are seen on otherwise white pupae |
|
•
|
Colonies are weak with a spotty brood pattern and other brood disease symptoms are evident |
|
•
|
The drone or worker brood has punctured cappings |
|
•
|
Disfigured, stunted adults with deformed legs and wings are found crawling on the combs or on the ground outside |
Because mite populations increase in proportion to the available bee larvae,
varroa can quickly overrun a colony and often colonies are dead by the
fall. Interrupted brood rearing during the winter slows the population
increase of varroa in temperate regions, but in warmer climates, colonies
can be destroyed within months. Treated apiaries can still perish if the
beekeeper is not diligent; re-infestation occurs due to the robbing of
varroa-infested and weakly defended colonies.
Control of Varroa
While long-range, non-chemical controls are vigorously being sought, beekeepers
need immediate relief from existing mite infestations. Fluvalinate (Apistant),
a pyrethroid, is a United States-registered pesticide for varroa control.
Coumaphos (trade name: "CheckMite"), an organophosphate, received
Section 18 registration in Florida in 1999. Coumaphos has subsequently
been disqualified from use in the United States but is still widely applied
elsewhere. Both chemicals are used as pesticide-impregnated plastic strips
which are hung between frames of bees in a hive. Treatment time is in
the spring and again in the fall as needed, and only when there is no
honeyflow. Applied in this manner, fluvalinate is released slowly and
dispersed by adult bees. These chemical options for varroa pose a serious
problem because repeated exposure to the same pesticides select for resistant
mites. Reports of fluvalinate-resistant mites have surfaced in several
places around the world. This resistance crisis is being compounded by
contamination of hive product, especially wax and possibly honey, as well.
Essential oils and formic and also are used to try to
control varroa. They are considered inefficient and unsafe, respectively.
Bees that are resistant to Varroa have yet to be found. However, in several
locations around the globe there are honey bees that are relatively tolerant
to Varroa, often due to colony behavioral changes, such as increased grooming
and hygienic behavior. Today, there is little doubt that Varroa acts in
concert with the viruses it vectors to cause widespread colony collapse.
