By: Clarence Collison
VARROA MITE POPULATION GROWTH
A factor that might contribute to the growth of Varroa populations is the migration of mites into colonies on foragers from other hives.
The population growth and virulence of the Varroa mite (Varroa destructor, Anderson and Trueman) depends on numerous factors. The most obvious ones are race and strain of bee (Fuchs and Bienefeld 1991; Moretto et al. 1991; Kulincevic et al. 1992), climate (De Jong et al. 1984; Ritter and De Jong 1984; Moretto et al. 1991) and possibly Varroa biotype ( Delfinado-Baker and Houck 1989). In cold and temperate climates, the number of mites increases about 10-fold per year and infested colonies collapse after about four years (Fries et al. 1991; Korpela et al. 1992). In tropical climates the parasite seems to be less virulent (Ritter and De Jong 1984; Engels et al. 1986).
In April and October mite-free honey bee colonies were artificially infested with 50 Varroa mites each and treated with the pesticide Apistan® after a period of 24 weeks. Population growth was studied in 24 colonies from April to October and six colonies from October to April. The proportion of Varroa mites that invaded the test colonies after the initial inoculation was monitored in colonies that were constantly treated with Apistan®. Conservative calculations suggested that the initial mite population in a honey bee colony increases on average about 300-fold during one year in central California. This result excludes the contribution of additional mites that invaded the colonies (Kraus and Page 1995).
Varroa mite population growth was monitored in honey bee colonies from 1993 to 2002 in Baton Rouge, LA. Monitoring occurred in colonies with queens from miscellaneous U.S. sources that had not been selectively bred for varroa resistance (Harris et al. 2003). Mite populations were measured at the beginning and end of short field tests that started in the late Spring of each year. Multiple regression analyses showed that Varroa mite population growth rate was dependent on two of six variables measured: 1) percentage of reproducing female mites and 2) proportion of total mites in capped brood. The population growth rate was not dependent on 3) mortality of mites in brood cells, 4) growth of the bee population, 5) capped brood area at the end of the test and 6) duration of the test. (Regression analysis is a statistical process for estimating the relationships among variables). Analysis of commonality indicated that the percentage of reproducing female mites explained ≈ 26% of the total variation in r (the correlation coefficient) and the proportion of total mites in capped brood explained 6%. The joint expression of both variables accounted for another 4%. Thus residual error reflected most of the total variation in r, which suggested possible climatic or environmental effects on mite population growth. The lowest growth rates occurred in three consecutive years of drought in Louisiana. Measures of ambient temperature and relative humidity correlated to growth of mite populations among different years. Reduced growth rates were probably the result of diminished reproductive rates by Varroa mites during periods of hot and dry weather.
The population dynamics of Varroa mites were studied in colonies during the Summer in the region of Thessaloniki, Greece. The reproductive rate of the mite was estimated by examining the progeny of 364 females in worker cells and 131 in drone cells containing pupae with dark eyes and light brown thorax (i.e. nine days after the worker cells and 10 days after the drone cells were sealed). The proportions of non-egg laying mites on the two kinds of brood were about 19% and 4%, respectively. The reproductive rate was 2.92 for mites in worker cells and 3.66 for mites in drone cells. The rate for females reaching adulthood from each original female mite, for a single passage through the brood cells, was 0.71 for worker and 1.70 for drone brood (Ifantidis 1984).
Mite infestation density may also effect population growth within colonies. Infestation levels by adult Varroa mites and offspring numbers were recorded in worker brood cells (day 14-20) and drone brood cells (day 17-23) under German climatic conditions. The numbers of offspring likely to develop to adulthood were calculated according to a scheme based on the developmental stage of each pupa and the time interval to uncapping of the brood cells. With increasing numbers of Varroa infesting a cell, the numbers of female offspring per Varroa decreased within both cell types. Apparently, this decrease was based on fewer offspring produced per Varroa rather than by complete non-reproduction of some mites. Reproduction within combs, calculated according to the mite distributions, was even lower. This density-dependent reproduction is likely to influence population growth considerably. With increasing cell infestation, the proportion of males increased, thus reducing the bias of the sex ratio towards females (Fuchs and Langenbach 1989).
Since it has been shown that environmental factors can impact Varroa mite population growth, the reproductive behavior of female mites invading worker brood cells during the Winter months (January to mid-March) was investigated in four colonies in the UK. The number of viable offspring produced during a reproductive cycle, per mite, was only 0.5 during Winter compared with 1.0 during the Summer. This was mainly due to a large increase in the population of non-reproductive mites (Winter 20%, Summer 8%). This increase can be explained by the high level of male offspring mortality observed in Winter (42% vs. 18% in Summer), which results in nearly half of the newly reared female mites being unfertilized. Since mites that do reproduce lay a similar number of eggs in Winter (X̄ = 4.7) as in Summer (X̄ = 4.9), and the level of mortality suffered by the first female offspring is similar in Winter (7%) as in Summer (6%), it is probably not the internal physiological state of the host which causes the high level of Winter non-reproduction, as has been previously suspected (Martin 2001).
Varroa mites have relatively low reproductive rates, so populations should not increase rapidly, but often they do. One factor that might contribute to the growth of Varroa populations is the migration of mites into colonies on foragers from other hives. DeGrandi-Hoffman et al. (2016) measured the proportion of foragers carrying mites on their bodies while entering and leaving hives, and determined its relationship to the growth of Varroa populations in those hives at two apiary sites. They also compared the estimates of mite population growth with predictions from a Varroa population dynamics model that generates estimates of mite population growth based on mite reproduction. Samples of capped brood and adult bees indicated that the proportion of brood cells infested with mites and adult bees with phoretic mites was low through the Summer but increased sharply in the Fall especially at site one. The frequency of capturing foragers with mites on their bodies while entering or leaving hives also increased in the Fall. The growth of Varroa populations at both sites was not significantly related to the colony estimates of successful mite reproduction, but instead to the total number of foragers with mites (entering and leaving the colony). There were more foragers with mites at site one than site two, and mite populations at site one were larger especially in the Fall. The model accurately estimated phoretic mite populations and infested brood cells until November when predictions were much lower than those measured in colonies.
This transfer of mites to foragers is a shift in the mite behavior from attaching to nurse bees for reproduction (Kraus 1993; Kuenen and Calderone 1997) to foragers for possible dispersal. The frequency of this behavioral shift seems to increase in the Fall, and might occur for several reasons. Varroa populations are at their highest levels in the Fall and brood production is decreasing. There are fewer brood cells to infest so more mites are on worker bees perhaps including foragers (Sakofski et al. 1990). In hives that are highly infested with mites, the chemical profile of nurses and foragers can overlap causing mites to attach to foragers (Cervo et al. 2014). There are indications that foragers carrying Varroa have low returning rates to their own colonies (Kralj and Fuchs 2006) and could be drifting to other hives. The drifting could be due to parasitism alone or infection by viruses that Varroa transmit such as Deformed Wing Virus (DWV) or Israeli Acute Paralysis Virus (IAPV). Both viruses affect learning and memory (Li et al. 2013; Iqbal and Mueller 2007). DWV and IAPV titers increase with the growth of the mite population throughout the season reaching their highest levels in the Fall (Francis et al. 2013). Left untreated, these colonies collapse over the Winter. Viruses vectored by Varroa that affect forager orientation causing them to drift could provide a mechanism for both the virus and the mite to disperse in the Fall from colonies that are likely to die over the Winter.
Cervo et al. (2014) investigated the factors regulating the dispersal of Varroa mites. They showed that at low mite abundance, mites remain within the colony and promote their reproduction by riding nurses that they distinguish from foragers by different chemical cuticular signatures. When mite abundance increases, the chemical profile of nurses and foragers tends to overlap, promoting mite departure from exploited colonies by riding pollen foragers.
Varroa mites can disperse and invade honey bee colonies by attaching to “drifting” and “robbing” honey bees that move into non-natal colonies. Frey and Rosenkranz (2014) quantified the weekly invasion rates and the subsequent mite population growth from the end of July to November 2011 in 28 honey bee colonies kept in two apiaries that had high (HBD) and low (LBD) densities of neighboring colonies. At each apiary, half (seven) of the colonies were continuously treated with acaricides to kill all Varroa mites and thereby determine the invasion rates. The other group of colonies was only treated before the beginning of the experiment and then left untreated to record Varroa population growth until a final treatment in November. The mite invasion rates varied among individual colonies but revealed highly significant differences between the study sites. The average invasion rate per colony over the entire 3.5 month period ranged from 266 to 1,171 mites at the HBD site compared with only 72 to 248 mites at the LBD apiary. In the untreated colonies, the Varroa population reached an average final infestation in November of 2,082 mites per colony (HBD) and 340 mites per colony (LBD). All colonies survived the winter, however, the higher infested colonies lost about three times more bees compared with the lower infested colonies. Therefore, mite invasion and late-year population growth must be considered more carefully for future treatment plans in temperate regions.
The growth rate (r) of Varroa mite populations in Russian and Italian honey bee colonies was monitored from 2001 to 2003 in Baton Rouge, LA. Over this period, De Guzman et al. (2007) consistently showed lower mite growth in the Russian than in the Italian colonies. In 2001, instantaneous growth rates per week (r7) were r7=0.191± 0.011 for mites in Italian colonies and r7 = 0.137± 0.012 in Russian honey bees for 24.3 weeks. These growth rates were equivalent to 159.1- and 61.6-fold increase, respectively. Divergence in r7 values also was observed in 2002 when Russian colonies supported a lower growth rate of r7 =0.061± 0.016 (9.3-fold increase) than the Italian colonies (r7 = 0.122± 0.01 or a 31.7-fold increase) did after 26 weeks. The lowest rate of r7 =0.021± 0.011 (a 1.4-fold increase) was recorded for Russian honey bees in 2003, whereas the Italian bees in that year supported r7 = 0.145± 0.009 (an 18.9-fold increase after 19 weeks). This low growth rate of mite populations in Russian colonies may be attributed to several factors. Notably, as this study showed, Russian bees were less attractive to Varroa mites. Furthermore, the Russian stock supported lower proportions of brood infested and fewer multiply infested cells in both worker and drone brood, reduced mite reproduction, and extended phoretic period.
Arechavaleta-Velasco and Guzman-Novoa (2001) conducted a study to determine the existence of phenotypic and genotypic variation in the ability of honey bee colonies to restrain the population growth of the Varroa mite and to access the relative effect of four characteristics that many confer tolerance to honey bees toward the mite. Fifty-eight colonies infested with an equal number of mites were sampled monthly during six months to determine their levels of infestation on adult bees and in worker brood. At the end of this period, 16 colonies were selected to study the effect of grooming behavior, hygienic behavior, brood attractiveness and host-induced non-reproduction. The infestation-levels in adult bees varied significantly between colonies (range 6.6-44.7%), but no differences were found in the brood infestation levels. The variation between colonies was partially genetic in origin. Grooming behavior explained most of the variation (r2 =0.38). Negative correlations were found between the mite population growth and both the total number of mites and the number of injured mites collected from the bottom-boards (r=-0.65 and r=-0.76, respectively. Differences were found for hygienic behavior but the effect of this mechanism was not clear. No differences were found among colonies for brood attractiveness, or for the effect of the brood on the mite’s reproduction.
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