J. Acarol. Soc. Jpn., 25(S1): 109-117. March 25, 2016 © The Acarological Society of Japan http://www.acarology-japan.org/ 109

Effects of tracheal infestation on Japanese honey , Apis cerana japonica

Taro MAEDA* National Institute of Agrobiological Sciences, 1-2 Ohwashi, Tsukuba, Ibaraki 305-0851; Japan

ABSTRACT

The tracheal mite, woodi (: ), is an endoparasite of honey . The feed on bee hemolymph in the tracheas of adult bees. Mite infestations cause serious damage to bee colonies. The distribution of these mites is now worldwide, from Europe to South and North America. The first recorded A. woodi infestation in the Japanese native honey bee, Apis cerana japonica, occurred in 2010. In a previous study, to determine the distribution of A. woodi in Japan, we sampled more than 350 colonies of A. cerana japonica. We found mite infestation from central to eastern Japan. On the other hand, Apis mellifera in Japan has not suffered serious mite damage. Here, to determine the effects of mite infestation on Japanese native honey bees, we investigated seasonal prevalence, mite load in the tracheal tube, and the relationship between mite prevalence and K-wing (disjointed wings). Similar to European honey bees, Japanese honey bees had a high prevalence of mite infestation in winter and a low prevalence in summer. The average mite load was about 21 or 22 mites (all stages) per trachea when all bees were infested by tracheal mites (100% mite prevalence). This mite load did not differ from that in A. mellifera. The K-wing rate was positively correlated with mite prevalence. Further comparative investigations focusing on the mechanisms of infestation will be needed to elucidate the differences in mite susceptibility between Japanese and European honey bees. Key words: , Asian honeybee, K-wing, seasonal prevalence, Tarsonemidae

INTRODUCTION

Acarapis woodi Rennie (Acari: Tarsonemidae) is a tiny endoparasite of honey bees. Most studies of A. woodi have been conducted on the European honey bee, Apis mellifera L. (Hymenoptera: Apidae). The mites are found in the tracheal systems of the bees. Acarapis woodi spends most of its life and several generations in a single host (Bailey and Ball, 1991). All larval and adult mites feed on bee hemolymph by piercing the walls of the trachea and air sacs (Pettis and Wilson, 1996). A. woodi was first discovered on the Isle of Wight, in England (Rennie,

Tetsuo GOTOH and DeMar TAYLOR (eds.), Acarology XIV: Proceedings of the International Congress. Journal of Acarological Society of Japan 25 (Suppl. 1): 1-192. * Corresponding author: e-mail: [email protected]; tel: +81-29-838-6289; fax: +81-29-838-6289. This work was supported by JSPS KAKENHI Grant Number 26290074. DOI: 10.2300/acari.25.Suppl_109 110 Taro MAEDA

Fig. 1. Pie graphs showing rates of infestation of Japanese honey bees by tracheal mites (infested colonies/tested colonies) in each prefecture. White portions, intact colonies; black portions, mite- infested colonies. Pie size represents sample size (n＝1 to 10; 11 to 20; 21 to 50; ≥51). Data from the work of Maeda (2015) were used to create this figure.

1921), and infestation of A. mellifera with the mite has now spread all over the world, with the exception of Sweden, Norway, Denmark, New Zealand, and Australia (reviewed by Sammataro et al., 2000). Many studies have revealed the various types of damage caused by infestation of A. mellifera with these tracheal mites (reviewed by Sammataro et al., 2000, 2013). For example, a lightly infested beehive shows low honey production and low pollination capacity (Eischen, 1987; Eischen et al., 1989; Otis and Scott-Dupree, 1992). A heavy load of A. woodi results in colony loss during winter (Eischen, 1987; Eischen et al., 1989; McMullan and Brown, 2009). Such damage is derived from injuries to individual bees, including nutritional loss, damage to tracheal Tracheal mite infestation in Japanese honey bees 111 systems, reduction of tracheal airflow, and paralysis of flight muscles (Bailey and Lee, 1959; Eischen, 1987; McMullan and Brown, 2009). There were no reports of the occurrence of A. woodi in Japan until 2010 (Kojima et al., 2011). A subsequent survey revealed that many colonies of the Japanese honey bee, Apis cerana japonica, were infested with A. woodi (Fig. 1; Maeda, 2015). A. cerana japonica is a subspecies of the Asian honey bee; it differs from the European honey bee in terms of more placid behavior, greater disease tolerance, higher resistance to natural enemies such as hornets, higher tendency to abscond, and active foraging in cold temperatures (e.g., Chen et al., 2000; Ono et al., 1987; Peng et al., 1987; Sasaki et al., 1991; Sugahara and Sakamoto, 2009; Yoshiyama and Kimura, 2009). The Japanese honey bee is a vitally important pollinator (Kevan, 1995). For example, in one study A. cerana japonica was the most common flower-visiting insect (17%) on common buckwheat (Taki et al., 2010). To conserve the honey bees and pollinator fauna of Japan, it is important that we clarify the damage done by the tracheal mite in Japanese honey bees. Diagnosis of tracheal mite infestation is difficult, because the mites are too small to see with the naked eye, and they stay inside the bees. Signs such as dwindling populations, bees crawling on the ground near their hives, and bees with disjointed hindwings (called K-wings) are often used in the absence of other reliable evidence (Sammataro et al., 2000). Here, we investigated seasonal changes in tracheal mite prevalence (% of bees infested within each colony), and mite load in the tracheal tubes, to determine the effects of mite infestation on A. cerana japonica. In addition, to test the possibility of using K-wing prevalence as a mite- infestation index, we checked the relationship between K-wing prevalence and tracheal mite prevalence.

MATERIALS AND METHODS

1. Seasonal prevalence of the tracheal mite Mite prevalence in 27 colonies of A. cerana japonica (% of bees infested within each colony) was investigated monthly from April 2013 to April 2014. Sampling of 19 colonies began in April or May 2013. Ten of the colonies were infested by A. woodi at the beginning of the investigation or had been placed less than 100 m away from infested colonies. The other nine colonies were placed where there were no infested colonies nearby. Two more colonies placed near the infested colonies were added in June and July. Investigation of six more colonies (either infested or with neighboring infested colonies) started in November and December. Worker bees collected from each colony (n＝10 to 68) were frozen in a freezer. Mite infestation was detected by dissecting the defrosted bees under a microscope. The dissecting procedure was modified from that of Sammataro (2006) and Sammataro et al. (2013); details have been given by Maeda (2015).

2. Mite load in tracheal tubes The numbers of A. woodi (mite load) in the tracheas of bees were counted under the microscope after dissection. Mite load was expected to vary with mite prevalence. To investigate the mite loads of heavily infested bees, bees were sampled from two managed colonies of A. cerana japonica where the mite prevalence was 100%. Forty-two workers were sampled from 112 Taro MAEDA

Fig. 2. Mite prevalences (infested bees/sampled bees) from April 2013 to April 2014. (A) Seven colonies collapsed by August; (B) three colonies survived through the summer (black dots), and the four added colonies (white dots) collapsed by the following March; (C) four colonies survived until the following April. one of the colonies in November. The other colony provided samples for dissection in December 2013 (n＝24).

3. K-wing rate and mite prevalence Workers bees were collected from 12 colonies of Japanese honey bees. The number of bees sampled ranged from 20 to 42 per colony. K-wing symptoms were determined visually. Tracheal mite infestation in Japanese honey bees 113

“K-wing” is defined as a state in which the wings are held asymmetrically over the back; the hindwing is disjointed from its forewing and sits farther forward, across the normal position (Shutler et al., 2014). Although the position of the hindwing is abnormal, the wing morphology is normal. Mite prevalence (% of bees infested within each colony) was determined by dissecting all sampled bees under a microscope.

RESULTS

1. Seasonal prevalence of tracheal mite Nine colonies placed away from mite-infested colonies were not infested by A. woodi during the experimental period. Four of the colonies survived through the experimental period. The other five colonies absconded because of disturbance by human activity, attack by hornets, takeover by European honey bee, or collapse of the honeycomb in hot weather. Seven colonies infested by the mites in the spring had collapsed by August (Fig. 2A) because of colony diminishment or absconding after colony deterioration. The other three colonies survived the summer with low mite prevalences (0.0%, 7.7%, and 26.7%; Fig. 2B). These three colonies died in November, December, and the following March. The four colonies in which observation began in the fall had collapsed by the following March (Fig. 2B). The first sign that these colonies would undergo rapid loss of bees was bees crawling on the ground around the hives. The residual small populations died in the cold weather. The colonies left large amounts of honey in their hives. Only four colonies survived until April 2014 (Fig. 2C), and only one colony swarmed.

2. Mite load in tracheal tubes Both tracheal tubes of all bees were infested by A. woodi. The maximum number of mites found in the tracheal tube of a worker bee was 65 (Fig. 3). The average mite loads of the two colonies were 22.3±2.3 (mean±S. E.) and 21.3±1.6 (mean±S. E.).

3. K-wing rate and mite prevalence Four of 12 colonies showed no infestation with tracheal mites. In addition, these four colonies had no K-wing bees. Mite prevalence in the other eight colonies ranged from 2.3% to 96.2%, and K-wing rates ranged from 0% to 26.9% (Fig. 4). A significant linear relationship was found between mite prevalence and K-wing rate (P＝0.0030). However, among individual bees, 88.9% of mite-infested bees (96/108) did not display the K-wing sign. Furthermore, nine bees had K-wing even though they were not infested with tracheal mites.

DISCUSSION

Mite prevalence in the European honey bee generally increases in winter, because the bees survive longer̶and the mites lay more eggs̶than in summer (Dawicke et al., 1992; Otis et al., 1988). The seasonal prevalence of A. woodi in Japanese honey bees was the same as in European honey bees, namely a high prevalence from fall to spring and low in summer. Many A. mellifera 114 Taro MAEDA

Fig. 3. Frequency distributions of tracheal mite load (all stages). A) Forty-two bees from a managed colony in Tsukuba, Japan, in November 2013. B) Twenty-four bees from a managed colony in Tsukuba, Japan, in December 2013. Mite prevalence in both colonies was 100% at the time of sampling. colonies infested with tracheal mites collapse during winter and early spring (Dawicke et al., 1992; Eischen, 1987). A. cerana japonica colonies collapsed in summer, as well as in winter. The colony that collapsed in winter left behind a large amount of honey in its hives. On the other hand, in summer, there was no honey in the hives. Colony death in summer could thus be due to brood reduction, shortage of honey stock, damage by the greater wax moth Galleria mellonella, or absconding (personal observation). Absconding occurs even in uninfested colonies, as shown in this study. It is therefore difficult to pinpoint the most important determinant of colony collapse. However, all collapsed colonies showed, first, a reduction in population, with the death of many bees crawling on the ground around the hives. Although infection with tracheal mites might not have been lethal, it caused serious damage to the bee colonies, even in spring and summer. Tracheal mites affect honey bees on an individual as well as a colony level (reviewed by Tracheal mite infestation in Japanese honey bees 115

Fig. 4. Relationship between mite prevalence and K-wing rate. Each dot represents the result for one colony. Regression line is shown (y＝0.1928x＋0.0065; r2＝0.6016; P＝0.0030, n＝12).

McMullan and Brown, 2009). For example, accumulation of mites in the tracheae in the thorax may decrease gas-exchange for flight; increased levels of mite infestation might therefore reduce the probability of a bee returning to the hive in cool weather (Harrison et al., 2001). The mite load in the tracheal tubes of A. mellifera varies according to the season and the age of the bees (Pettis and Wilson, 1996). For example, infested bees 20 to 30 days old were found to have 14 to 33 mites (including all stages) in their tracheas in winter. Here, we found 21.3 mites in the tracheal tubes of A. cerana japonica in November and December̶values not very different from those in A. mellifera. These severe infestations might result in colony collapse in the Japanese honey bee. It is desirable to detect A. woodi infestations as early as possible to minimize the damage caused. Bees crawling in front of the hives are one of the typical signs of mite infestation, but this signs occurs when the mite infestation rate is already very high (Sammataro et al., 2013). K-wing is the other candidate index of mite infestation, and it is used by beekeepers (Sammataro et al., 2013). Shutler et al. (2014) demonstrated that K-wing is positively associated with both black queen cell virus (BQCV) and colony strength. However, the cause of K-wing is still unknown (Shutler et al., 2014). Here, we revealed that the K-wing rate was positively correlated with mite prevalence in Japanese honey bees. This result suggests that the K-wing rate could be useful in early detection of mite infestation. On the other hand, our finding that nine uninfested bees had K-wing suggests that mite infestation is not the only cause of this condition. There have been many studies of the effects of tracheal mite on European honey bees since this epidemic disease first appeared almost 100 years ago (reviewed by Sammataro et al., 2000, 116 Taro MAEDA

2013). Since the first report of A. woodi in Japan in 2010 (Kojima et al., 2011), many managed and feral colonies of the native Japanese honey bee, A. cerana japonica, have suffered serious damage from these tracheal mite parasites. On the other hand, commercially managed European honey bees in Japan have not suffered serious damage (Maeda et al., 2015). Considering the pandemic distribution of the tracheal mite and the substantial damage it causes, the current interspecific difference in mite prevalence in Japan is intriguing. Comparative studies of the mechanisms of why Japanese native bees are susceptible to A. woodi but European bees in Japan are not infested would give us a strong hint as to how to control tracheal mite infestation.

ACKNOWLEDGEMENTS

I thank the apiarists all over Japan for sampling the bees. This work was supported by JSPS KAKENHI Grant Number 26290074.

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