Varroa Mite

The 1.5mm-long parasite, Varroa destructor, is the apex pest of honey bees worldwide, hindering the beneficial insects' ability to pollinate a range of fruit, vegetable, flower and nut crops (Image courtesy of the US Dept. Of Agriculture, Agriculture Research Service)

The 1.5mm-long parasite, Varroa destructor, is the apex pest of honey bees worldwide, hindering the beneficial insects’ ability to pollinate a range of fruit, vegetable, flower and nut crops (Image courtesy of the US Dept. Of Agriculture, Agriculture Research Service)

Varroa is a genus of parasitic mites associated with honeybees. This pinhead-size pest attacks developing bee larvae and adult bees, resulting in deformed bees, reduced lifespan and ultimately, the destruction of the colony or hive. MIKE NICHOLS reports on the New Zealand experience and the impact of Varroa mites on honeybee populations, the honey industry and crop pollination. He highlights the potential of using domesticated bumblebees for pollinating horticultural crops.

Agriculture in general and horticultural specifically requires pollinating insects if it is to achieve the ever-increasing productivity that the world demands, if the increasing population is to be adequately fed in the future. Worldwide, the usual pollinating insect is the honeybee (Apis spp), which lives in well organised colonies comprising a single queen (fertile female), some drones (males), and many thousands of worker bees (sterile females) whose main role is to collect honey and nectar from flowers to feed the colony. It is this nectar and honey collection that ensures that pollen is transferred from flower to flower, and thus, those crops requiring this type of transfer are pollinated. Of course, there are also many wind-pollinated crops, such as sweet corn, the grasses, and many nuts, but pollen transfer by insects is a very important part of crop production.

Female Varroa mite—Australia is the last Varroa-free bastion, (Image courtesy Honey Naturally)

Female Varroa mite—Australia is the last Varroa-free bastion, (Image courtesy Honey Naturally)

There is one major problem that faces the honeybee in Australia. It is called Varroa, a small mite, which can decimate honeybee populations. It is already in New Zealand and Papua New Guinea, and it is not a question of if it will arrive on the Australian mainland, but when! This will be a dilemma for the Australian authorities. For example, within New Zealand attempts were made to stop the spread of the pest with strong quarantine measures to prevent the transfer of hives between districts, but still it spread, even across the Cook Strait. Whether this was by man or by feral bees is a matter of conjecture. Research efforts are now directed at the use of miticides (short-term, because we know that resistance to chemicals will eventually and inevitably occur), and selective breeding to produce, hopefully in the long-term, tolerance to Varroa. It is estimated the value of the honeybee to New Zealand horticulture is close to NZ$2 billion, and in Australia the estimate is likely to be much higher.

There is in New Zealand a steadily developing interest in the potential of using domesticated bumblebees for pollinating horticultural crops.
My first involvement with bumblebees was in 1959, when I was based in the small South Island town of Oamaru. Also based in the same Department of Agriculture office were two apiculturists, who were examining the effect of bumblebees on the pollination of red clover in the high country near the Southern Alps. My next involvement with bumblebees was not until  the mid-70s, when there was a developing market for melons to be grown in a greenhouse in New Zealand for export to Japan. In a study involving growing the plants at three distinct minimum temperatures, I discovered just how difficult it was to pollinate melons with a small paint brush by transferring pollen from the male flowers to the female flowers. After the first few flowers, the brush simply gummed-up with nectar. There were, however, some flowering shrubs growing nearby, which were attractive to bumblebees, so every few days I caught one or two bumblebees and released them in the greenhouses. The result was excellent pollination, and a very successful crop.

Bumblebee on tomato truss.

Bumblebee on tomato truss.

At that time, a certain Nelson Pomeroy was studying at Massey towards his PhD, researching bumblebees, so he came and visited and asked whether I had considered using bumblebees for pollinating greenhouse tomatoes, as the Dutch were looking at this technology. To my undying embarrassment, I replied that “tomatoes did not require cross pollination”—not being aware that the role of the bumblebee was simply to vibrate the flower, and make the pollen ‘fly’. Dr Pomeroy (as he later became) went on to establish Biobees to produce bumblebees for the greenhouse tomato producer.
Like Australia, the bumblebee is not a native to New Zealand. In fact, it was first introduced in 1885 to assist in red clover seed production. Similarly, the honey bee (Apis mellifera) is not a native to either New Zealand or Australia. Bumblebees do exist in Australia in the wild, in Tasmania, where they have become established either from intentional importation, or chance.

It is interesting to consider how both New Zealand and Australia would have far fewer pests and diseases if current biosecurity standards had existed in the 18th and 19th century. There was certainly considerable trade of both plants and produce between Australia and New Zealand in the 19th century. A plaque on the wharf at Nelson (New Zealand) gives some idea of the range of plant material brought in by the earlier settlers. One must suspect that the only reason many pests and diseases did not become established, was simply that the local environment was not suitable. This is probably why that major Australian concern (Fire Blight) does not exist in Australia, but in certain (wet) seasons can be a problem in some parts of New Zealand.

With the potential for declining honeybee populations in New Zealand, there is concern about the future pollination of horticultural crops, and an interest is developing in the potential for using bumblebees to supplement (at this stage) the honeybee. In fact, bumblebees are far more reliable than honeybees for pollination. They will forage in much wetter, colder and windier conditions, and will pollinate flowers, which do not produce nectar, whereas honeybees do not. Thus, they are far superior to honeybees for kiwifruit pollination, for example.  In fact, it has been estimated that one bumblebee is worth 50 honeybees, in terms of pollination potential. The downside is, however, that bumblebee colonies comprise only a few bees (20-400), whereas a honey bee colony can be as many as 50,000 bees. Also, honeybee colonies are perennial, while bumblebee colonies last for only three to five months.

Koppert hives fitted with prototype wireless operated doorways.

Koppert hives fitted with prototype wireless operated doorways.

Although bumblebee hives have been available in New Zealand since 1988, it is only recently that an interest has developed in the possibility of using bumblebees for pollinating crops other than greenhouse tomatoes. To date, this has mainly involved research to examine methods to ensure that a higher proportion of bumble queens overwinter successfully. Currently, only about 30% of the hibernating queens survive in natural conditions. Providing a suitable environment for the bumblebees is a key factor, and this involves such fundamental, but essential factors, as care with the use of agrochemicals (particularly the use of neonicotoid insecticides), providing undisturbed areas for nesting sites, and having a wide diversity of flower-rich plants nearby, because unlike honeybees, the bumblebee stores only small amounts of pollen and nectar. Of course, it is not impossible that Varroa mites prove too difficult to control, that we will see bumblebee hives becoming the standard method for pollinating fruit crops in the future. Under these circumstances, we will need to investigate what exactly makes bumblebees ‘tick’, in a similar manner that has occurred with honeybees over the past 150 years, with the possibility to ensure a longer life for each domesticated colony, and prolonging the life of each queen.

Of course, this will require a massive change in biosecurity policy in Australia, but given the importance of fruit and other insect-pollinated crops in Australia, nothing may be impossible if Varroa becomes established.

What’s the risk to Australia?
The most obvious threat is to Australia’s bee and honey industries. The Varroa mite would decimate Australia’s feral bee population and cause a rapid increase in demand for pollination services. It is estimated that Varroa mite could cost Australian plant industries between $21.3 million and $50.3 million per year over thirty years (Source: CSIRO Submission no. 33, p.10, to the House of Representatives Standing Committee Inquiry into the Future Development of the Australian Honeybee Industry). Apart from reduced honey production, apiarists would need to repeatedly treat their hives to ensure their survival.

However, the major part of the cost of Varroa would probably be felt not by the honeybee industry but by other industries with crops that rely on honeybees for pollination, including almonds, avocadoes, cotton, stone fruits, pome fruit, melons and pumpkins.

Varroa mites were discovered in New Zealand in 2000 and have already had a major economic impact, with significant control costs and losses of bees, hives, honey production, crop yields and export revenue.

About the author
Dr Nichols is a retired University teacher from Massey University, New Zealand, and a regular contributor to Practical Hydroponics & Greenhouses magazine. Email:   Ω

January 2014 / Issie 139