Leafcutting Bees and Lucerne

May 2018

The challenges of increasing the numbers of leafcutting bees, super-pollinators of lucerne and other crops

Leafcutting bees have the potential to increase the yield of lucerne seed crops by about 600%. They are also very effective pollinators of carrot seeds and blueberries and cranberries. Dr Ron van Toor is one of several people breeding the bees in an attempt to have enough for a business offering pollination services to the industry.

Megachile rotundata, the lucerne (alfalfa) leafcutting bee, is a native of Europe but is also found in North America where it is used to pollinate seed crops of lucerne, caged carrots and oil seed rape, and also blueberries, and cranberries.

The bees do not form hives but live as individuals and nest in holes in trees. Their life cycle starts in early summer and ends in mid autumn when they form pupae for overwintering.

DSIR scientist Dr Barry Donovan imported quantities of the bees in the 70s for work on improving the yield of lucerne seed crops. The bees proved successful in Canterbury and Marlborough and he bred them up to the point where it would have been possible to establish large-scale commercial pollination of lucerne for seed production for export, and possibly for exporting surplus bees.

However, that was not to be. Government science policy changed and in 1992 the DSIR was wound up. The substantial leafcutting bee stocks were handed over to AgResearch Ltd, which was working in lucerne seed production. Unfortunately, within a few years only 26 kg of cells were left. However, Barry retained the records and knowhow and started to breed them up again. Ron van Toor took over management of the leafcutting bees from AgResearch in 1997.

Continuing rasing leafcutting bees was a good move. Early in the 2000s New Zealand’s honey bee population began to face the challenges of Varroa mites, colony collapse disorder and another parasite Lotmaria passim.

Honey bees to pollinate New Zealand's key fruit, vegetable and seed crops were in short supply.

Some years later the situation changed.  Beekeepers began ramping up bee breeding and the number of registered hives more than doubled to over 810,000 s a result of the steep rise in the price of Manuka honey.

Paradoxically, this led to even fewer hives being available for pollination services because beekeepers found Manuka honey production farm more profitable.

Donovan has now retired from mass-rearing of bees but his stocks have been taken over by three enthusiasts.  One of them, Dr Ron van Toor, is a crop protection specialist with entomological experience.  He sees great potential for leafcutting bees to increase the yield of lucerne crops dramatically and help re-establish the industry.  They could also benefit other crops like carrot seeds, cranberries and blueberries.

“Barry has reported that in the 1960-1970s lucerne was grown here for seed and farmers were getting an average 75 kg of seed per hectare, but when leafcutting bees were introduced into the fields in the 1980s, seed yields exceeded 500 kg per hectare with 30,000 bees per hectare,” says Ron. This increase has been attributed mostly to leafcutting bees.

“Today lucerne seed is mostly imported from Australia because it is cheaper compared with what can be produced here without leafcutting bees. Also, we are struggling to get enough bees to support a lucerne seed industry.”

One of the reasons bee numbers are slow to increase is that they have a short breeding season. They are solitary insects and nest in holes in trees or in habitats provided by a beekeeper. Females need a blind ended tube ideally about 6mm in diameter and 100mm long in which they use leaves to construct robust cells about the size of a large vitamin capsule. Pollen and nectar are stored in each cell as food for the larva. The female lays one egg in each cell. The larva hatches from the egg and consumes the food supply. After moulting a few times, it spins a tough cocoon and pupates, hibernating over winter. It emerges from the nest as an adult. Males die shortly after mating, but females survive for another few weeks, during which they build new nests.

Beekeepers can manage the bees’ emergence to coincide with when crops are in flower. For lucerne, mating and cell formation typically take place in January and February when lucerne is flowering. By late March the females have mostly finished nesting and flowering is over. Most of the females by then have died. The prepupae in the cells hibernate over winter until warm weather in late November or December stimulates them into hatching as male and female bees and the cycle starts again. The cells are measured by the kilo with each kilo having about 10,000 live bee prepupae.

Bee breeders use laminated boards with matching 3mm grooves placed together to form 6mm nesting tubes. Stacks of these are put in small shelters within the lucerne crop and female bees fill the tubes with cells. In April the boards are taken apart and the cells removed to a large container for storage at around 4 ºC in a chiller where they are kept until the following December. They are then taken out and incubated in trays for three weeks at about 28ºC. As the bees start to emerge from the cells the trays are taken back to the shelter in the crop where they begin feeding, pollinating and reproducing.

The process of creating the cells is quite intricate. Often, the first four eggs in the tube will be female, and the remaining six male. For each cell the female packs in about 15 leaf discs to make a cocoon for the developing egg, furnishes it with pollen and nectar, seals that cell up and then continues on until the hole is filled with about 10 cells. The following summer the males are first to emerge. They wait for the females and once mating is complete the males die and the females begin making cells.

In favourable conditions there can be a six-fold increase in cells from one season to the next, but there are many hazards that beset leafcutting bees.  Irrigation is one problem, says Ron.

The bees are small and can be damaged by raindrops. They drop to the ground and succumb quite quickly to predation or hypothermia. Normally they can sense when rain is coming and take shelter but if an irrigator is operating on a sunny day they don’t sense that it is raining and so they are quite vulnerable,” he says.

“One option for farmers is to irrigate at night but lucerne is often grown in dry regions and regular irrigation may be required to get a high seed yield.”

Other factors that reduce bee numbers include mob stocking of lucerne stands near the seed production site – the bees are susceptible to trampling – and nearby trees can offer alterative accommodation to the nesting tubes provided by beekeepers. Bare soil nearby may also be a problem – bees like to sun themselves on the ground and may be run over by tractors and cultivation equipment. As if those were not enough hazards to contend with, there is also a parasitic wasp that can infest and destroy bee larvae.

Although raising leaf cutting bees presents difficulties Ron sees a future for them in the pollination industry.

“In the 1970s and 80s Barry started off by importing cells into New Zealand in small numbers and then gradually built up to about 270 kg. Unfortunately since then the number of good sites for propagating his leafcutting bees has diminished but I'm sure that if we had time on our side and some dedication to research and solving the problems we could make this a viable business,” he says.

“These bees do best on lucerne but other crops like carrots and blueberries are suitable hosts and we haven't yet tried oil seed rape and cranberries that are used in North America.” 

“Given the disease and supply problems with honey bees there is a natural demand from industry for alternative pollinators and I think there is a good future for leaf cutting bees particularly on crops where full pollination is important and honey bees are not available or are less effective.”

Carrot seed production is a case in point. New Zealand produces around half the world’s carrot seeds and limited work here so far has shown that leafcutting bees do visit carrot flowers. In shadehouses in United States, 150 leafcutter bees provided a pollination service equivalent to 3,000 honeybees!

They also have the advantages of being easily handled and transported (they don’t sting unless squashed on to skin), and their emergence can be timed to coincide with the flowering of broad-acre crops including white clover and Lotus species.

However, any research on lucerne leafcutting bees is inherently slow. It takes twelve months to do one trial because propagation occurs only once a year. For beekeepers the only income is from the pollination business as the bees don’t produce honey. Being able to offer pollination services requires large numbers of bees, typically 30,000 per hectare of crop.

In North America the lucerne leafcutting bee industry sells bee cells to farmers and growers to pollinate a range of crops. With this firmly in mind Ron and a couple of other enthusiasts are working hard at bee propagation, working towards a viable industry for New Zealand.