Bee trials on maize in the field

“We are looking at how bee colonies that can gather pollen from only one type of maize develop.”

The maize test area in the middle of August. Gauze-covered tents, distributed over the whole maize test area, represent the restricted habitat for the bees during the maize flowering season, each with two bee colonies. For the maize plants the space in the tents is rather cramped; their tops are bent under the three-meter high net. This flowering season is the crucial time for the bee trials of Stephan Härtel and his co-workers from the University of Bayreuth, since only then can the bees gather the maize pollen.

Dr. Stephan Härtel from Chair for Animal Ecology I at the University of Bayreuth, since 2010 Chair for Zoology III at the University of Würzburg is in charge of the bee project

During the maize-flowering period two honeybee colonies are placed in each of the flight tents.

Gauze tents cover 32 tracts of test land cultivated with four types of maize, each containing two bee colonies

In the maize jungle: Stefanie Nadler checks the development stage of selected maize plants.

An artificial hive for honeybees

In front of the man-made housing for the bees, the beehive, is a ‘dead trap’. Bees remove dead animals from the hive. In this way they are easy to collect and count.

One of three frames in the beehive inside which the bees build their honeycombs.

The honeycombs are built in such a frame. On the left is the cell of a queen

In the Bee Institute at Celle, other bee colonies are waiting for their transfer to the maize field. Not all maize strains are flowering yet.

Young bees

Young, one- to two-day-old bees

Bees with markings

Young, one- to two-day-old bees are marked before they are transferred to the maize field. Thus they can be recognised and examined to see how much maize pollen they have taken up.

Harmen Hendriksma performing a colony rating: Individual honeycombs are taken out and the population development estimated following the Liebefelder estimation method.

The bee with the green marking is the queen

The queen is the large bee with the green marking.

On a cloudy and windy day such as this it is quiet in the tent, only a few bees are out and about. Stephan Härtel makes a note of the development stage of the maize that his co-worker Stefanie Nadler skilfully determines for selected maize plants by pulling down the male flowers and checking the pollen load.

The bees in this tent can only gather pollen from the type of maize that is growing on this tract of land. Four different maize types are being cultivated on 32 tracts of land in this test area: genetically modified Bt maize, the conventional wild-type strain, and two other conventional strains. Each tract of land is covered by a tent enclosing two bee colonies. The scientists want to find out whether there is a difference in the development of bee colonies depending on which pollen they feed on. In particular they are interested in whether consuming pollen from genetically modified Bt-maize has any undesirable effects on the honey bees.

Pollen contains proteins that bees require, in particular for feeding their larvae. “In the test tents, maize pollen represents the only source of protein, which means that the bees are ‘forced’ to gather only maize pollen”, says Stephan Härtel pointing to the male flowers at the top of the maize plants. “The experimental design therefore represents the worst-case conditions. These can also occur in nature, for example in large monocultures”.

Maize is a wind-pollinated plant and does not produce nectar. Therefore, the bees under the test nets are also provided with a standard sugar solution, which can be filled into a chamber in the beehive. To ensure that the bees cannot drown, hydroculture pebbles are floated on the sugar solution. In addition, a trough of water is placed in each tent tract. A sufficient water supply is important for the regulating the temperature in the beehive.

“No dead bees today”, comments Stefanie Nadler briefly looking into the “dead trap” that scientists have developed for this experiment. Dead bees that are removed from the colony by the other members of their community are caught in the lower white container in front of the beehive.

“We record all the different parameters of the colony: for example, what is the mortality rate among the honeybees, what does the larval development look like, how much pollen has been collected?”, explains Stephan Härtel, and starts a five-minute observation of the flight activity of the test colony. He records how many bees fly out and how many return with pollen. Only a few carry a yellow shiny pollen freight on their legs.

Marking bees

Loud humming fills the small room at the LAVES Institute for Bee Research in Celle. Hundreds of young bees are crawling and jostling in constant movement on the countless honeycombs that are built in the wooden frames. Every now and then, the cover of a still closed comb is broken open and a young bee emerges.

While the colleagues out in the field fulfil their daily quota, Harmen Hendriksma and a co-worker are in the Bee Institute to mark the young freshly emerged bees. They take each individual animal and give them a coloured spot so that they can be recognised later in the test area. The young, one- to two-day-old bees need a large amount of protein during this stage in their development. In the test tent they will only have the maize pollen as a source of protein. When they are removed from the colony some time later, it is possible to tell exactly from their digestive tract how much maize pollen they have taken up.

The bee colonies for the experiments have been breed in the Bee Institute at Celle. They are “Celle bees”, a line of bees with a long breeding tradition. To be able to compare the bee colonies it is important that they all have a similar genetic background. All queen bees are daughters of one mother and are mated with drones breed in one population.

Colony rating

Back on the field, in the afternoon, Harmen Hendriksma carefully opens one of the beehives. During the breeding cycle of three weeks, five colony ratings are carried out. In each hive are three removable frames, on one side of which is a prepared comb nucleus onto which the bees can build. The frames are taken out individually and, according to a particular method, the Liebefelder estimation method, the development of the colony is determined. Opening the hive makes the bees agitated. When the bees are brushed off to take a look into the honeycomb, the bee researcher risks a sting or two. Harmen Hendriksma has got used to bee stings; he says that he has already been stung about fifteen times this week.

Eggs can be recognised at the bottom of the honeycomb cells. The scientists follow the fate of individual marked eggs up to the emergence of the bees and calculate from that the breeding index, an important value for determining the development of the population.

Future approaches

Next year, Stephan Härtel and his co-workers want to infect part of the bee population with bee pests, the Varroa destructor mite and the gut parasite Nosema, to see how the weakened population develop on the different maize types.

To determine how much maize pollen is actually carried to the beehives by the honeybees, the scientists are going to place standardised young bee populations in different agricultural settings in which both the proportion of cultivated maize as well as other possible pollen sources differ. Using pollen traps they want to determine the spectrum of pollen that is the bees exploit.

The bee researchers are investigating another interesting aspect. They want to know whether different bee parasites are sensitive to Bt proteins. The Bt maize growing on the test area contain three different Bt proteins that are effective against maize pests the European corn borer, a butterfly, and the Western corn rootworm, a beetle. Economically significant bee parasites such as the lesser wax moth and the greater wax moth as well as the small hive beetle can thus be affected in their development by absorbing residues of Bt proteins stored inside the honeycombs.