Each spring, when the March sun begins to warm the earth, still cold from the winter, the apple-blossom weevil crawls out of its winter quarters. Its goal: fruit orchards. With its proboscis, the weevil pierces the apple-blossom buds to get at maturation feeding. Later, the females will lay their eggs inside. The blossom perishes before it can open. The fruit farmer has to contend with greater or smaller losses–depending on the intensity of the attack.

Claudia Hausmann, biologist at the ETH Institute for Plant Sciences, applied entomology, has developed a method to test and monitor the infestation of fruit orchards by the apple-blossom weevil. The method provides producers with valuable information to help them to decide whether and when they need to combat the weevils with chemical means. For her work, which is of great theoretical and practical value in European fruit production, Hausmann has received this year's Hans Vontobel Prize.

Exploiting penchant for warmth

The surveillance system is strikingly simple. It takes advantage of the fact that the apple-blossom weevil can sense warmth and heads for protected places. Hausmann developed a shelter trap that exploits this preference. In principle, the trap is a hollow cylinder, coated with a layer of isolating foil inside and a dark cover on the outside and that is then fixed to the trunk of the tree. This is where the weevil seeks shelter from the cold.

Tests and comparison with other, partially self-made, traps, showed the scientist that the number of weevils found in the new traps corresponded very accurately to the level of infestation in the orchard overall. A classical method is to knock on the branches so that the weevils fall off. The problem with this is that even though the fruit farmer knows that he has Anthonomus pomorum he still doesn't know how strong the infestation is overall and whether the damage threshold–important for the possible deployment of insecticide–has been reached. "The new traps specifically attract the apple-blossom weevil and now make it possible to determine the exact point in time of maximal immigration," explains Claudia Hausmann. Moreover, the traps also work on cold or wet days when the presence of the six-millimetre long creepy-crawlies can hardly be ascertained by traditional methods. If the weather is wet, knocking on the branches won't dislodge the weevils because they stick to them.

Microclimate important for activity

The PhD student of the applied entomology group, laid the fundament for the new testing method in the laboratory. In trials she discovered that the temperature plays a central role in the orientation ability of this weevil. In a special test arena with a temperature pattern from cool to warm the weevils–both males and females–always sought out the warmest micro-living spaces.

Claudia Hausmann with her beetle trap.

Just 6 mm long with an appetite for apple-blossom buds: the apple-blossom beetle Anthonomus pomorum (Picture: Slide from the ETH entomological collection) large

The same behaviour can also be observed in nature–and for makes sense, biologically, because Anthonomus becomes active in spring but the body temperatures of insects is determined by the surrounding temperature. In order to escape from the cool pre-spring temperatures the weevil seeks out places with favourable microclimatic conditions. Visual orientation also seems to play a role in the search, and Claudia Hausmann also investigated this aspect.

The female of the species apparently recognises certain colours and silhouettes and uses these for orientation. Exactly how she uses these capabilities to find a fruit orchard on leaving her winter hideout is something that needs to be examined more closely.

Soon a commercial application?

Meanwhile, research institutions in Switzerland and France have signalled their interest in putting these fundamental findings into practice. Before such a product can be brought to market, says Hausmann, trials must be conducted over large areas. This is necessary in order to make exact predictions as to when the damage threshold really had been reached. "The research institutions have to test the traps by working together with the farmers," she says.

Up to 80 per cent crop loss

Apple-blossom weevils can do considerable damage in an apple orchard. Before the second World War this weevil destroyed up to 80 per cent of the harvest. With the application of wide-spectrum insecticides in the post-war period the incidence dropped sharply. Today, Anthonomus pomorum is on the move again because increasing numbers of cultivators are going over to integrated production methods, hardly using wide-spectrum insecticides anymore and, instead, specifically target individual pests. Biodiversity benefits from this approach, but so does the apple-blossom weevil. This makes an early warning system that can alert farmers to infestation reliably even more valuable.