New hope for the many households affected by food moths: Swarms of Trichograms from mail order put an end to pests.
The mail puts the animals into the post box, that is how they come into the house: 15,000 eggs on paperboards. The cards are to be distributed in the kitchen, says the package insert.
Then nature takes its course.
In the following days tiny Trichograms crawl from the eggs and start working immediately. They walk around the supplies and search for food moths that have become a plague for many households.
After three weeks a new delivery arrives, then another. After ten weeks there should be no moth alive.
The use might cost some effort; not everybody is happy to share its home with thousands of insects. But more and more people take the step.
The company AMW Nützlinge in the Hessian Pfungstadt breeds the insects for mail order. A swarm of Trichograms costs up to 40 euros. And the sales are rising. Last year AMW supplied almost 10,000 households, says director Sylvia Melchior: "Fife years ago the numbers were not even half of it."
Trichograms are parasites with beneficial preferences. If a female finds a moth egg it places an own egg with the ovipositor. The larva that emerges from the egg eats the foreign egg empty. In the end a complete insect leaves the shell, ready for new actions. The moths have little chances against ten thousands of parasites that advance in always new offensives against their clutches of eggs.
The aggressors of the species Trichogramma evanescens, hardly half a millimeter large, belong strictly speaking to the Trichogramma wasps. Almost all these insects are specialized on foreign eggs and larvae. And now slowly the word gets around that some parasites are as well suitable for households. Especially in the Internet consulters find encouraging reports.
It is very hard to get rid of the plague once you have food moths in your flat. The parasites infest all types of dry supplies: Cereals, noodles, dried fruit, nuts, but also chocolate, fruit tea and spices. Closed bags cannot stop them. The initially hair-fine moth larvae even squeeze through leaks in screw caps.
If you want to make the kitchen mothproof, you should store the supplies in tight plastic containers. But the pests can also live some time on a few cereal flakes behind the cupboard. Their eggs can hardly been detected with the naked eye and are also well hidden; even the predrilled holes for shelves in the kitchen cupboard can serve as breeding place.
But moths can hardly hide from the ichneumon flies. The perfect haunters track down their hosts everywhere. The persistence of the animals is amazing. "They walk around until they find some traces of odor. A few wing scales are enough", says AMW biologist Olaf Zimmermann. The wasps even scent the special signal odor that a already copulated moth female gives off to keep off other males. This odor is especially exciting for the small searchers. They deduce that soon there will be fresh eggs. Researcher even observed how the mites sometimes climb onto their hosts und let themselves take along directly to the oviposition.
This sounds like a questionable lot of swarming and struggle for survival. In truth the highly motivated creatures are hardly noticed in the kitchen. The ichneumon flies do not like to fly; they are so tiny that every air draft can take them away - biologists talk about aerial plankton. Therefore the parasites prefer to hunt for eggs by foot; their life is short enough. And when they finish their destruction work and do not find new moth eggs anymore they depart this life after some days without causing a stir. Some might escape before through the open window, the other discretely join the house dust.
In private households the career of the small pest exterminators has just begun. With gardeners and farmers, however, their use is already very common. "Ichneumon flies of the species Trichogramma are probably the most widely bred beneficial insects worldwide", says Johannes Steidle, professor for Zoology at the Universität Hohenheim. "Many studies have shown that they effectively decimate harmful butterfly species."
In Southwestern Germany farmers disperse wasps of the species Trichogramma brassicae across their corn fields. Otherwise the voracious corn borer rages unchallenged. The larval of this butterfly do not only devour the leaves; they also eat the stems from the inside until the plants bend.
But now millions of ichneumon flies combat against the borer. High-wheeled straddle-type tractors roll through the rows of corn plants. Wide tube works are installed on them from which the parasites are dropped like paratroopers. About every 14 meters a unit goes down. This way thousands of hectares are vaccinated each year.
For that reason the company AMW developed slotted globule from maize starch, each containing thousand action forces sleeping in their eggs. The hatched Trichogramma wasps scrawl out through the slot and immediately start attacking the eggs of the borer.
Some small companies ensure the adequate supply. In the incubators of AMW the parasites mature in corked glass tubes. Sheets of papers full of eggs from the Angoumois grain moths are handed to the females for oviposition. These moths are specially kept as hosts; they easily grow on grains.
The art of breeding consists in synchronizing carefully the cycle of generations: Private households receive a new delivery of moth eggs already populated by ichneumon flies at intervals of two, three weeks. The cards contain eggs of different maturity levels; new wasps should hatch every day as possible so that the pressure of persecution does not decline.
Some years ago the chemical company BASF also dabbled in the breeding of ichneumon flies but the demand of supervision for the insects was too high. And there was already a remedy against the corn borer in sight: Genetic Engineering. The so-called Bt corn, distributed by the company Monsanto, has been developed for the defense of the corn borer. A gene of the bacterium Bacillusthuringiensis is integrated in this corn and produces a protein lethal for the pest.
The radical opposition against genetically engineered corn could not been predicted then. If it were up to the Hohenheim biologist Steidle the whole controversy would have not been necessary. The pest can be well kept in check with ichneumon flies believes Steidle. The ichneumon flies do not completely extinguish their victims in the field but they press heavy on them.
"But the intellectual effort is comparatively high", says Steidle. The farmer has to control the development of the corn borer with counting traps and has to know how many ichneumon flies to send how and when into the field - much depends on the right time.
Already in the 1920s a Berlin biologist discovered the special suitability of the Trichogramma. But his discovery was soon forgotten, because chemistry had just begun its triumphant success. And the breeding of beneficial insects is still a niche business, operated by small companies mostly founded by biologists.
But just since the public reacts more and more sensitive on chemical maces and genetic engineering things gain momentum. More and more useful insects that can be easily breeded and are robust enough for shipping are discovered. The Hohenheim biologist Steidle himself has made ready for the market a Trichogramma that proceeds against the feared corn weevil. This bug only remains in grain storages where it empties the grains from the inside. Up to now normally gas helped.
By Manfred Dworschak
DER SPIEGEL 47/2008