A recent meta study on the effects of Bt-corn on non-target organisms has generated clear cut results according to its author. Bt-corn exhibits the desired effect on the targeted pest and seems to do no unwanted harm to other organisms in the cornfield.
 
Genetically modified crops were planted in large scale in 23 countries in 2007. Alone Bt-corn, which produces proteins from genes of the soil bacteria, Bacillus thuringiensis (Bt), was planted on 28 million hectares. These so-called Bt toxins, of which Bt-corn has two, protect the plant from being eaten by insect larvae. Varieties of the corn with Bt-toxins targeting butterfly larvae, in particular the European corn borer, Ostrinia nubilalis, have been in cultivation since 1996.

In addition, the last several years has seen the development of a Bt-corn variety producing a toxin targeting another agricultural pest. It protects the roots of corn stalks from the Western Corn Rootworm, Diabrotica virgifera. In the USA this pest is considered to be by far the most important corn pest and each year more than 10 million hectares of land are sprayed with insecticide to combat it. Since its introduction into Europe during the mid 80´s, it has been spreading steadily and has now become a threat to European corn production.

Danger for other insects?

Because the target of the toxins produced by Bt corn is an insect, the question has been raised as to whether other insects might also be harmed when cultivating this crop. Recently, all data from field studies on Bt-corn published in scientific journals in the years 1992 to 2006 have been gathered together and quantitatively analyzed, a so-called meta study, by LaReesa Wolfenbarger and her colleagues at the University of Nebraska.

A meta study, or meta analysis, allows compilation of the results of individual studies and can be applied for detecting and better understanding even minor ecological phenomena. The method is therefore used in many of the different areas of ecology.

Although the majority of studies included in the meta study by Wolfenbarger were on Bt-corn resistant to butterfly larvae, several studies on corn varieties resistant to the Western Corn Rootworm were included as well.

Many predatory and parasitic insect species important for down regulating the numbers of certain agricultural pests are among the beneficial organisms which could be potentially harmed by Bt-plants, as well as by insecticides. In the course of their lifecycle, predators such as green lacewings, Family Chrysopidae, or ladybugs, Family Coccinellidae, consume large numbers of harmful insects and mites; and parasites like the Ichneumon wasp use their ovipositor to pierce and deposit their eggs inside of various insect pests. Development of the wasp larvae then leads to the death of the host.

It is therefore in the interests of the biological control of agricultural pests, generally, that these beneficial predators and parasites not be adversely affected by efforts to protect crops from specific pest species. In this meta analysis the non-target organisms were sub-divided into several categories: predators, Ichneumon wasps, plant eaters (potential agricultural pests), species with a broad spectrum diet and species that contribute to the decomposition of plant material.

Lessons from field studies

In a first step, studies were analyzed in which neither Bt-corn nor the non-genetically engineered corn control was treated with insecticide. There were no observed differences in the numbers of non target organisms in the fields, with one exception. Ichneumon wasps that deposit their eggs exclusively in the European corn borer were significantly reduced in number. This is not unexpected. The Bt-corn is planted with the specific goal of eliminating European corn borer infestation as completely as possible.

It is unavoidable that beneficial species relying on the European corn borer as their sole nutrition source will also be adversely affected when these pest numbers are successfully reduced. This side effect, however, is not to be considered an unwanted effect of the Bt-corn because it results regardless of how the pest numbers are reduced.

The meta analysis of Wolfenbarger and her colleagues also showed that the proportion of predators to herbivores was not affected by which type of corn was being grown, Bt- or non-engineered. This is an indication that the regulatory function of the beneficial organisms is intact; very important for the control of pests unaffected by the Bt toxins.

A scientifically based, environmental impact study must consider not only the ecological consequences of the Bt-corn. It must also consider the impact on the environment of the conventional methods used to control pest numbers. Feed corn, generally, is not widely sprayed with insecticide. In the USA, only 1 out of 10 such fields were sprayed in 2005. That means that planting Bt-corn in these fields would not lead to a significant reduction in the amount of pesticide being applied.

Sweet corn, on the other hand, is given multiple chemical treatments throughout its cultivation. If one compares the prevalence of non target organisms in untreated Bt-corn fields to their prevalence in pesticide-treated, non-Bt corn fields, the results are very clear. Important beneficial species, including predatory insects, spiders and Ichneumon wasps were significantly more numerous in the Bt-corn field than in the pesticide treated fields. This encourages natural pest eradication and supports long term biological regulation of pests.

According to the author, when one looks at all available data there are no indications that Bt-corn exhibits unexpected or unwanted effects on non-target organisms in corn fields. When replacing corn being treated with pesticide, the Bt-corn even raises biodiversity significantly.

Bt-cotton and Bt-carrots

In addition to Bt-corn, the authors of this meta study also performed analyses of the available data for Bt-cotton and Bt-potatoes. While Bt-potatoes are no longer grown, Bt-cotton is being cultivated on millions of hectares in a long list of countries. Bt-potatoes were protected from the potato beetle but new pesticide developments have made them obsolete. The Bt-toxins of Bt-cotton are directed against a number of important butterfly pests.

The analyses of the data for these two crops largely corresponded to the results observed with Bt-corn. However, particularly in the cotton, the observed biodiversity increase in fields planted with the Bt-crop compared to fields treated with broad activity pesticide, was even more substantial than the increase observed with Bt-corn cultivation. This is significant because cotton is among the most highly sprayed crops in the world.

Aside from the predatory insects, there are, however, also agricultural pests that profit from the reduction in pesticide, and which then cause more damage. This point underscores how important it is to view Bt-crops as a method for controlling agricultural pests to be applied in conjunction with other methods in order that the desired effect can be long lasting.