THE LACEWING STORY: EFFECTS OF BT
ON NON-TARGET INSECTS
1. This paper was prepared to react to a widely
published allegation that the Bt gene, which is introduced into
crops as a source of resistance to some insect pests, also kills
useful insects. The allegations stem from research done by a team
of scientists from the Swiss Federal Research Station for Agroecology
and Agriculture in Zurich, Switzerland under the leadership of
Dr Angelika Hilbeck. The subject species of the work was the lacewing
Chrysoperla carnea, a common predator insect. The results of this
work were published in the scientific journal Environmental Entomology,
volume 27, pages 480-487.
2. The work of Dr Hilbeck is not new. She started
work on lacewings and Bt in 1994, and was assisted by Novartis
(then Ciba-Geigy) with material of Bt maize. The objective of
the study was to find out what happens to a predator insect (in
this case the lacewing larva) when it feeds on an insect that
has been eating from plants containing Bt genes. The prey species
for the lacewings in the study was the European Corn Borer (ECB),
which is the target pest of Bt maize, and which is also a common
prey for lacewings in the field.
3. From 1995 on, Dr. Hilbeck started circulating
the results of her first experiments, alleging that she saw more
mortality among lacewings feeding on ECB that had been raised
on Bt maize than among lacewings feeding on ECB that had been
raised on non-Bt maize. Her conclusion was that the Bt in the
maize was affecting the lacewings. Both company scientists and
academic colleagues immediately challenged the results. It was
pointed out that ECB larvae <<raised>> on Bt maize
were dying or dead by the time the lacewings ate them. The experiment
did not take into account that a predator eating dying prey is
going to starve by lack of sufficient food, and/or become poisoned
by the internally generated toxins in the decomposing prey. The
experiment amounts to feeding a dog with rotting meat until he
4. Proper control treatments were suggested,
such as powdering healthy ECB larvae with Bt just before feeding
them to the lacewings, to distinguish between the insect food
effect and the Bt effect. Dr. Hilbeck ignored these suggestions.
She repeated the above experiment, but included a parallel trial
with another prey insect (Spodoptera littoralis, which is less
sensitive to Bt). The rationale was that in this second trial,
the S. littoralis larvae would not be dying and therefore would
constitute healthy prey.
5. The result was that again more lacewing larvae
died when fed on prey that had eaten Bt plants than on the controls.
This led to the conclusion that Bt affects a non-target insect
(lacewings), and it was published in Environmental Entomology.
6. We would like to deliver three comments on
(a) An assessment of the experimental work
done, and the conclusions reached.
(b) An assessment of the way in which it
(c) An assessment of the wider context into
which these results, and other evaluations on the effects of Bt
are to be seen.
A. THE EXPERIMENTS
It is still not understood why the
team did not do the obvious control experiment of feeding the
lacewing larvae healthy prey powdered with Bt.
In this study, it was assumed that
Spodoptera Littoralis was a good control insect, because Dr Hilbeck
state that it is insensitive to Bt. It is actually well known
that S Littoralis is sensitive to Bt albeit less than ECB. It
is quite clear why she still chose this insect as a control, instead
of using another leaf eating insect prey of the lacewing.
There have been several other similar
studies to investigate possible effects on non-target species.
Until now these have come out without showing any effect. Dr Hilbeck
mentions some of these studied in passing in the introduction
of her paper, but without mentioning which insects were tested.
This is significant, because at least two of the studies are about
lacewings (Pilcher et al.: Environmental Entomology; volume
26; pages 446-454; 1997) (Sims et al.: Southwest Entomology;
volume 20, pages 493-500; 1995), and they reach the opposite conclusion
of the Hilbeck study. Dr Hilbeck is aware of this, as she mentions
the papers in her references, but in her text, she refers to the
work of Pilcher and of Sims without mentioning that his work was
on the same lacewing. Therefore, she also does not attempt to
discuss why she reaches the opposite conclusion of Pilcher or
Sims. In scientific publishing, this is unusual. More unusual
still is the fact that the referees of the paper have apparently
not picked this up and asked her to address this question. It
is unlikely that the referees were unaware of Pilcher's work,
since it had been published in the same journal as Hilbeck's work,
less than one year earlier.
Dr Hilbeck claims that her work is
different from previously published studies in that it tries to
study effects of long term feeding. This is correct, but then
it obviously becomes crucially important to ensure that the target
insects do not die or suffer from the poor health of their prey.
Dr Hilbeck concludes that the lacewings
suffer from a Bt related effect. In a way this is correct. If
lacewings are fed during their whole development on prey that
is dying or diseased, then they suffer as well. In nature, lacewings
are non-specialised predators, eating a wide range of insects.
They always have healthy prey to eat, and would likely be totally
unaffected by the sick ECB larvae. This is actually the conclusion
of field observations of Pilcher et al.
B. THE PUBLICATION
This study has an unusual publication history.
Although the experiments were done from 1994 to 1996, it was sent
to Environmental Entomology on 28 July 1997, and accepted on 25
November 1997. It was published in April 1998.
The results of the paper were widely circulated
unofficially after the paper was accepted in November 1997. It
was cited and commented upon in New Scientist and many other general
public media, probably with limited access to the actual paper.
From then on, the story of the lacewings started leading its own
life, with everyone using it without anyone seeing the actual
study. Since the paper itself has only been officially published
in April, it is only now that the professional community of entomologists
can comment on it.
In the meantime, the "lacewing problem"
has become part of the common wisdom on insect resistant crops,
and it is highly unlikely that later criticisms on the quality
of the study will reach the same audience as the original study
C. WIDER CONTEXT
Bt has been fed to a very wide range of insects,
both as a spray and through genetically modified plants. The very
fact that the only effect ever noticed is in the above study,
which can be heavily criticised for its methodology, is an indication
that major effects are highly unlikely. It is impossible to prove
this, as it is impossible to prove a negative. What is possible
though is to compare Bt plants with other practices to control
insect pests, and to do a comparative analysis of damage to non-target
insects. This has never been done.
Companies develop insect resistance through
genetic engineering as an alternative to the use of chemical insecticides.
In the furore about the lacewing data, it has been largely overlooked
that in the standard treatment of ECB today, most or all lacewings
in the field are killed by the treatment. The use of Bt genes
is a massive improvement over these older technologies in terms
of specificity. Until now nothing indicates that lacewings or
any other predator insects suffer from the use of Bt maize. But
even if there were to be found a small effect, it is already clear
that this effect would always be much less than anything we are
doing today for control of insect pests.