Agriculture problems - Cotton
Transgenic cotton: are there benefits for conservation?
One concern about the risks of transgenic cotton is the escape of transgenes through pollen dispersal (via wind, insects) from transgenic crop plants to their relatives.
Dispersal of pollen from transgenic cotton plants to surrounding non-transgenic plants has been observed, although cotton is mainly self-pollinating (Umbeck et al., 1991; Llewellyn and Fitt, 1996). Biothai for instance has reported that conventional farmers found transgenic cotton in their fields last year (Bangkok Post, 1999).
Out-cross of herbicide-tolerant genes
The escape of transgenes through pollen is regarded as a environmental concern in the case of herbicide resistance genes, resulting in "superweeds" which are resistant even to broad-spectrum herbicides. According to the list published by Keeler et al., 1996 cotton (Gossypium hirsutum) is likely to hybridise with wild congeners. However, no member of the genus Gossypium ssp. is known as a weed worldwide.
Selection of herbicide tolerant weeds
A more indirect but not less adverse effect in transgenic cotton farming is due to repeated application of herbicide on the same area contributing to naturally developing herbicide resistance.
Herbicide resistant weeds can be observed within a short period as a result of high selection pressure. For instance, overuse of the herbicide group Triazine has led to resistance in more than 55 weeds [UBA, 1999]. This phenomenon is also well known for the herbicide glyphosate (Roundup) [Anken, 1999; UBA, 1999].
Hence, it can generally be assumed that arable weeds develop a specific herbicide resistance after intensive application of one herbicide.Comments: Higher herbicide dosage will be needed to control weeds in the future, raising higher impacts on the environment by herbicides.
Side-effects by broad-spectrum herbicides
There is little knowledge about long-term use and subsequent side effects of broad-spectrum herbicides for non-target species and for biodiversity. This feature is getting more important when these herbicides are used intensively. A recent review from the German government gives strong evidence that harmful effects on arthropods can not be excluded (Pesticides Trust, 1999).
Compensation by other pests
Whilst introducing Bt-cotton to reduce the most damaging pests in cotton farming, increasing populations of other pests (i.e. tarnished plant bug (Lygus hesperus), boll weevil (Anthonomous grandis) have been reported in several areas. For instance, Bt-cotton in North Carolina sustained less damage from bollworms compared to conventional fields, while damage from other pests (e.g. stink bug) was approximately four times higher than in conventional fields [Bacheler, 1999 cited after Gianessi and Carpenter, 1999].
An other survey conducted in 1998 revealed an increasing number of insecticide treatments for pests not controlled by Bt-varieties on Bt-cotton fields than on conventional fields (Mullin et al., 1999 cited after Gianessi and Carpenter, 1999).
Resistance of target insects
The development of insecticide resistance is a naturally occurring phenomenon. For instance, in Arkansas, pyrethroid resistance by the tobacco budworm has progressed to the point of basically no control in 1998 [Williams et al., 1999 cited after Gianessi and Carpenter, 1999]. The resistance spectrum can also encompass many of the newer organo-phosphate and carbamate insecticides. Even broad-spectrum resistance to these synthetic insecticides has been reported.
The occurrence of resistance in Arkansas has led to the development of resistance management plans to reduce selecting pressure on insects for Bt-cotton. They are currently in place for the U.S., where selection pressure for resistant insecticides is particularly high due to monocultures and missing crop rotation practice: cotton growers who plant Bt-cotton varieties are required to plant a portion of non-transgenic cotton as a refuge for insects in order to avoid the development of resistance to Bt in insect populations.
According to the USDA's Cotton Research Laboratory this insect management plan does not properly fulfil its intentions at least for the Pink Cotton Bollworm (Pectinophora gossypiella) after preliminary results from laboratory work [Liu et al., 1999].Comments: If one looks at the history of pest resistance it is very likely that resistance to the Bt incorporated in cotton will occur within the next years. When conventional Bt control agents have been used intensively in cotton farming in the past insecticide resistance was observed. Thus, insecticide use will increase again and an easily biodegradable insecticide such as Bt won’t be effective anymore.
Opinion: As Bt represents a good larvicide that does not target the non damaging moth stage of Pink Cotton Bollworm (Pectinophora gossypiella), it seems to be reckless to risk insecticide resistance by large scale farming of Bt-cotton.
Side-effects on non-target species
Several laboratory studies have revealed strong evidence that the Bt-Protein Cry1Ac (also incorporated into Bt-cotton) expressed by transgenic plants may harm more non-target species than assumed before [Losey et al., 1999]. Moreover Hilbeck et al., (1998) reported in a laboratory based study that predatory larvae of green lacewings (Chrysoperla carnea) - an important beneficial insect in cotton fields - were fed on caterpillars that had in turn been feeding on maize leaves expressing a Bt-toxin.
The lacewings given the Bt fed caterpillars to eat died in higher numbers (62% mortality) than those given Bt-free caterpillars (37% mortality). Even though, the results of the cited research are based on laboratory studies and were not confirmed yet on farm they indicate some adverse effects by Bt-technology and it should be kept an eye on these findings in the future.