Refuge Crops Research Articles

The natural refuge policy for Bt cotton (Gossypium L.) in Pakistan – a situation analysis

Bt cotton (event Cry1Ac) was formally commercialized in Pakistan in 2010. However, there has been an increasing trend of planting unauthorized Bt cotton germplasm in farmers' fields since 2003 with a high rate of adoption in the core cotton areas especially in the province Punjab. The transgenic cotton technology has provided the growers with substantial economic benefits and has reduced their dependence on pesticides for pest control, especially against <em>Helicoverpa armigera</em> (Hubner). However, keeping in view the capacity of this insect to develop resistance against novel chemical formulations, it is easily speculated that Bt toxin, too, is no exception. Refuge crop policy for mono transgenic crop events has helped in delaying the rate of resistance evolution in the target pests. Thus, in Pakistan, where planting of structured refuge crops along Bt cotton fields is not mandatory, the effectiveness and durability of Bt cotton technology may decrease due to a number of factors which are discussed in this review.

Mating of Helicoverpa armigera (Lepidoptera: Noctuidae) moths and their host plant origins as larvae within Australian cotton farming systems

Transgenic (Bt) cotton dominates Australian cotton production systems. It is grown to control feeding damage by lepidopteran pests such as Helicoverpa armigera. The possibility that these moths might become resistant to Bt remains a threat. Consequently, refuge crops (with no Bt) must be grown with Bt cotton to produce large numbers of Bt-susceptible moths to reduce the risk of resistance developing. A key assumption of the refuge strategy, that moths from different host plant origins mate at random, remains untested. During the period of the study reported here, refuge crops included pigeon pea, conventional cotton (C3 plants), sorghum or maize (C4 plants). To identify the relative contributions made by these (and perhaps other) C3 and C4 plants to populations of H. armigera in cotton landscapes, we measured stable carbon isotopes (δ(13)C) within individual moths captured in the field. Overall, 53% of the moths were of C4 origin. In addition, we demonstrated, by comparing the stable isotope signatures of mating pairs of moths, that mating is indeed random amongst moths of different plant origins (i.e. C3 and C4). Stable nitrogen isotope signatures (δ(15)N) were recorded to further discriminate amongst host plant origins (e.g. legumes from non-legumes), but such measurements proved generally unsuitable. Since 2010, maize and sorghum are no longer used as dedicated refuges in Australia. However, these plants remain very common crops in cotton production regions, so their roles as 'unstructured' refuges seem likely to be significant.

Contamination and management of resistance evolution to high-dose transgenic insecticidal crops

For maize and cotton, transgenic varieties that express toxins derived from Bacillus thuriengensis (Bt) are now planted in several countries. To slow resistance evolution, the “high-dose/refuge” strategy is broadly implemented in which resistance is recessive and some fields (or areas within fields) are planted exclusively with Bt crops and other fields planted exclusively with non-transgenic refuge crops for susceptible insects. This strategy, however, could potentially be undermined by contamination. Here, we investigate general models of resistance evolution for high-dose events in which fields are contaminated due to the inadvertent mixing of seeds, volunteer plants, or pollen flow between Bt and non-Bt varieties coupled with seed-saving by farmers. Contamination of the refuge by Bt plants increases selection for resistance, thereby speeding resistance evolution. Nonetheless, in most situations this effect is small. Contamination of Bt fields by non-transgenic plants might be expected to have the opposite effect and always reduce the rate of resistance evolution. While this is often the case, it is not always so. If larvae move among plants within a field, then high movement rates may reverse the effect of contamination of Bt fields to slow resistance evolution. Furthermore, if the dispersal rates of adult females between Bt and refuge fields are low, then contamination of Bt fields may speed resistance. These results suggest that contamination has the potential to undermine the efficacy of the high-dose/refuge strategy, yet depending upon the particular pest and situation, contamination may not be a concern.

Natural refuge crops, buildup of resistance, and zero-refuge strategy for Bt cotton in China

In the context of genetically modified crops expressing the Bacillus thuringiensis (Bt) toxin, a 'refuge' refers to a crop of the same or a related species that is planted nearby to enable growth and reproduction of the target pest without the selection pressure imposed by the Bt toxin. The goal of this study is to discuss the role of natural refuge crops in slowing down the buildup of resistance of cotton bollworm (CBW), and to evaluate China's no-refuge policy for Bt cotton. We describe in detail the different factors that China should consider in relation to the refuge policy. Drawing on a review of scientific data, economic analyses of other cases, and a simulation exercise using a bio-economic model, we show that in the case of Bt cotton in China, the no-refuge policy is defensible.

Enhancement of refuges for Helicoverpa armigera (Lepidoptera: Noctuidae) used in the resistance management plan for cotton (Gossypium hirsutum L.) containing Bollgard II® traits

Helicoverpa armigera (Hubner) egg densities were sampled across interfaces between cotton containing Bollgard II® traits and pigeonpea or unsprayed non-transgenic cotton refuges to determine the effectiveness of the refuge crops for Bollgard II® resistance management plans (RMPs) against H. armigera. Additionally, a commercially available moth-attraction technology (Magnet®) was included at interfaces with both refuge crops to determine whether the use of such an attractant technology enhances the effectiveness of the refuges. Where the refuge crop was unsprayed non-transgenic cotton, there was no consistent change in the egg-density profile across the interface, suggesting that the Helicoverpa moths were not detecting and responding to the transition from Bollgard II® to non-transgenic cotton. However wherever Magnet was applied, there was a consistent increase in egg density on or near the Magnet-treated cotton rows. This suggests that moth-attraction technology has potential to increase the efficiency of non-transgenic cotton refuges by increasing oviposition there so that the refuge produces more individuals unselected for Bt trait resistance. However, it is not yet known how best to deploy insect-attraction technology in non-transgenic cotton refuges. Pigeonpea was a very attractive refuge crop, with Helicoverpa egg densities an order of a magnitude higher than the adjacent Bollgard II® crop and with no spillover of this increased oviposition into cotton. Application of moth attractant to the edge of the pigeonpea refuge did not alter the egg-density profile across the crop interface, suggesting that the combination of moth-attraction technology with pigeonpea refuges would not increase the effectiveness of the refuge. These studies identify avenues for improving the effectiveness of refuge crops for the management of Bt resistance in H. armigera.

Dynamically optimal strategies for managing the joint resistance of pests to Bt toxin and conventional pesticides in a developing country

In this study we discuss why planting non-Bacillus thuringiensis (non-Bt) cotton as a refuge crop in China (and other developing countries) may not be economically optimal. To show this, we develop a bioeconomic model to run simulations that will help find the optimal strategies for managing the joint resistance of pests to the Bt toxin and conventional pesticides. We show that the approach of not requiring non-Bt cotton as a refuge is defensible given initial conditions and parameters calibrated to China's cotton production environment. Of special importance is the existence of natural refuge crops. The nature of transaction costs associated with implementing a refuge policy is also considered.

Production of Helicoverpa spp. (Lepidoptera, Noctuidae) from different refuge crops to accompany transgenic cotton plantings in eastern Australia

The adoption of transgenic (Bt) cotton varieties has markedly reduced feeding damage by noctuid pests, Helicoverpa armigera and H. punctigera, in Australian cotton crops. However, the potential for Bt resistance to evolve within these insect pests is a major concern. To reduce the likelihood of Bt resistance occurring, it is mandatory for growers of Bt cotton to also grow refuge crops which produce large numbers of Bt-susceptible moths. We present here findings from a series of field studies which sought to identify the relative Helicoverpa productivity of different refuge crop options. The abundance of Helicoverpa pupae (during the cotton season) was compared under crops of Ingard® (transgenic, single Bt gene) cotton, sprayed conventional cotton, and various unsprayed refuge crops (cotton, sorghum, maize, pigeon pea) from 1996 to 2003 in the major cotton-producing regions of northern New South Wales, plus St George and Dirranbandi in southern Queensland. Unsprayed, conventional cotton was used as the ‘control’ refuge in these studies. Productivity of adult Helicoverpa was estimated by surveying for pupae and discounting those that were parasitised and dead. Some supplementary experiments were also conducted to evaluate specific issues, in particular the value of additional refuge crops sown late in 2000–01 (due to accidental over-planting of Ingard® cotton that year) and the potential productivity of mixed plantings of various refuge crops which individually flowered at different times throughout the cotton season. Unsprayed refuges generally produced many more pupae than sprayed, conventional cotton and Ingard® cotton. Overall, pigeon pea, which has the advantage of prolonged flowering and thus on-going attractiveness to Helicoverpa, was the most reliable and productive refuge option, producing about twice as many pupae as unsprayed cotton. The seed mix refuge is perhaps a viable alternative option, although logistically more difficult for growers to adopt. Laboratory rearing of live pupae until moth emergence indicated that parasitism can be a substantial source of mortality in some refuge crops, especially sorghum. The most common parasitoid of Helicoverpa pupae was the ichneumonid wasp, Heteropelma scaposum. Such parasitism can greatly reduce productivity of adult Helicoverpa from refuges and needs to be taken into account when assessing relative refuge ‘value’, although such refuges will obviously contribute to the abundance of these beneficial species across the landscape.

Modeling the development of resistance by stalk-boring lepidopteran insects (Crambidae) in areas with transgenic corn and frequent insecticide use.

We simulated the population dynamics and population genetics of two bivoltine species of corn borers, the European corn borer, Ostrinia nubilalis (Hübner), and the southwestern corn borer, Diatraea grandiosella Dyar, in a hypothetical region of irrigated transgenic and nontransgenic corn where insecticide was applied only to the nontransgenic refuge crop. Over the 100-yr time horizon, resistance developed quickly in both species and to both transgenic corn and the insecticide when the allele for resistance to the respective toxin was dominant. When the allele for transgenic resistance was not dominant and the refuge location was constant over the time horizon, spraying the refuge to control southwestern corn borer had no effect on how quickly resistance to the transgenic corn developed. In contrast, the European corn borer developed resistance to transgenic corn much sooner when the refuge was sprayed once per year, and the time to 3% resistance allele frequency decreased as efficacy of the insecticide increased. Only when the refuge was treated less than once every 5 yr (10 generations) did the frequency of application decline enough to permit resistance management for the European corn borer to approximate the effectiveness of an unsprayed refuge. A consistently sprayed refuge <40% of the corn acreage was an inadequate resistance management strategy for the European corn borer even when a low efficacy insecticide (70% mortality) was used. When assumptions about European corn borer adult behavior were changed and the adults behaved similarly to adult southwestern corn borer, the development of resistance to the transgenic crop was slowed significantly.

Abundance of Helicoverpa (Lepidoptera: Noctuidae) pupae under cotton and other crops in central Queensland: Implications for resistance management

AbstractPlanting of refuge crops and post‐harvest cultivation of soil in winter are key elements of resistance management strategies (RMS) for Bt‐transgenic (Ingard®) and conventional cottons in southern Queensland and New South Wales. As part of a larger project to examine the feasibility of growing Ingard® in central Queensland (CQ), field assessments were conducted during the 1996–97 growing season to examine the adequacy of the southern Bt‐cotton RMS under local environmental conditions. The suitability of cotton and a number of other field crops as refuges for Bt‐cotton was assessed in terms or their relative Helicoverpa pupal productivity. The practicality and potential effectiveness of post‐harvest cultivation under CQ conditions were also assessed. Field assessments show that pigeon pea has the greatest potential as a refuge for Bt‐cotton. Unsprayed cotton, sorghum and maize also produced substantially high pupal densities and hence are suitable refuge options, but they will require larger areas to be planted relative to pigeon pea. Post‐harvest cultivation in cotton fields is largely ineffective for resistance management under CQ conditions. A Bt‐cotton RMS for CQ is proposed. The CQ strategy includes refuge crop options contained in the southern strategy and the use of late season trap‐crops of pigeon pea as an alternative to post‐harvest cultivation.

Invasions on Large and Small Scales: Management of a Well-established Crop Pest, the Colorado Potato Beetle

Understanding the movement of invading organisms is critical to predicting invasion dynamics. The Colorado potato beetle, Leptinotarsa decemlineata (Say), is an invasive species on multiple spatial and temporal scales, and can serve as a model for studies of invasion dynamics. It is the major insect defoliator of potato in North America, and successful management requires an understanding of CPB invasions of individual fields. Its origin, spread, and biology, especially the cycle of annual invasions of agricultural potato fields, are described. Approaches to reducing the size of colonizing populations include rotation, delay of planting, and treatments of field margins. Rotation and sub-lethal insecticide treatments can slow the establishment of invasions within fields. These approaches interact with a late season diapause switch away from reproduction to reduce the impact of CPB. The refuge approach to delaying the fixation of resistance alleles is designed to encourage alleles for susceptibility to invade treated areas. We present data from an experimental refuge crop planted adjacent to a field treated with imidacloprid, an insecticide for which there is high variation in resistance. The treated field was four times as resistant as the untreated side, and a cline in resistance was formed from the untreated to the treated portion of the field. The cline width of about 100 m provides an empirical basis for designing refuges to enhance the spread of alleles for susceptibility into treated areas and prevent fixation of resistance in the summer generation.