Planting patterns of in-field refuges observed for Bt maize in Minnesota

Ecological Modelling of Insect Movement in Cropping Systems

The spatio-temporal dynamics of insect pests in agricultural landscapes involves the potential of species to move, invade, colonise, and establish in different areas. This study revised the dispersal of the important crop pests Diabrotica speciosa Germar and Spodoptera frugiperda (J.E. Smith) by using computational modelling to represent the movement of these polyphagous pests in agricultural mosaics. The findings raise significant questions regarding the dispersal of pests through crops and refuge areas, indicating that understanding pest movement is essential for developing strategies to predict critical infestation levels to assist in pest-management decisions. In addition, our modelling approach can be adapted for other insect species and other cropping systems despite discussing two specific species in the current manuscript. We present an overview of studies, combining experimentation and ecological modelling, discussing the methods used and the importance of studying insect movement as well as the implications for agricultural landscapes in Brazil.

Effects of seed mixture sowing with transgenic Bt rice and its parental line on the population dynamics of target stemborers and leafrollers, and non-target planthoppers

The widespread planting of insect-resistant crops has caused a dramatic shift in agricultural landscapes, thus raising concerns about the potential impacts on both target and non-target pests. In this study, we examined the potential effects of intra-specific seed mixture sowing with transgenic Bt rice (Bt) and its parental non-transgenic line (Nt) (100% Bt rice [Bt₁₀₀ ], 5% Nt+95% Bt [Nt₀₅ Bt₉₅ ], 10% Nt+90% Bt [Nt₁₀ Bt₉₀ ], 20% Nt+80% Bt [Nt₂₀ Bt₈₀ ], 40% Nt+60% Bt [Nt₄₀ Bt₆₀ ] and 100% Nt rice [Nt₁₀₀ ]) on target and non-target pests in a 2-year field trial in southern China. The occurrence of target pests, Sesamia inferens, Chilo suppressalis and Cnaphalocrocis medinalis, decreased with the increased ratio of Bt rice, and the mixture ratios with more than 90% Bt rice (Bt₁₀₀ and Nt₀₅ Bt₉₅ ) significantly increased the pest suppression efficiency, with the lowest occurrences of non-target planthoppers, Nilaparvata lugens and Sogatella furcifera in Nt₁₀₀ and Nt₀₅ Bt₉₅ . Furthermore, there were no significant differences in 1000-grain dry weight and grain dry weight per 100 plants between Bt₁₀₀ and Nt₀₅ Bt₉₅ . Seed mixture sowing of Bt rice with ≤10% (especially 5%) of its parent line was sufficient to overcome potential compliance issues that exist with the use of block or structured refuge to provide most effective control of both target and non-target pests without compromising the grain yield. It is also expected that the strategy of seed mixture sowing with transgenic Bt rice and the non-transgenic parental line would provide rice yield stability while decreasing the insecticide use frequency in rice production.

Bt Maize Seed Mixtures for Helicoverpa zea (Lepidoptera: Noctuidae): Larval Movement, Development, and Survival on Non-transgenic Maize

In 2012 and 2013, field trials were conducted near Rosemount, MN, to assess the movement and development of Helicoverpa zea (Boddie) larvae on non-Bt refuge corn plants within a seed mixture of non-Bt and Bt corn. The Bt corn hybrid expressed three Bt toxins-Cry1Ab, Cry1F, and Vip3A. As the use of seed mixtures for insect resistance management (IRM) continues to be implemented, it is necessary to further characterize how this IRM approach impacts resistance development in ear-feeding Lepidopteran pests. The potential for Bt pollen movement and cross pollination of the non-Bt ears in a seed mixture may lead to Bt toxin exposure to larvae developing on those refuge ears. Larval movement and development by H. zea, feeding on non-Bt refuge plants adjacent to either transgenic Bt or non-Bt plants, were measured to investigate the potential for unintended Bt exposure. Non-Bt plants were infested with H. zea eggs and subplots were destructively sampled twice per week within each treatment to assess larval development, location, and kernel injury. Results indicate that H. zea larval movement between plants is relatively low, ranging from 2-16% of larvae, and occurs mainly after reaching the second instar. Refuge plants in seed mixtures did not produce equivalent numbers of H. zea larvae, kernel injury, and larval development differed as compared with a pure stand of non-Bt plants. This suggests that there may be costs to larvae developing on refuge plants within seed mixtures and additional studies are warranted to define potential impacts.

Strong oviposition preference for Bt over non-Bt maize in Spodoptera frugiperda and its implications for the evolution of resistance

BACKGROUND

Transgenic crops expressing Bt toxins have substantial benefits for growers in terms of reduced synthetic insecticide inputs, area-wide pest management and yield. This valuable technology depends upon delaying the evolution of resistance. The 'high dose/refuge strategy', in which a refuge of non-Bt plants is planted in close proximity to the Bt crop, is the foundation of most existing resistance management. Most theoretical analyses of the high dose/refuge strategy assume random oviposition across refugia and Bt crops.

RESULTS

In this study we examined oviposition and survival of Spodoptera frugiperda across conventional and Bt maize and explored the impact of oviposition behavior on the evolution of resistance in simulation models. Over six growing seasons oviposition rates per plant were higher in Bt crops than in refugia. The Cry1F Bt maize variety retained largely undamaged leaves, and oviposition preference was correlated with the level of feeding damage in the refuge. In simulation models, damage-avoiding oviposition accelerated the evolution of resistance and either led to requirements for larger refugia or undermined resistance management altogether. Since larval densities affected oviposition preferences, pest population dynamics affected resistance evolution: larger refugia were weakly beneficial for resistance management if they increased pest population sizes and the concomitant degree of leaf damage.

CONCLUSIONS

Damaged host plants have reduced attractiveness to many insect pests, and crops expressing Bt toxins are generally less damaged than conventional counterparts. Resistance management strategies should take account of this behavior, as it has the potential to undermine the effectiveness of existing practice, especially in the tropics where many pests are polyvoltinous. Efforts to bring down total pest population sizes and/or increase the attractiveness of damaged conventional plants will have substantial benefits for slowing the evolution of resistance.

Bt pollen dispersal and Bt kernel mosaics: integrity of non-Bt refugia for lepidopteran resistance management in maize

Field trials were conducted at Rosemount, MN in 2009 and 2010, to measure pollen movement from Bt corn to adjacent blocks of non-Bt refuge corn. As the use of Bt corn hybrids continues to increase in the United States, and new insect resistance management (IRM) plans are implemented, it is necessary to measure the efficacy of these IRM plans. In Minnesota, the primary lepidopteran pests of corn include the European corn borer, Ostrinia nubilalis (Hübner) and corn earworm, Helicoverpa zea (Boddie). The primary IRM plan in transgenic corn is the use of hybrids expressing a high dose of insecticidal proteins and an insect refuge containing hybrids not expressing insecticidal proteins that produce susceptible insects. Wind-assisted pollen movement in corn occurs readily, and is the primary method of pollination for corn. The combination of pollen movement and viability determines the potential for cross pollination of refuge corn. In 2009 and 2010, cross pollination occurred with the highest frequency on the north and east sides of Bt corn fields, but was found at some level in all directions. Highest levels of cross pollination (75%) were found within the first four rows (3 m) of non-Bt corn adjacent to Bt corn, and in general decreasing levels of cross pollination were found the further the non-Bt corn was planted from the Bt corn. A mosaic of Bt cross-pollinated kernels was found throughout the ear, but in both years the ear tip had the highest percentage of cross-pollinated kernels; this pattern may be linked to the synchrony of pollen shed and silking between Bt and non-Bt corn hybrids. The dominant wind direction in both years was from WNW. However, in both years, there were also prevailing winds from SSW and WSW. Further studies are needed to quantify Bt levels in cross-pollinated kernels, measure the Bt dose of such kernels and associated lepidopteran pest survival, and measure the impact of Bt pollen on lepidopteran pests, particularly when considering the seed mixture refuge configuration.