Tritrophic interactions among Bt maize, an insect pest and entomopathogens: effects on development and survival of western corn rootworm

Agricultural systems often provide a model for testing ecological hypotheses, while ecological theory can enable more effective pest management. One of the best examples of this is the interaction between host-plant resistance and natural enemies. With the advent of crops that are genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt), a new form of host-plant resistance has been introduced to agroecosystems. How Bt crops interact with natural enemies, especially insect pathogens in below-ground systems, is not well understood, but provides a unique opportunity to study below-ground tritrophic interactions. In this study, we used two species of entomopathogenic fungi and three species of entomopathogenic nematodes to determine how this community of soil-borne natural enemies might interact with Bt maize (event 59122, expressing the insecticidal protein Cry34/35Ab1) to affect survival and development of western corn rootworm (Diabrotica virgifera virgifera), which is an obligate root feeder and a serious pest of maize. We ran two experiments, one in a greenhouse and one in a growth chamber. Both experiments consisted of a fully crossed design with two maize treatments (Bt maize and non-Bt maize) and two entomopathogen treatments (present or absent). The community of entomopathogens significantly increased mortality of western corn rootworm, and Bt maize increased larval developmental time and mortality. Entomopathogens and Bt maize acted in an independent and additive manner, with both factors increasing the mortality of western corn rootworm. Results from this study suggest that entomopathogens may complement host-plant resistance from Bt crops

Adaptation by Western Corn Rootworm (Coleoptera: Chrysomelidae) to Bt Maize: Inheritance, Fitness Costs, and Feeding Preference

We examined inheritance of resistance, feeding behavior, and fitness costs for a laboratory-selected strain of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), with resistance to maize (Zea maize L.) producing the Bacillus thuringiensisBerliner (Bt) toxin Cry3Bb1. The resistant strain developed faster and had increased survival on Bt maize relative to a susceptible strain. Results from reciprocal crosses of the resistant and susceptible strains indicated that inheritance of resistance was nonrecessive. No fitness costs were associated with resistance alleles in the presence of two entomopathogenic nematode species, Steinernema carpocapsae Weiser and Heterorhabditis bacteriophora Poinar. Larval feeding studies indicated that the susceptible and resistant strains did not differ in preference for Bt and non-Bt root tissue in choice assays

Field-Evolved Resistance to Bt Maize by Western Corn Rootworm

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests. However, the evolution of resistance could cut short these benefits. A primary pest targeted by Bt maize in the United States is the western corn rootwormDiabrotica virgifera virgifera (Coleoptera: Chrysomelidae). We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury. In all cases, fields experiencing severe rootworm feeding contained Cry3Bb1 maize. Interviews with farmers indicated that Cry3Bb1 maize had been grown in those fields for at least three consecutive years. There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins. This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary

Entomopathogenic fungi in cornfields and their potential to manage larval western corn rootworm Diabrotica virgifera virgifera

Entomopathogenic ascomycete fungi are ubiquitous in soil and on phylloplanes, and are important natural enemies of many soil-borne arthropods including larval western corn rootworm, Diabrotica virgifera virgifera, which is a major pest of corn. We measured the prevalence of Beauveria bassiana andMetarhizium anisopliae sensu lato in ten cornfields in Iowa, USA by baiting with larval insects. B. bassianaand M. anisopliae s.l. were present in 60% ± 6.3% and 55% ± 6.4% of soil samples, respectively. Subsequent laboratory bioassays found that some M. anisopliae s.l. strains collected from cornfields killed a greater proportion of D.v. virgifera larvae than a standard commercial strain

Effect of Bt Maize and Soil Insecticides on Yield, Injury, and Rootworm Survival: Implications for Resistance Management

A 2-yr field experiment was conducted to determine the effects on Diabrotica spp. (Coleoptera: Chrysomelidae) of an insecticidal seed treatment (Poncho 1250, (AI) /clothianidin) and a granular insecticide (Aztec 2.1G, (AI)/tebupirimphos and cyfluthrin) alone and in combination with maize producing the insectidical toxin Cry3Bb1 derived from the bacterium Bacillus thuringiensis (Bt). Yields for Bt maize plots were significantly greater than for non-Bt maize; however, insecticides did not significantly affect yield. Insecticides significantly decreased root injury in non-Bt maize plots, but there were no significant differences in root injury between Bt maize with or without either insecticide. Maize producing the Bt toxin Cry3Bb1 and the soil-applied insecticide Aztec significantly decreased survival of western corn rootworm (Diabrotica virgifera virgifera LeConte), while only Bt maize significantly decreased survival of the northern corn rootworm (Diabrotica barberi Smith & Lawrence). For both species, Bt maize and each of the insecticides delayed emergence. In the absence of density-dependent mortality, Bt maize imposed 71 and 80% reduction in survival on the western corn rootworm and the northern corn rootworm, respectively. The data from this study do not support combining insecticide with Bt maize because the addition of insecticide did not increase yield or reduce root injury for Bt maize, and the level of rootworm mortality achieved with conventional insecticide was likely too low to delay the evolution of Bt resistance. In addition, delays in emergence from Bt maize combined with insecticides could promote assortative mating among Bt-selected individuals, which may hasten resistance evolution

Interactions Among Bt Maize, Entomopathogens, and Rootworm Species (Coleoptera: Chrysomelidae) in the Field: Effects on Survival, Yield, and Root Injury

A 2 yr field study was conducted to determine how a blend of entomopathogens interacted with Bt maize to affect mortality of Diabrotica spp. (Coleoptera: Chrysomelidae), root injury to maize (Zea maize L.) and yield. The blend of entomopathogens included two entomopathogenic nematodes, Steinernema carpocapsae Weiser and Heterorhabditis bacteriophora Poinar, and one entomopathogenic fungus, Metarhizium brunneum(Metschnikoff) Sorokin. Bt maize (event DAS59122–7, which produces Bt toxin Cry34/35Ab1) decreased root injury and survival of western corn rootworm (Diabrotica virgifera virgifera LeConte) and northern corn rootworm (Diabrotica barberi Smith & Lawrence) but did not affect yield. During year 1 of the study, when rootworm abundance was high, entomopathogens in combination with Bt maize led to a significant reduction in root injury. In year 2 of the study, when rootworm abundance was lower, entomopathogens significantly decreased injury to non-Bt maize roots, but had no effect on Bt maize roots. Yield was significantly increased by the addition of entomopathogens to the soil. Entomopathogens did not decrease survival of corn rootworm species. The results suggest that soil-borne entomopathogens can complement Bt maize by protecting roots from feeding injury from corn rootworm when pest abundance is high, and can decrease root injury to non-Bt maize when rootworm abundance is low. In addition, this study also showed that the addition of entomopathogens to soil contributed to an overall increase in yield

Applying an Integrated Refuge to Manage Western Corn Rootworm (Coleoptera: Chrysomelidae): Effects on Survival, Fitness, and Selection Pressure

The refuge strategy can delay resistance of insect pests to transgenic maize producing toxins from Bacillus thuringiensis (Bt). This is important for the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), because of its history of adaptation to several management practices. A 2-yr study across four locations was conducted to measure the effects of integrated refuge (i.e., blended refuge) on western corn rootworm survival to adulthood, fitness characteristics, and susceptibility to Bt maize in the subsequent generation. The treatments tested in this study were as follows: a pure stand of Bt maize (event DAS-59122-7, which produces Bt toxins Cry34Ab1/Cry35Ab1), a pure stand of refuge (non-Bt maize), and two variations on an integrated refuge consisting of 94.4% Bt maize and 5.6% non-Bt maize. Within the two integrated refuge treatments, refuge seeds received a neonicotinoid insecticidal seed treatment of either 1.25 mg clothianidin per kernel or 0.25 mg thiamethoxam per kernel. Insects in the pure stand refuge treatment had greater survival to adulthood and earlier emergence than in all other treatments. Although fecundity, longevity, and head capsule width were reduced in treatments containing Bt maize for some site by year combinations, Bt maize did not have a significant effect on these factors when testing data across all sites and years. We found no differences in susceptibility of larval progeny to Bt maize in bioassays using progeny of adults collected from the four treatments

The Impacts of Migration on Multidimensional Child Well-Being: Comparative Analysis Between Moldova and Georgia

Using novel household survey data collected between September 2011 and December 2012 on migrant- and non-migrant households in Moldova and Georgia, this paper proposes a method for measuring and comparing multidimensional child well-being in a migration context. While a growing body of literature addresses the effects of migration for children “left behind”, relatively few studies have empirically analysed if and to what extent migration implies different well-being outcomes for children. To compare the outcomes of children in current- and non-migrant households, the present paper defines a multidimensional well-being index comprised of six dimensions of wellness: education, material living standards, protection, physical health, emotional health, and communication access. The results of both bivariate and multivariate analysis suggest that migration bears limited consequences for different domains of well-being. In both Moldova and Georgia children in migrant households were found to have a slightly lower probability of attaining material well-being, but in Georgia migration was linked to higher probabilities of children attaining wellbeing in physical health, communication access, and on total index level. The results suggest that when migration has any statistically significant effect on child well-being, it is generally positive and relatively low in magnitude. The impacts of migration appear to differ widely between Moldova and Georgia, however. While migration was seen to have limited effect on the well-being of children in Moldova, it seemed to bear more consequences for children in Georgia, which likely reflects different migration trajectories, mobility patterns, and levels of maturity of each migration stream

“Children with an Absent Parent: Are They Worse-off?”

While it is taken as a given that the absence of a parent, main caregiver, or other family member can significantly affect many aspects of child well-being, the precise relationship between parental absenteeism through migration and child well-being outcomes is still uncertain. Within the field of migration studies increasing attention has been paid to the “left behind”, individuals who remain in the country of origin following the emigration of a household member. Available evidence on the lives of children left behind is scattered, however, due to the limited scope and depth of previous studies, many of which address the phenomenon through small-scale, qualitative studies. Migration of a household member could have both positive and negative effects on the well-being of children who remain in the country of origin: the transfer of remittances and availability of additional resources could enable the household to make increased investments in the education and health of children while enabling them to meet daily consumption needs without problem. At the same time the absence of a care giver could imply less supervision and greater emotional challenges for children. There are generally no universally positive or negative impacts of migration on well-being outcomes. Kandel and Kao (2001) note that there is a tendency to over-simplify potential positive benefits of migration, and nuance is often lost by failing to balance greater material resources against losses of less-easily measured impacts (such as parental supervision)