Travelling waves of resistance in a bi-trophic pest adaptation model

We consider a reaction-diffusion system for spatial spread of pest resistance to host plant resistance genes which is based on the Lotka-Volterra predator-prey equations, with logistic growth of the resource level and a diffusion term added to account for spatial spread of the pest. The model is phenotype specific, in which a pest subpopulation's fitness comes down to a balance between its resource assimilation rate and its respiration rate. We derive an expression for the rate of spatial spread of the resistant pest types from an initial point source, and discuss its relevance for adaptive pest resistance management strategies. Using results for an analogous single-species reaction-diffusion model in heterogeneous media, we consider the likely impact of pest-susceptible plant refugia on the speed of the travelling wave of resistant pests, and simultaneously the expected trade-off, in terms of crop yield decrease, when refugia are included. We also explore the possibility that resistance breaking by the pest population is not an inevitable phenomenon, particularly when refugia of the appropriate size are used.     

A spatially explicit model simulating western corn rootworm (Coleoptera: Chrysomelidae) adaptation to insect-resistant maize

A stochastic spatially explicit computer model is described that simulates the adaptation by western corn rootworm, Diabrotica virgifera virgifera LeConte, to rootworm-resistance traits in maize. The model reflects the ecology of the rootworm in much of the corn belt of the United States. It includes functions for crop development, egg and larval mortality, adult emergence, mating, egg laying, mortality and dispersal, and alternative methods of rootworm control, to simulate the population dynamics of the rootworm. Adaptation to the resistance trait is assumed to be controlled by a monogenic diallelic locus, whereby the allele for adaptation varies from incompletely recessive to incompletely dominant, depending on the efficacy of the resistance trait. The model was used to compare the rate at which the adaptation allele spread through the population under different nonresistant maize refuge deployment scenarios, and under different levels of crop resistance. For a given refuge size, the model indicated that placing the nonresistant refuge in a block within a rootworm-resistant field would be likely to delay rootworm adaptation rather longer than planting the refuge in separate fields in varying locations. If a portion of the refuge were to be planted in the same fields or in-field blocks each year, rootworm adaptation would be delayed substantially. Rootworm adaptation rates are also predicted to be greatly affected by the level of crop resistance, because of the expectation of dependence of functional dominance on dose. If the dose of the insecticidal protein in the maize is sufficiently high to kill >90% of heterozygotes and approximately 100% of susceptible homozygotes, the trait is predicted to be much more durable than if the dose is lower. A partial sensitivity analysis showed that parameters relating to adult dispersal affected the rate of pest adaptation. Partial validation of the model was achieved by comparing output of the model with field data on population dynamics, and with field data documenting rootworm adaptation to cyclodienes and organophosphates.     

Impact of small fitness costs on pest adaptation to crop varieties with multiple toxins: a heuristic model

A deterministic two-locus model was used to examine how small fitness costs to individuals carrying resistance alleles could impact the risk of panmictic insect pest populations adapting to crop varieties that produced two distinct toxins. Parameters examined were (1) level of toxicity of each toxin, (2) initial frequencies of alleles for adaptation to the toxins, (3) percentage of population feeding on nontoxic plants, and (4) level of fitness cost associated with adaptation to each of the two toxins. Resistance to each toxin was assumed to be biochemically independent, controlled by a resistance coding allele at a single locus, and inherited as a partially recessive trait in the field. When plants are extremely toxic to the pest, effective refuge size is 10%, and there is a fitness cost to resistance alleles only when in homozygous form (5%), the pest population is never predicted to adapt to either toxin as long as the initial frequencies of the resistance alleles are below 0.05. Even if the initial frequency of the allele for adapting to one toxin is 0.95 when a two-toxin cultivar completely replaces a one-toxin cultivar, the model predicts that a low equilibrium allelic frequency will develop for both resistance alleles, as long as the frequency of the allele for adapting to the second toxin is initially 0.001 or less. If cultivars with one and two toxins are planted, the model predicts that resistance will develop. Nonrandom mating and stochastic variation within subpopulations also could lead to evolution of resistance.     

A situation in which a local nontoxic refuge promotes pest resistance to toxic crops

In order to delay the development of pest resistance to genetically engineered insecticidal crop varieties, it is current practice to grow "refugees" of non-toxic plants close to insecticidal crops. We model such a toxic/nontoxic crop complex as an open system with a small stream of toxin-susceptible immigrants. We find that, for intermediate values of the dominance of a pest gene for resistance to the toxin, the local refuge can spoil the benefit that is provided by the immigrant stream. We provide formulas for some important boundaries in parameter space.     

A spatial model of the development of pest resistance to a transgenic insecticidal crop: European corn borer on Bt maize

A mathematical model was constructed to describe the evolution of resistance to the Bacillus thuringiensis toxin (Bt) in an insect pest (European corn borer) population on a transgenic crop (Bt corn). The model comprises a set of partial differential equations of the reaction-diffusion type; local interactions of three competing pest genotypes formed by alleles of Bt resistance and susceptibility are described as in the Kostitzin model, and the spread of insects is modeled as diffusion. The model was used to evaluate the influence of pest characteristics on the efficacy of the high-dose/refuge strategy aiming to prevent or delay the spread of Bt resistance in pest populations. It was shown, by contrast, that a model based on Fisher-Haldane-Wright equations and formally incorporating a diffusion term cannot adequately describe the evolution of Bt resistance in a spatially inhomogeneous pest population. Further development of the proposed demo-genetic model is discussed.     

Role of Spatial and Temporal Refuges in the Evolution of Pest Resistance to Toxic Crops

Toxic plants have been used for years in agriculture to control major crop pests. However, the continuous exposure of targeted pests to toxins dramatically increases the rate of resistance evolution (Gassman et al. in Annu Rev Entomol 54:147–163, 2009a; Tabashnik et al. Nat Biotechnol 26:199–202, 2008). To prevent or delay resistance, non toxic host plants can be used as refuges. Our study considers spatial and temporal refuges that are respectively implemented concurrently or alternatively a toxic crop. A conceptual model based on impulsive differential equations is proposed to describe the dynamics of the susceptible and resistant pest populations over time. The mathematical study enlightens threshold values of the proportion of the spatial refuge and key parameters that should help to understand evolution of pest resistance to toxic crop.     

Myths, models and mitigation of resistance to pesticides

Resistance to pesticides in arthropod pests is a significant economic, ecological and public health problem. Although extensive research has been conducted on diverse aspects of pesticide resistance and we have learned a great deal during the past 50 years, to some degree the discussion about ''resistance management'' has been based on ''myths''. One myth involves the belief that we can manage resistance. I will maintain that we can only attempt to mitigate resistance because resistance is a natural evolutionary response to environmental stresses. As such, resistance will remain an ongoing dilemma in pest management and we can only delay the onset of resistance to pesticides. ''Resistance management'' models and tactics have been much discussed but have been tested and deployed in practical pest management programmes with only limited success. Yet the myth persists that better models will provide a ''solution'' to the problem. The reality is that success in using mitigation models is limited because these models are applied to inappropriate situations in which the critical genetic, ecological, biological or logistic assumptions cannot be met. It is difficult to predict in advance which model is appropriate to a particular situation; if the model assumptions cannot be met, applying the model sometimes can increase the rate of resistance development rather than slow it down. Are there any solutions? I believe we already have one. Unfortunately, it is not a simple or easy one to deploy. It involves employing effective agronomic practices to develop and maintain a healthy crop, monitoring pest densities, evaluating economic injury levels so that pesticides are applied only when necessary, deploying and conserving biological control agents, using host-plant resistance, cultural controls of the pest, biorational pest controls, and genetic control methods. As a part of a truly multi-tactic strategy, it is crucial to evaluate the effect of pesticides on natural enemies in order to preserve them in the cropping system. Sometimes, pesticide-resistant natural enemies are effective components of this resistance mitigation programme. Another name for this resistance mitigation model is integrated pest management (IPM). This complex model was outlined in some detail nearly 40 years ago by V. M. Stern and colleagues. To deploy the IPM resistance mitigation model, we must admit that pest management and resistance mitigation programmes are not sustainable if based on a single-tactic strategy. Delaying resistance, whether to traditional pesticides or to transgenic plants containing toxin genes from Bacillus thuringiensis, will require that we develop multi-tactic pest management programmes that incorporate all appropriate pest management approaches. Because pesticides are limited resources, and their loss can result in significant social and economic costs, they should be reserved for situations where they are truly needed--as tools to subdue an unexpected pest population outbreak. Effective multi-tactic IPM programmes delay resistance (= mitigation) because the number and rates of pesticide applications will be reduced.     

Sexual selection expedites the evolution of pesticide resistance

The evolution of insecticide resistance by crop pests and disease vectors causes serious problems for agriculture and health. Sexual selection can accelerate or hinder adaptation to abiotic challenges in a variety of ways, but the effect of sexual selection on resistance evolution is little studied. Here, we examine this question using experimental evolution in the pest insect Tribolium castaneum. The experimental removal of sexual selection slowed the evolution of resistance in populations treated with pyrethroid pesticide, and also reduced the rate at which resistance was lost from pesticide‐free populations. These results suggest that selection arising from variance in mating and fertilization success can augment natural selection on pesticide resistance, meaning that sexual selection should be considered when designing strategies to limit the evolution of pesticide resistance.     

Plant morphological traits associated with field resistance to cardamom thrips (Sciothrips cardamomi) in cardamom (Elettaria cardamomum)

Cardamom thrips (Sciothrips cardamomi) is a major insect pest of cardamom (Elettaria cardamomum) causing severe economic losses to the crop in all cardamom producing countries. The present control measures rely heavily on chemical insecticides, which in addition to increased input costs also lead to pesticide residues in the produce and environmental hazards. Although the crop is of high commercial value, limited efforts have been made to identify sources of resistance to this major pest. Exploiting sources of resistance along with integrated pest management tactics will help to reduce pesticide usage in this crop promoting sustainable production. The present study aimed at identifying sources of resistance against cardamom thrips and the associated plant morphological traits conferring resistance against the pest. Field screening of 180 cardamom germplasm accessions for 3 years at Appangala, Karnataka, India, resulted in identification of eight accessions resistant to cardamom thrips. Differences in panicle type and the nature of adherence of leaf sheath to the pseudostem explained a significant amount of the variance in resistance and therefore are likely to play a major role in conferring resistance against this pest. Multiple regression analysis of the different traits indicated that accessions with prostrate panicles having leaf sheath loosely adhered to the pseudostems were found to have significantly less thrips damage, when compared with other panicle and leaf sheath types. However, persistence of flower bract did not have a significant additive effect on imparting resistance against thrips. Metabolomic analysis of the accessions may provide further insight into the existence of supplementary biochemical mechanisms, if any, in imparting resistance. The identified traits and accessions can be exploited in future breeding programmes for developing thrips resistant cardamom varieties.     

Dry Matter Production in a Tomato Crop: Measurements and Simulation

Simulation of dry matter production by the explanatory glasshousecrop growth model SUKAM (Gijzen, 1992, Simulation Monographs),based on SUCROS87 (Spitters, Van Keulen and Van Kraalingen,1989, Simulation and systems management in crop protection),was validated for tomato. In the model, assimilation rates arecalculated separately for shaded and sunlit leaf area at differentcumulative leaf area in the canopy, taking into account thedifferent interception of direct and diffuse components of light.Daily crop gross assimilation rate (P_gd) is computed by integrationof these rates over total crop leaf area and over the day. Leafphotochemical efficiency and potential gross assimilation rateat saturating light depend on temperature and CO₂ concentrationand are approximated as being identical in the whole canopy.Crop growth results from P_gd minus maintenance respiration rate(R_m; dependent on temperature and crop dry weight), multipliedby the conversion efficiency (carbohydrates to structural drymatter; C_f). Growth experiments (periodic destructive harvest) with differentplanting dates and plant densities and two data-sets from commerciallygrown crops, were used for model validation. Hourly averagesfor global radiation outside the glasshouse, glasshouse temperatureand CO₂ concentration, together with measured leaf area index,dry matter distribution (for calculation of C_f) and organ dryweights (for calculation of R_m) were the inputs to the model. Dry matter production (both level and dynamic behaviour) wassimulated reasonably well for most experiments, but final drymatter production was under-estimated by about 27% for the commerciallygrown crops. At low irradiance and with large crop dry weight,growth rate was under-estimated, probably as a result of over-estimationof R_m. This could almost completely explain the large under-estimationfor the commercially grown crops, which had large dry weight.Final dry matter production was over-estimated by 7-11% if dailyaverages instead of hourly input of climatic data were used. It is concluded that SUKAM is a reliable model for simulatingdry matter production in a tomato crop, except for those situationswhere R_m has a large influence on crop growth rate (low irradianceand large crop dry weight). An improved estimate of R_m wouldtake into account the influence of metabolic activity. A preliminaryattempt to relate maintenance costs to relative growth rate(a measure for metabolic activity), showed promising results.Copyright1995, 1999 Academic Press Crop growth, dry matter production, glasshouse, maintenance respiration, metabolic activity, model, relative growth rate, respiration, simulation, tomato, model validation     

Evaluation of pest control efficiencies for different banker plant systems with a simple predator–prey model

The banker plant system has been introduced for the biological control of various pest species in Japanese greenhouses. With the banker plant system, non-crop plants infested with a host insect (a non-commercial crop pest) are placed in the greenhouse to provide alternative resources for the parasitoids or predators. We want to evaluate the effectiveness for controlling pests on the crop in a quantitative way by immigrating predators from the banker plant. Therefore, we developed a simple model for the interaction of the pest and predator in the crop and included the banker plant only as a source for predators. For three different pest-predator systems we parameterised the model and used these models to predict under what conditions biological control in a banker plant system is successful. We defined successful as keeping the pest below the economic injury level of the crop estimated from damage analysis. Because the crop is mostly grown during a period that lasts less than a year our analysis should not only focus on the equilibrium dynamics. In contrast, it should also focus on the transient dynamics. Our main analytical result, from the equilibrium analysis, is that for successful control the maximum lifetime consumption of immigrating predators should exceed the daily prey growth at half the value of the maximum consumption rate. For practical purpose this translates into the fact that the immigration of predators at a low initial pest density is crucial for successful control.     

Boundaries of sustainability in simple and elaborate models of agricultural pest control with a pesticide and a non-toxic refuge

In two models of pest control using a pesticidal crop along with a non-pesticidal refuge to prevent the development of resistance, we numerically compute the bifurcations that bound the region in parameter space where control is sustainable indefinitely. An exact formula for one of the bifurcation surfaces in one of the models is also found. One model is conceptual and as simple as possible. The other is realistic and very detailed. Despite the great differences in the models, we find the same distinctive bifurcation structure. We focus on the parameters that determine: (i) the restriction of pest exchange between the crop and the refuge, which we call 'screening' the refuge, and (ii) the recessiveness of the resistance trait. The screened refuge technique is seen to work in the models up to quite high values of fitness of resistant heterozygotes, that is, even when resistance is not strongly recessive.     

Modelling the interplay between pest movement and the physical design of trap crop systems

Abstract 1 The interplay between pest movement and trap crop physical design is modelled in a situation where the pest moves by a random walk with spatially variable mobility. Questions addressed are: (i) how does the proportion of trap crop area of the total field area influence the equilibrium distribution of pests among the crop and the trap crop and (ii) how do crop patch size and shape influence the speed of pest redistribution from the crop to the trap crop. 2 When pest mobility in the trap crop is clearly lower than that in the crop, the pest population in the crop decreases very sharply for small trap crop proportions. When mobility in the trap crop is slightly closer to that in the crop, the pest population in the crop decreases much more gradually with increasing trap crop proportion. Thus finding a trap crop that the pest distinctly prefers over the crop appears to be crucial for developing efficient trap crop systems. 3 The rate of decay in the pest population in the crop increases with increasing perimeter to area ratio of the crop patch. Hence, designing field layouts to increase the perimeter to area ratio of crop patches may be beneficial.     

Thermal adaptation of soil microbial respiration to elevated temperature

In the short‐term heterotrophic soil respiration is strongly and positively related to temperature. In the long‐term, its response to temperature is uncertain. One reason for this is because in field experiments increases in respiration due to warming are relatively short‐lived. The explanations proposed for this ephemeral response include depletion of fast‐cycling, soil carbon pools and thermal adaptation of microbial respiration. Using a > 15 year soil warming experiment in a mid‐latitude forest, we show that the apparent ‘acclimation’ of soil respiration at the ecosystem scale results from combined effects of reductions in soil carbon pools and microbial biomass, and thermal adaptation of microbial respiration. Mass‐specific respiration rates were lower when seasonal temperatures were higher, suggesting that rate reductions under experimental warming likely occurred through temperature‐induced changes in the microbial community. Our results imply that stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted.     

Differential Regulation of Mitochondrial Pyruvate Carrier Genes Modulates Respiratory Capacity and Stress Tolerance in Yeast

Mpc proteins are highly conserved from yeast to humans and are necessary for the uptake of pyruvate at the inner mitochondrial membrane, which is used for leucine and valine biosynthesis and as a fuel for respiration. Our analysis of the yeast MPC gene family suggests that amino acid biosynthesis, respiration rate and oxidative stress tolerance are regulated by changes in the Mpc protein composition of the mitochondria. Mpc2 and Mpc3 are highly similar but functionally different: Mpc2 is most abundant under fermentative non stress conditions and important for amino acid biosynthesis, while Mpc3 is the most abundant family member upon salt stress or when high respiration rates are required. Accordingly, expression of the MPC3 gene is highly activated upon NaCl stress or during the transition from fermentation to respiration, both types of regulation depend on the Hog1 MAP kinase. Overexpression experiments show that gain of Mpc2 function leads to a severe respiration defect and ROS accumulation, while Mpc3 stimulates respiration and enhances tolerance to oxidative stress. Our results identify the regulated mitochondrial pyruvate uptake as an important determinant of respiration rate and stress resistance.     

The value of asymmetric signal processing in klinokinesis

Klinokinesis is a behavioral mechanism in which an organism moves toward or away from a stimulus source by altering its frequency of change of direction without biasing its turns with respect to the stimulus field. Previous studies of a variety of organisms have demonstrated that rates of adaptation (or other information processing features) for increases and decreases in stimulus intensity are often very different from one another. In order to determine if such asymmetric signal processing could improve the efficiency of klinokinesis, computer modeling studies were performed. The model involved a simple generic version of klinokinesis in 2 dimensions with the rate of adaptation for increasing intensity varied independently of the rate for decreasing intensity. The effects of three types of noise that limit the performance of the model were tested-intensity noise, motor noise, and developmental noise. The results demonstrated that, with all three types of noise, the two adaptation rates had quite different effects on efficiency. The overall pattern of effects was different for each type of noise. In the cases of intensity noise and motor noise, the optimum combination of adaptation rates had a 3-to 5-fold higher rate for decreases in attractant than for increases, which is similar to what has previously been found with bacteria and nematodes.     

Respiration and Dry Matter Accumulation Around the Time of Anthesis in Field Stands of Winter Wheat (Triticum aestivum)

Rates of CO₂ efflux from the aerial parts of eight winter wheat(Triticum aestivum L. em Thell) genotypes were determined duringspike emergence, anthesis and early grain filling over two seasons.Dry weight data were also recorded at flag leaf full expansionand anthesis Respiration rates on a ground area basis did not differ (P >0–05) among the times of day when measurements were made.In contrast, highly significant day-to-day effects (P > 0–001)were evident. Daily respiratory efflux was consistently highduring spike emergence, declining to lower values before andafter anthesis in each year. Changes in daily respiration wereanalysed in terms of a two-parameter respiration model. Modelpredictions suggested that changes in chemical composition ofthe dry matter formed during ontogeny could explain the day-to-dayeffect Although no genotypic differences in respiration (ground areabasis) and crop growth rate were found, significant negativecorrelations between crop d. wt and respiration rate per unitd. wt were evident. Genotypic differences in crop d. wt alsobecame more distinct from the first harvest to the second. Theseobservations are discussed in terms of genotypic variation inmaintenance respiration Wheat, respiration, genotype, ontogeny     

The potential of crop management practices to reduce pollen beetle damage in oilseed rape

A major problem associated with agricultural intensification over recent decades has been the development of insecticide resistance in crop pest populations. This has been a particular issue for control of the pollen beetle (Brassicogethes aeneus syn. Meligethes aeneus), a major pest of oilseed rape throughout Europe. Sustained and often prophylactic use of pyrethroid insecticides has led to the development of insecticide-resistant beetle populations, and alternatively, more environmentally benign integrated pest management strategies are sought for the pest. The population dynamics of pollen beetles and their natural enemies, and the damage caused by the pest, are influenced by processes acting at multiple scales, from the regional or landscape scale down to the local field or within-field scale. In this review, we focus on the within-field scale, and how crop management factors, including tillage, crop plant density, crop nutrition and crop rotations may be optimised and incorporated into integrated pest management strategies for more sustainable and effective control of the pest.     

Genetic engineering, integrated pest management and the evolution of pests

It is well known that pest populations can evolve resistance to pesticides. The bollworm complex (Heliothis spp.), which attacks cotton plants, is a particularly good example of a highly adaptable pest. To achieve long-term success, genetic engineering projects for the production of 'insecticidal' crop plants, including cotton, need to take account of the adaptive potential of pests. Using the bollworm complex as a model, it is possible to show how an integrated approach to pest management, involving genetic engineers, evolutionary biologists and ecologists, could lead to the development of safe and effective transgenic cultivars.     

Using lower trophic level factors to predict outcomes in classical biological control of insect pests

Importation of exotic natural enemies for biological control of insect pests entails risks to the environment. Pre-release estimates of the likelihood of achieving successful control would be helpful in avoiding ineffective importations. Based on strong evidence of multi-trophic level interactions in terrestrial ecosystems, we tested whether variation in ecological and biological factors found at the plant and herbivore trophic levels (levels one and two) could be used to create a simple, empirically based formula, capable of estimating the probability of successful biological control against holometabolous insect pests. We constructed a database consisting of 828 records of biological control attempts against 91 pest insect species and used stepwise logistic regression to test whether five basic features of the ecosystem, crop, and pest (habitat type, crop use, pest order, pest feeding niche, and damage severity) were correlated with rates of successful control. Natural enemy characteristics were not included in the model. The final model included 10 significant independent variables, nine of which were two-way interactions; all five basic ecosystem features appeared in significant interactions. The model provided good estimates of historical success rates against pest species in the data set. In a further test, the model was able to correctly rank amenability to biological control for 10 pest species not included in the original data set. These results provide evidence that lower trophic level factors can be useful in the search for a predictive formula for biological control.