The effect of mating system on growth of Arabidopsis lyrata in response to inoculation with the biotrophic parasite Albugo candida

The effects of variation in host reproductive systems on response to pathogens are not well understood. We inoculated individuals from outcrossing and inbreeding populations of North American Arabidopsis lyrata with Albugo candida (white blister rust) to test the effect of mating system and heterozygosity on disease response. We observed three host infection phenotypes, classified as fully resistant, partially resistant and fully susceptible. Overall, inbreeding populations had more susceptible and fewer partially resistant individuals than outcrossing populations, but the highest proportion of resistant individuals was found in two of the inbreeding populations. Mating system did not affect relative growth rate of inoculated plants, but there were strong effects of population and infection phenotype. We conclude that mating system per se does not determine the resistance of natural A. lyrata populations to infection by Albugo, but that the increased variability in responses among inbreeding populations may be due to reduced effective population size.     

Variation in performance of western flower thrips populations on susceptible and partially resistant cucumber

Biotypic variation is of major concern in breeding for host plant resistance to insects. The existence or development of aggressive biotypes can lead to a rapid break-down of host plant resistance. Therefore the study of biotypic variation should be included in breeding programs for resistance to insects. In the present study we measured the reproduction of randomly collected females of ten different populations of the insect herbivore Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) on one susceptible and two resistant cucumber (Cucumis sativus L.) accessions. Significant differences between thrips populations were observed on all three cucumber accessions. None of the populations had a significantly higher reproduction than the Dutch reference population NL1. For three populations, the Dutch population NL1, a population from New Zealand (NZ), and an Italian population (IT), partial life history parameters, such as reproduction rate, developmental time and survival were determined and the relative rate of increase r _r was calculated. On all three cucumber accessions the r _r-value of population NZ was lower than of populations NL1 and IT. It is concluded that there is biotypic variation in F. occidentalis with regard to performance on cucumber plants with different levels of resistance. Reproduction is a good criterion for differentiating biotypes of F. occidentalis on cucumber.     

Phenotypic- and Genotypic-Resistance Detection for Adaptive Resistance Management in Tetranychus urticae Koch

Rapid resistance detection is necessary for the adaptive management of acaricide-resistant populations of Tetranychus urticae. Detection of phenotypic and genotypic resistance was conducted by employing residual contact vial bioassay (RCV) and quantitative sequencing (QS) methods, respectively. RCV was useful for detecting the acaricide resistance levels of T. urticae, particularly for on-site resistance detection; however, it was only applicable for rapid-acting acaricides (12 out of 19 tested acaricides). QS was effective for determining the frequencies of resistance alleles on a population basis, which corresponded to 12 nonsynonymous point mutations associated with target-site resistance to five types of acaricides [organophosphates (monocrotophos, pirimiphos-methyl, dimethoate and chlorpyrifos), pyrethroids (fenpropathrin and bifenthrin), abamectin, bifenazate and etoxazole]. Most field-collected mites exhibited high levels of multiple resistance, as determined by RCV and QS data, suggesting the seriousness of their current acaricide resistance status in rose cultivation areas in Korea. The correlation analyses revealed moderate to high levels of positive relationships between the resistance allele frequencies and the actual resistance levels in only five of the acaricides evaluated, which limits the general application of allele frequency as a direct indicator for estimating actual resistance levels. Nevertheless, the resistance allele frequency data alone allowed for the evaluation of the genetic resistance potential and background of test mite populations. The combined use of RCV and QS provides basic information on resistance levels, which is essential for choosing appropriate acaricides for the management of resistant T. urticae.     

Evaluation of the poultry red mite, Dermanyssus gallinae (Acari: Dermanyssidae) susceptibility to some acaricides in field populations from Italy

Red mite field populations from seven naturally infested Italian caged laying poultry farms were investigated for their susceptibility to acaricide formulations available on the market, containing amitraz, carbaryl and permethrin. A minimum of 3,000 mites of all stages were collected from each farm and were tested with five acaricide concentrations (5, 10, 20, 50, 100%) plus an untreated control (0%). Field red mite populations were found to be tolerant even with the highest concentrations with carbaryl and permethrin for six (86%) and three (42%) of the investigated farms, respectively (P < 0.05). Furthermore, six (86%) of the investigated farms showed a red mite population susceptible to amitraz at any concentration. Out of the seven field populations tested with amitraz, one population is becoming less tolerant whereas another was the most tolerant to carbaryl and permethrin at any concentration. Data show that the lack of effectiveness of some acaricides is spreading in Europe and call for the adoption of alternative management strategies to avoid development of resistance.     

Fitness costs associated with low-level dimethoate resistance in Phytoseiulus macropilis

Phytoseiulus macropilis Banks (Acari: Phytoseiidae) is an effective predator of tetranychid mites, but there are no data on its response to pesticides. We investigated the resistance of the predatory mite P. macropilis to the acaricides abamectin and dimethoate, and we examined the fitness costs associated with resistance. Two populations were tested: one from conventional cultivation and another from an area not commercially exploited. After the application of acaricides to the predator, we determined the lethal effects of the acaricides, the instantaneous rate of population increase (r _i ), the predation on Tetranychus urticae Koch (Acari: Tetranychidae) and its ability to locate prey in an olfactometer. P. macropilis exhibited resistance to dimethoate only. The low level of resistance (9.4x) of the predator did not affect their ability to locate prey. However, the dimethoate resistant population was not as effective in contatining prey population when in lower density and exhibited a more pronounced decrease of r _i in the presence of this acaricide, due to the reduced oviposition of the predator, a likely consequence of the different genetic background of this population.     

Baseline Susceptibility and Cross Resistances of Some New Acaricides in the European Red Mite, Panonychus ulmi*

We have developed baseline susceptibility values for four new acaricides: bifenazate, acequinocyl, spirodiclofen and etoxazole for eggs or adult females of a susceptible laboratory population of Panonychus ulmi (Koch) (Acari: Tetranychidae). Further, we have tested diagnostic concentrations of each acaricide against populations resistant to organochlorine, organotin or IGR-type acaricides. Resistance to etoxazole (ca. 4-fold) was identified in populations resistant to the IGRs clofentezine and hexythiazox. *The Canadian Crown’s right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

截形叶螨对哒螨灵、阿维菌素和阿维·哒螨灵的抗性选育和抗性稳定性研究

【目的】明确截形叶螨Tetranychus truncatus Ehara对哒螨灵、阿维菌素和阿维·哒螨灵3种田间常用药剂产生抗性的速率和稳定性,为叶螨的抗性综合治理提供一定的理论依据。【方法】采用室内生测法,对截形叶螨进行药剂的抗性筛选、衰退和再恢复规律研究。【结果】经过连续30代的药剂汰选,截形叶螨对哒螨灵、阿维菌素和阿维·哒螨灵3种药剂产生了不同程度的抗药性,抗性指数分别达到197.50、19.56和12.57;停止喷药后,其抗性都有所下降,其中截形叶螨对哒螨灵的抗性最不稳定,培育至30代后,抗性衰退率达到63.54%,对阿维菌素的抗性较为稳定,抗性衰退率为23.30%;再次恢复用药后,截形叶螨对哒螨灵、阿维菌素和阿维哒螨灵抗性再度回升,以抗哒螨灵品系的抗性恢复最快,药剂汰选30代后,增长率达到了58.47%,阿维·哒螨灵次之(增长率为38.67%),抗阿维菌素的品系抗性恢复最慢,增长率仅为22.86%。【结论】截形叶螨对哒螨灵抗性不稳定,停止用药后,敏感性易恢复,对阿维菌素和阿维·哒螨灵的抗性较稳定,一旦抗性产生不易衰退,故田间应用时应交替轮换用药。     

Toxicity of three acaricides to Tetranychus urticae (Tetranychidae: Acari) and Orius insidiosus (Anthocoridae: Hemiptera)

Management for twospotted spider mite, Tetranychus urticae Koch, populations in peanut, Arachis hypogaea L., relies on acaricides. The outcomes of acaricide applications are most predictable when complete information on their toxicity and specificity is available. Specifically, the degrees to which acaricides impact different stages of T. urticae and natural enemies combined determine the overall efficacy of an acaricide application. The objectives of this study were to determine stage-specific direct and residual efficacies of three acaricides (fenpropathrin, etoxazole, and propargite) against T. urticae, and the direct and residual toxicity of the acaricides to Orius insidiosus (Say) adults. Direct toxicity of acaricides to T. urticae was measured on peanut cuttings. All acaricide treatments caused significant mortality to a mixed stage population of T. urticae, and mortality did not differ among the acaricides 7 d after treatment. When toxicity to eggs was tested, the proportion of eggs that hatched for all acaricide treatments was significantly lower than the control, with etoxazole and propargite causing 100% mortality. Exposure to acaricide residues caused < 30% mortality of T. urticae adults 1 and 2 d after treatment and was not significantly different from the control. Fenpropathrin and propargite caused 100% mortality and etoxazole caused > 50% mortality of O. insidious adults after direct exposure to the acaricides. Residual toxicity of acaricides to O. insidiosus adults varied but remained toxic to O. insidiosus longer than to T. urticae. Fenpropathrin had the longest residual effect on O. insidiosus adults, causing > 95% mortality after 14 d; etoxazole and propargite caused < 30% mortality after 14 d.     

Esterase profile of Rhipicephalus (Boophilus) microplus populations collected from Northern India exhibiting varied susceptibility to deltamethrin

Rhipicephalus (Boophilus) microplus is an economically important ectoparasite of cattle. Chemical acaricides remain the most practical method for control of these pests. During past two decades there have been increasing reports of resistance development against synthetic pyrethroids in tick populations of this species throughout the world. A study was conducted to determine the level of susceptibility of R. (B.) microplus to deltamethrin collected from different geographical locations of northern India. LPT bioassay results revealed LC(50) values of deltamethrin ranging from 0.035 to 0.00037?% A.I. Esterase profile of the tick larval extracts using native PAGE, revealed 5 bands of esterase activity designated EST-5 to EST-1A. Inhibitory tests recognized EST-1, EST-2 and EST-3 as Acetylcholinesterases (AchEs), EST-4 and EST-5 as Carboxylesterases (CaEs). The band intensity varied between tick populations of various locations, being more intense in case of the resistant populations. An extra band of esterase activity (EST-1A) was obtained in larval extracts of ticks from 3 locations. This increased esterase activity may be involved in the resistance development in these tick populations. Acaricide resistance is a multi-factorial phenomenon, thus other causes of increased resistance like sodium channel mutation and reduced drug penetration (e.g. cuticle thickening) and behavioural changes (e.g. avoiding the pesticides) are to be tested in future in order to confirm the basic cause of the resistance development in these acaricide resistant tick populations.     

Effects of cotton cultivar on fitness costs associated with resistance of pink bollworm (Lepidoptera: Gelechiidae) to Bt cotton

Fitness costs associated with insect resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) reduce the fitness on non-Bt refuge plants of resistant individuals relative to susceptible individuals. Because costs may vary among host plants, choosing refuge cultivars that increase the dominance or magnitude of costs could help to delay resistance. Specifically, cultivars with high concentrations of toxic phytochemicals could magnify costs. To test this hypothesis, we compared life history traits of three independent sets of pink bollworm, Pectinophora gossypiella (Saunders), populations on two cotton cultivars that differed in antibiosis against this cotton pest. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. Confirming previous findings with pink bollworm feeding on cotton, costs primarily affected survival and were recessive on both cultivars. The magnitude of the survival cost did not differ between cultivars. Although the experimental results did not reveal differences between cultivars in the magnitude or dominance of costs, modeling results suggest that differences between cultivars in pink bollworm survival could affect resistance evolution. Thus, knowledge of the interaction between host plants and fitness costs associated with resistance to Bt crops could be helpful in guiding the choice of refuge cultivars.     

Variation in host resistance and pathogen selective value in the interaction between Pinus sylvestris and the fungus Crumenulopsis sororia

There have been many studies of plant pathogen evolution in systems showing gene-for-gene control of host resistance. However little is known about situations, exemplified by Scots pine, Pinus sylvestris, and its fungal pathogen Crumenulopsis sororia, where variation in host resistance is quantitative. In a field experiment genetically marked isolates of C. sororia from three natural populations were reciprocally inoculated on 1- and 2-year-old branch tissue of P. sylvestris in the three sites from which they had been collected. Quantitative variation in host resistance was measured by comparing the performance of the same inocula on different host populations, individuals and tissues. The selective value of isolates derived from different populations was estimated by comparing the frequency of genotypes in lesion re-isolations with those in the initial inoculum mixtures. Host resistance varied significantly among populations, individuals within populations and between 1- and 2-year-old branch tissue of P. sylvestris. Large differences in the relative selective values of C. sororia isolates from different populations were detected. The selective value of pathogens was independent of the host population on which they were inoculated. However, their selective value did depend on the age of the tissue on which they grew. The implications of these results for modelling evolution in pathogen-host interactions that lack gene-for-gene determination of host resistance are discussed.     

Influence of acaricide resistance on cattle-fever tick (Boophilus spp.) infestations in semi-arid thornshrublands: a simulation approach

Cattle-fever tick (Boophilus microplus and B. annulatus) populations that develop acaricide resistance become more difficult to control or eradicate. We used a simulation model to assess the direct and indirect effects of interactions among season, habitat type, grazing strategy, and acaricide resistance on the ability to eradicate Boophilus infestations in semi-arid thornshrublands of Texas, USA. Season of infestation appeared to have the strongest effect, with infestations begun on 27 September (autumn) tending to die out sooner than those begun on 1 March (spring) and to remain undetected. Habitat type had the next strongest effect, with infestations surviving much longer as canopy cover increased from uncanopied buffelgrass (Cenchrus ciliaris) habitats to mesquite (Prosopis glandulosa)-canopied grass habitats. Acaricide resistance had a moderate effect; as expected, highly resistant tick populations survived longer than those with no acaricide resistance. The importance of grazing strategy varied with changes in habitat type: as canopy cover increased, infestation duration increased faster under continuous grazing than under rotational grazing strategies. Importance of grazing strategy also varied with acaricide resistance: detected tick populations with no and slight acaricide resistance subjected to acaricide treatments tended to survive longer under rotational grazing than continuous grazing, due to reduced contact with a treated host. Populations with moderate and high resistance behaved more like untreated populations, tending to survive longer under continuous, rather than rotational, grazing, because they experienced less mortality on a treated host. Assuming acaricide treatments at 2-week intervals and maintenance of cattle in infested pastures, results indicate that, for each habitat type, infesting ticks have a threshold of acaricide resistance below which one can eradicate them faster with continuous grazing than with rotational grazing. As canopy cover increases, this threshold appears to shift from high resistance (in grass) to slight resistance (in mesquite).     

Effects of gossypol on fitness costs associated with resistance to Bt cotton in pink bollworm

Fitness costs associated with insect resistance to Bacillus thuringiensis (Bt) crops may help to delay or prevent the spread of resistance alleles, especially when refuges of non-Bt host plants are present. The potential for such delays increases as the magnitude and dominance of fitness costs increase. Here, we examined the idea that plant secondary chemicals affect expression of fitness costs associated with resistance to Bt cotton in Pectinophora gossypiella (Saunders). Specifically, we tested the hypotheses that gossypol affects the magnitude or dominance of fitness costs, by measuring performance of three independent sets of pink bollworm populations fed artificial diet with and without gossypol. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. No individuals completed development on diets with gossypol in one set, suggesting that these individuals partially lost the ability to detoxify this chemical. In the other two sets, costs affecting survival did not support the hypotheses, but costs affecting pupal weight did. Adding gossypol to diet increased the magnitude and dominance of costs affecting pupal weight. In one of the two sets with survivors on diet with gossypol, costs affecting development time were less recessive when gossypol was present in diet. These results indicate that gossypol increased the magnitude and dominance of some fitness costs. Better understanding of the effects of natural plant defenses on fitness costs could improve our ability to design refuges for managing insect resistance to Bt crops.     

Detecting Resistance to Organophosphates and Carbamates in the Cattle Tick Boophilus Microplus, with a Propoxur-based Biochemical Test

Rapid and sensitive detection of resistance to insecticides in arthropods is needed. In the cattle tick, Boophilus microplus, resistance to a variety of acaricides is widespread. The most commonly used assay for resistance, the larval packet test, takes at least two, but generally six weeks for a one-host tick like B. microplus to complete and may take up to three months to complete for three-host ticks. Here we describe a test for resistance to organophosphate acaricides that can be used on larvae and adult ticks which takes less than 24 hours. The test measures the difference in acetylcholinesterase (AChE) activity in homogenates of ticks in the presence and absence of propoxur, a carbamate acaricide. We found clear discrimination of organophosphate-susceptible and organophosphate-resistant adults with 100 M propoxur. AChE from susceptible ticks had almost no activity at this concentration of propoxur whereas AChE from resistant ticks had 67% of its potential activity. AChE from heterozygote ticks could also be distinguished from AChE from homozygous-susceptible and homozygous-resistant ticks. This is the first biochemical test for resistance to an acaricide. Rapid, sensitive tests like ours will allow resistance to organophosphates to be detected soon after it develops in the field, thus, the spread of resistance might be slowed and the useful life of acaricides extended.     

Integrated Control of Acaricide-Resistant Boophilus Microplus Populations on Grazing Cattle in Mexico Using Vaccination with Gavac™ and Amidine Treatments

Throughout most of the twentieth century, tick infestations on cattle have been controlled with chemical acaricides, typically administered by dipping or spraying. This approach can cause environmental and residue problems and has created a high incidence of acaricide resistance within tick populations in the field. Recently we developed a vaccine against Boophilus microplus employing a recombinant Bm86 antigen preparation (Gavac), (Heber Biotec S.A., Havana, Cuba) which has been shown to induce a protective response in vaccinated animals. Here we show for the first time under field conditions a near 100% control of B. microplus populations resistant to pyrethroids and organophosphates, by an integrated system employing vaccination with Gavac and amidine treatments. This method effectively controls tick infestations while reducing the number of chemical acaricide treatments and consequently the rise of B. microplus populations resistant to chemical acaricides.     

First report of amitraz and cypermethrin resistance in Rhipicephalus sanguineus sensu lato infesting dogs in Mexico

Engorged female Rhipicephalus sanguineus sensu lato (Ixodida: Ixodidae) were collected from dogs in the state of Yucatán, Mexico. Fourteen tick populations were collected from dogs at seven veterinary clinics, four residential homes and three cattle farms. The larval immersion test was used in the progeny of collected adult females to test susceptibility to amitraz and cypermethrin. Dose–mortality regressions, 50% lethal concentrations (LC₅₀), confidence intervals and slope were estimated by probit analysis. For amitraz, 12 tick populations (85.7%) were classified as resistant and low inter‐population variation in the phenotypic level of resistance was evident [resistance ratios (RRs) at LC₅₀: 1.0–13.0]. For cypermethrin, 12 tick populations (85.7%) were classified as resistant and substantial inter‐population variation in the phenotypic level of resistance was evident (RRs at LC₅₀: 1.0–104.0). Thus, amitraz resistance in R. sanguineus s.l. is common, but generally occurs at low levels; however, alarmingly high levels of cypermethrin resistance are present in R. sanguineus s.l. populations in dogs in Yucatán, Mexico. The intensive use of both acaricides to control ectoparasites on dogs is likely to lead to more serious resistance problems that may cause high levels of control failure in the future.     

Genetic basis of resistances to chlorfenapyr and etoxazole in the two-spotted spider mite (Acari: Tetranychidae)

We studied the genetic basis of resistance to two new acaricides, chlorfenapyr and etoxazole, which have different chemical structures and modes of action in the two-spotted spider mite, Tetranychus urticae Koch. The resistance ratios calculated from the LC50s of resistant and susceptible strains were 483 for chlorfenapyr and >100,000 for etoxazole. Mortality caused by the two acaricides in F1 progeny from reciprocal crosses between the resistant and susceptible strains indicated that the modes of inheritance of resistance to chlorfenapyr and etoxazole were completely dominant and completely recessive, respectively. Mortality in F2 progeny indicated that for both acaricides, the resistance was under monogenic control. Repeated backcross experiments indicated a linkage relationship among the two acaricide resistances and malate dehydrogenase, although phosphoglucoisomerase was not linked with them. The recombination ratio between the resistances was 14.8%. From this result, we suggest that heavy spraying of the two acaricides will lead to apparent cross-resistance as a consequence of crossing over; the two resistance genes are so close to each other that it would be difficult to segregate them once they came together on the same chromosome.     

Inbreeding alters resistance to insect herbivory and host plant quality in Mimulus guttatus (Scrophulariaceae)

Previous studies have demonstrated genetic variation for resistance to insect herbivores and host plant quality. The effect of plant mating system, an important determinant of the distribution of genetic variation, on host plant characteristics has received almost no attention. This study used a controlled greenhouse experiment to examine the effect of self- and cross-pollination in Mimulus guttatus (Scrophulariaceae) on resistance to and host plant quality for the xylem-feeding spittlebug Philaenus spumarius (Homoptera: Cercopidae). Spittlebugs were found to have a negative effect on two important fitness components in M. guttatus, flower production and above ground biomass. One of two M. guttatus populations examined showed a significant interaction between the pollination and herbivore treatments. In this case, the detrimental effects of herbivores on biomass and flower production were much more pronounced in inbred (self) plants. The presence of spittlebug nymphs increased inbreeding depression by as much as three times. Pollination treatments also had significant effects on important components of herbivore fitness, but these effects were in opposite directions in our two host plant populations. Spittlebug nymphs maturing on self plants emerged as significantly larger adults in one of our host plant populations, indicating that inbreeding increased host plant quality. In our second host plant population, spittlebug nymphs took significantly longer to develop to adulthood on self plants, indicating that inbreeding decreased host plant quality. Taken together these results suggest that the degree of inbreeding in host plant populations can have important and perhaps complex effects on the dynamics of plant-herbivore interactions and on mating-system evolution in the host.     

Competitiveness and life‐history characteristics of Daphnia with respect to susceptibility to a bacterial pathogen

Costs of resistance, i.e. trade‐offs between resistance to parasites or pathogens and other fitness components, may prevent the fixation of resistant genotypes and therefore explain the maintenance of genetic polymorphism for resistance in the wild. Using two approaches, the cost of resistance to a sterilizing bacterial pathogen were tested for in the crustacean Daphnia magna. First, groups of susceptible and resistant hosts from each of four natural populations were compared in terms of their life‐history characteristics. Secondly, we examined the competitiveness of nine clones from one population for which more detailed information on genetic variation for resistance was known. In no case did the results show that competitiveness or life history characteristics of resistant Daphnia systematically differed from susceptible ones. These results suggest that costs of resistance are unlikely to explain the maintenance of genetic variation in D. magna populations. We discuss methods for measuring fitness and speculate on which genetic models of host‐parasite co‐evolution may apply to the Daphnia‐microparasite system.     

Influence of dispersal from almonds on the population dynamics and acaricide resistance frequencies of spider mites infesting neighboring cotton

Neighboring almond and cotton fields were sampled for spider mites in four locations in the San Joaquin Valley of California. The dominant species in the almonds wasTetranychus pacificus McGregor. In three cotton sites.T. pacificus was present in significantly higher densities near the almonds on at least one sampling date. In contrast.T. urticae Koch andT. turkestani Ugarov & Nikolski were equally abundant across the cotton fields. Almonds appeared to act as a continuous early-season source ofT. pacificus for cotton, with peaks in aerial dispersal from almonds occurring due to overcrowding, plant water stress, and applications of repellent acaricides. Cotton, which experienced little water stress, supported very high densities of spider mites and so acted primarily as a sink for spider-mite dispersal from almonds and other field crops throughout the growth-season. The frequencies of resistance expressed byT. pacificus andT. urticae were similar between neighboring crops, even if the acaricide had been registered for use only in almonds (cyhexatin) or cotton (dicofol). Thus, longterm acaricide selection and movement of spider mites between the two crops resulted in similar proportions of resistant individuals. In these study sites, large-scale dispersal ofT. pacificus from almonds rarely directly affected acaricide efficacy in cotton, because resistance frequencies were similar for spider mites from the two crops and because acaricide applications were usually made in cotton after dispersal from almonds was completed. In two cotton sites, field selection with dicofol was reversed by subsequent immigration of spider mites from neighboring field crops.