Bulb Mite Research Articles

Sexual and ecological selection on a sexual conflict gene.

Sexual selection and conflict can act on genes with important metabolic functions, potentially shaping standing genetic variance in such genes and thus evolutionary potential of populations. Here, using experimental evolution, we show how reproductive competition intensity and thermal environment affect selection on phosphogluconate dehydrogenase (6Pgdh)-a metabolic gene involved in sexual selection and conflict in the bulb mite. The S allele of 6Pgdh increases male success in reproductive competition, but is detrimental to S-bearing males' partners. We found that the rate of the S allele spread increased with the proportion of males in the experimental populations, illustrating that harm to females is more easily compensated for males under more intense sexual competition. Furthermore, we found that under equal sex ratio, the S allele spreads faster at higher temperature. While the direction of selection on 6Pgdh was not reversed in any of the conditions we tested, which would be required for environmental heterogeneity to maintain polymorphism at this locus, our study highlights that ecological and sexual selection can jointly affect selection on important metabolic enzymes.

Preliminary sampling of dry bulb mite, Aceria tulipae Keifer in native garlic in Ilocos Region, Luzon Island, Philippines

The objective of this study was to provide information on the population density and distribution of the dry bulb mite, Aceria tulipae on the leaves and bulbs of native garlic in Ilocos Region, Philippines. Preliminary data showed that the average number of A. tulipae is the same within the leaf and plant canopy. The total number of A. tulipae is highest at the middle section of the leaf. Aceria tulipae is also detected in cloves sampled from harvested bulbs. Results suggest that the leaves and bulbs of native garlic could be considered when designing a sampling method in the future.

Selection for Male Weapons Boosts Female Fecundity, Eliminating Sexual Conflict in the Bulb Mite

Extreme differences between the sexes are usually explained by intense sexual selection on male weapons or ornaments. Sexually antagonistic genes, with a positive effect on male traits but a negative effect on female fitness, create a negative inter-sexual correlation for fitness (sexual conflict). However, such antagonism might not be apparent if sexually selected male traits are condition-dependent, and condition elevates female fitness. Here we reveal a surprising positive genetic correlation between male weaponry and female fecundity. Using mite lines that had previously been through 13 generations of selection on male weapons (fighting legs), we investigated correlated evolution in female fecundity. Females from lines under positive selection for weapons (up lines) evolved higher fecundity, despite evolving costly, thicker legs. This is likely because male mites have condition-dependent weaponry that increases our ability to indirectly select on male condition. Alleles with positive effects on condition in both sexes could have generated this correlation because: the up lines evolved a higher proportion of fighters and there were positive correlations between weapon size and the male morph and sex ratios of the offspring. This positive inter-sexual genetic correlation should boost the evolution of male weapons and extreme sex differences.

Effectiveness of Soil Heat Treatment Using Covering Material Against the Bulb Mite on Chinese Chive

Effectiveness of Soil Heat Treatment Using Covering Material Against the Bulb Mite on Chinese Chive

Rhizoglyphus echinopus (Acari: Acaridae) associated with Atta sexdens (Insecta: Formicidae)

ABSTRACT Four deutonymphs of bulb mites (hypopus) from Rhizoglyphus echinopus (Fumouze & Robin) (Acari: Acaridae) were found attached to the head of Atta sexdens in Rio de Janeiro, Brazil. This mite species is commonly associated with ornamental plants and trees with bulbs, corms and tubers. The results of this study provided an insight into the phoretic relationship between mites and ants, indicating the role of the latter in the dispersion of the first. Despite the abundant and diverse mite fauna existing in ants, little is known about their diversity, biology, ecology and the nature of their associations.

Methyl benzoate fumigation for control of post‐harvest pests and its effects on apple quality

AbstractMethyl benzoate is a newly discovered natural insecticide. In this study, we evaluated methyl benzoate as a potential fumigant for controlling four pests including western flower thrips (WFT), Frankliniella occidentalis, lettuce aphid, Nasonovia ribisnigri, rice weevil, Sitophilus oryzae, and bulb mites, Rhizoglyphus spp., at different temperatures and effects on post‐harvest quality of apples. Methyl benzoate fumigations were effective against all four pests. Complete control of WFT and lettuce aphid was both achieved in 8, 16 and 24 hr at 25, 13 and 2°C, respectively. For rice weevil, complete control was achieved in 16 and 72 hr with and without rice at 25°C, respectively. Complete control of bulb mites was achieved in 64 hr at 25°C on peanut. In addition, a 24 hr methyl benzoate fumigation at 2°C had complete control of WFT but had no negative impact on visual quality of apples of three varieties 4 weeks after fumigation. These results suggest that methyl benzoate fumigation has potential to be an alternative treatment to methyl bromide fumigation, which has been phased out, for post‐harvest pest control for stored products and may also have potential to control other pests on fresh products.

충북지역 한지형 마늘 재배지에서 뿌리응애 발생양상 및 살충제 감수성

충북지역 한지형 마늘 재배지에서 뿌리응애 발생양상 및 살충제 감수성

Et tu, brother? Kinship and increased nutrition lower the incidence of cannibalism in male bulb mites

The killing of conspecifics returns indirect benefits when killers eliminate competitors for resources, and direct benefits when cannibals obtain nutrients directly after killing. Cannibals risk pathogen infection and both killers and cannibals risk reducing their inclusive fitness when killing kin. When competing with kin over food during a prolonged period of starvation, we surmise that this cost–benefit ratio will stay almost constant for killers, as their benefits are indirect so that they direct killing away from kin (unless in extremis), but that this ratio will reduce over time for cannibals, as their benefits are direct, such that they will start consuming kin when food stressed. We tested this hypothesis using the male-dimorphic bulb mite Rhizoglyphus robini in which males are armed ‘fighters’ that can kill and cannibalize or unarmed ‘scramblers’. In four-fighter groups that were either starved or fed for 2 weeks, and that comprised either kin (brothers) or nonkin (at most cousins), we assessed the number of dead and cannibalized fighters. The cumulative number of dead and cannibalized fighters was higher under starved than under fed conditions, and increased over time, rejecting our hypothesis. Instead, we found that fewer kin than nonkin were cannibalized, but fighters discriminated to a much lesser extent between kin and nonkin when killing. Hypothetically, cannibalism of kin incurs a greater risk of infection by pathogens to which cannibals are genetically susceptible. Our results highlight the importance of distinguishing killing from cannibalism, as they can have contrasting effects on the demography of populations.

The role of genetic diversity in the evolution and maintenance of environmentally-cued, male alternative reproductive tactics

BackgroundAlternative reproductive tactics (ARTs) are taxonomically pervasive strategies adopted by individuals to maximize reproductive success within populations. Even for conditionally-dependent traits, consensus postulates most ARTs involve both genetic and environmental interactions (GEIs), but to date, quantifying genetic variation underlying the threshold disposing an individual to switch phenotypes in response to an environmental cue has been a difficult undertaking. Our study aims to investigate the origins and maintenance of ARTs within environmentally disparate populations of the microscopic bulb mite, Rhizoglyphus robini, that express ‘fighter’ and ‘scrambler’ male morphs mediated by a complex combination of environmental and genetic factors.ResultsUsing never-before-published individual genetic profiling, we found all individuals across populations are highly inbred with the exception of scrambler males in stressed environments. In fact within the poor environment, scrambler males and females showed no significant difference in genetic differentiation (Fst) compared to all other comparisons, and although fighters were highly divergent from the rest of the population in both poor or rich environments (e.g., Fst, STRUCTURE), fighters demonstrated approximately three times less genetic divergence from the population in poor environments. AMOVA analyses further corroborated significant genetic differentiation across subpopulations, between morphs and sexes, and among subpopulations within each environment.ConclusionOur study provides new insights into the origin of ARTs in the bulb mite, highlighting the importance of GEIs: genetic correlations, epistatic interactions, and sex-specific inbreeding depression across environmental stressors. Asymmetric reproductive output, coupled with the purging of highly inbred individuals during environmental oscillations, also facilitates genetic variation within populations, despite evidence for strong directional selection. This cryptic genetic variation also conceivably facilitates stable population persistence even in the face of spatially or temporally unstable environmental challenges. Ultimately, understanding the genetic context that maintains thresholds, even for conditionally-dependent ARTs, will enhance our understanding of within population variation and our ability to predict responses to selection.

Life-history consequences of bidirectional selection for male morph in a male-dimorphic bulb mite

Intralocus sexual conflict (IASC) arises when males and females have different trait optima. Some males pursue different alternative reproductive tactics (ARTs) with different trait optima, resulting in different strengths of IASC. Consequently, for instance daughter fitness is differentially affected by her sire’s morph. We tested if—and which—other life-history traits correlatively change in bidirectional, artificial selection experiments for ARTs. We used the male-dimorphic bulb mite Rhizoglyphus robini, the males of which are high-fitness ‘fighters’ or low-fitness ‘scramblers’. Twice in each of the five generations of selection, we assessed clutch composition (number of mites of the various life stages present) and size (total number of offspring). Furthermore, we tracked offspring from egg to adulthood in the first and final generation to detect differences between selection lines in the size and duration of stages, and in maturation time. We found that selection for male morph increased the frequency of that morph. Furthermore, compared to fighter lines, scrambler lines produced more females, which laid larger eggs (in the final generations), and maintained a higher egg-laying rate for longer. Otherwise, our results showed no consistent differences between the selection lines in clutch size and composition, life stage size or duration, or maturation time. Though we found few correlated life-history trait changes in response to selection on male morph, the differences in egg laying rate and egg size suggest that IASC between fighters is costlier to females than IASC with scramblers. We hypothesize that these differences in reproductive traits allow fighter-offspring to perform better in small, declining populations but scrambler-offspring to perform better in large, growing populations.

Entomopathogenic nematodes as natural enemies for control of Rhizoglyphus robini (Acari: Acaridae)?

Bulb mites of the genus Rhizoglyphus are cosmopolitan pests of onion, garlic and ornamental plants. Despite the growing awareness against the use of pesticides, growers continue to use insecticides as the main control method. Thus, the search for environmentally safe alternative control methods, such as effective biocontrol agents, is of paramount importance. In the present study, the bulb mites were exposed to the infected juveniles of 20 strains of Steinernema and Heterorhabditis species applied at a single dose of 300 IJs per mite, and the invasion rate and mite mortality were assessed. Furthermore, the effect of the culture supernatants of the selected symbiotic bacteria of the genus Xenorhabdus on bulb mite mortality was tested. Our data show that both Steinernema and Heterorhabditis nematodes are able to invade and kill bulb mites, but in general, the invasion and resulting mortality were relatively low. The highest invasion rate of ca 30 IJs per mite was observed in Heterorhabditis taysearae. The mortality reached the maximum of 30% in the most efficient species, namely Steinernema huense, H. bacteriophora and H. amazonensis. The effect of the culture supernatants was generally low, though there were considerable differences in the efficacy among Xenorhabdus species and strains. To conclude we demonstrate that some EPN species are able to invade and kill bulb mites but their effect is in general quite low. Based on present data, EPNs and the metabolites of their symbiotic bacteria do not seem to represent a viable option for bulb mite biocontrol as a standalone approach.

An eco-evolutionary feedback loop between population dynamics and fighter expression affects the evolution of alternative reproductive tactics.

Surprisingly, little is known about how eco‐evolutionary feedback loops affect trait dynamics within a single population. Polymorphisms of discrete alternative phenotypes present ideal test beds to investigate this, as the alternative phenotypes typically exhibit contrasting demographic rates mediated through frequency or density dependence, and are thus differentially affected by selection.Alternative reproductive tactics (ARTs), like male fighters and sneakers, are an extreme form of discrete phenotype expression and occur across many taxa. Fighters possess weapons for male–male competition over access to mates, whereas sneakers are defenceless but adopt tactics like female‐mimicking. Because fighters in some species mortally injure conspecifics, this raises the question whether fighter expression can feed back to affect population size and structure, thereby altering the selection gradient and evolutionary dynamics of ART expression in an eco‐evolutionary feedback loop.Here, we investigated how the eco‐evolutionary feedback loop between fighter expression and population size and structure affects the evolution and maintenance of ARTs. We introduced intraspecific killing by fighters in a two‐sex, two‐ART population model parameterized for the male dimorphic bulb mite (Rhizoglyphus robini) that includes life‐history differences between the ARTs and a mating‐probability matrix analogous to the classic hawk–dove game.Using adaptive dynamics, we found that the intraspecific killing by fighters can extend the range of life‐history parameter values under which ARTs evolve, because fighters that kill other fighters decrease fighter fitness. This effect can be nullified when benefits from killing are incorporated, like increased reproduction through increased energy uptake.The eco‐evolutionary feedback effects found here for a dimorphic trait likely also occur in other fitness‐related traits, such as behavioural syndromes, parental care and niche construction traits. Current theoretical advances to model eco‐evolutionary processes, and empirical steps towards unravelling the underlying drivers, pave the way for understanding how selection affects trait evolution in an eco‐evolutionary feedback loop.

Exploring the impact of non-chemical methods on the control of bulb mite (Rhizoglyphus robini Claparáde) and yield of saffron (Crocus sativus L.)

The bulb mite (Rhizoglyphus robini) causes damage to a number of crop and ornamental plant species, including saffron (Crocus sativus). A split-plot experiment based on a randomised complete-block design was conducted between 2011 and 2015 to investigate the effects of solarisation using transparent plastic along with planting date and first irrigation on the population of bulb mite, yield of saffron, size of corms produced and extent of corm infection. Solarisation significantly decreased the mite population and increased the saffron yield (P<0.01). Increased yields were achieved by planting in June rather than in September. However, the use of irrigation immediately after planting in June increased the mite population and decreased saffron yields. A combination of solarisation in the first year and planting in June of the next year with no irrigation after planting resulted in a reduction of 94% in the population of the mites and a 3.48-fold increase in saffron yield. In Iran, saffron corms should be planted in June but not irrigated until September. If planting does occur in September then irrigation should be applied immediately after planting.

Male-limited secondary sexual trait interacts with environment in determining female fitness.

Selection for secondary sexual trait (SST) elaboration may increase intralocus sexual conflict over the optimal values of traits expressed from shared genomes. This conflict can reduce female fitness, and the resulting gender load can be exacerbated by environmental stress, with consequences for a population's ability to adapt to novel environments. However, how the evolution of SSTs interacts with environment in determining female fitness is not well understood. Here, we investigated this question using replicate lines of bulb mites selected for increased or decreased prevalence of a male SST—thickened legs used as weapons. The fitness of females from these lines was measured at a temperature to which the mites were adapted (24°C), as well as at two novel temperatures: 18°C and 28°C. We found the prevalence of the SST interacted with temperature in determining female fecundity. At 28°C, females from populations with high SST prevalence were less fecund than females from populations in which the SST was rare, but the reverse was true at 18°C. Thus, a novel environment does not universally depress female fitness more in populations with a high degree of sexually selected dimorphism. We discuss possible consequences of the interaction we detected for adaptation to novel environments.

Relative costs and benefits of alternative reproductive phenotypes at different temperatures \u2013 genotype-by-environment interactions in a sexually selected trait

BackgroundThe maintenance of considerable genetic variation in sexually selected traits (SSTs) is puzzling given directional selection expected to act on these traits. A possible explanation is the existence of a genotype-by-environment (GxE) interaction for fitness, by which elaborate SSTs are favored in some environments but selected against in others. In the current study, we look for such interactions for fitness-related traits in the bulb mite, a male-dimorphic species with discontinuous expression of a heritable SST in the form of enlarged legs that are used as weapons.ResultsWe show that evolution at 18 °C resulted in populations with a higher prevalence of this SST compared to evolution at 24 °C. We further demonstrate that temperature modified male reproductive success in a way that was consistent with these changes. There was a genotype-by-environment interaction for reproductive success – at 18 °C the relative reproductive success of armored males competing with unarmored ones was higher than at the moderate temperature of 24 °C. However, male morph did not have interactive effects with temperature with respect to other life history traits (development time and longevity).ConclusionsA male genotype that is associated with the expression of a SST interacted with temperature in determining male reproductive success. This interaction caused an elaborate SST to evolve in different directions (more or less prevalent) depending on the thermal environment. The implication of this finding is that seasonal temperature fluctuations have the potential to maintain male polymorphism within populations. Furthermore, spatial heterogeneity in thermal conditions may cause differences among populations in SST selection. This could potentially cause selection against male immigrants from populations in different environments and thus strengthen barriers to gene flow.

Trait-based predictions and responses from laboratory mite populations to harvesting in stochastic environments.

Predictions on population responses to perturbations are often derived from trait‐based approaches like integral projection models (IPMs), but are rarely tested. IPMs are constructed from functions that describe survival, growth and reproduction in relation to the traits of individuals and their environment. Although these functions comprise biologically non‐informative statistical coefficients within standard IPMs, model parameters of the recently developed dynamic energy budget IPM (DEB‐IPM) are life‐history traits like “length at maturation” and “maximum reproduction rate”. Testing predictions from mechanistic IPMs against empirical observations can therefore provide functional insights into the links between individual life history, the environment and population dynamics.Here, we compared the population dynamics of the bulb mite (Rhizoglyphus robini) predicted by a DEB‐IPM with those observed in an experiment where populations experienced daily food rations that were either positively correlated over time (red noise), negatively (blue noise) or uncorrelated (white noise). We also selectively harvested large adults in half of these populations. The model failed to generate detailed predictions of population structure as juvenile numbers were overestimated; likely because juvenile–adult interference competition was underestimated. The model performed well at the population level as, for both harvested and unharvested populations, simulations matched the observed, long‐term stochastic growth rate λs.We next generalised the model to investigate how stochastic change affects mite λs, which correlated well with the frequency f of experiencing periods of good environment, but, due to the relationship between f and noise colour ρ, did not correlate well with shifts in ρ. The sensitivity of λs to perturbations in life‐history parameters depended on the type of stochastic change, as well as population growth.Our findings show that responses to differential mortality depend on individual life‐history traits, environmental characteristics and population growth. As long‐term climate change causes ever greater environmental fluctuations, trait‐based approaches will be increasingly important in predicting population responses to change. We therefore conclude by illustrating what questions can be examined with mechanistic trait‐based models like the DEB‐IPM, the answers to which will advance our knowledge of the functional links between individual traits, the environment and population dynamics.

Proximate mechanisms of the differences in reproductive success of males bearing different alleles of Pgdh - a gene involved in a sexual conflict in bulb mite.

Enzyme polymorphism in phosphogluconate dehydrogenase (Pgdh) is a striking example of single gene polymorphism involved in sexual conflict in bulb mite Rhizoglyphus robini. Males homozygous for the S Pgdh allele were shown to achieve higher reproductive success than FF homozygous males, while negatively influencing fecundity of their female partners. Here, we investigate proximate mechanisms responsible for the increased reproductive success of SS males and find that the S allele is associated with shorter time until copulation, higher copulation frequency and increased sperm production. We also show that Pgdh alleles are probably codominant, with SS males gaining the highest reproductive success, FF males - the lowest - and FS-heterozygous males taking an intermediate position in all fitness parameters differentiating males of different genotypes. Additionally, we confirm the negative effect that S-bearing males impose on the fecundity of females they mate with, showing a clear pattern of interlocus sexual conflict. We discuss that this effect is probably associated with increased copulation frequency. Whereas, contrary to what we have predicted, the S allele does not cause increased general male mobility, we speculate that the S allele-bearing males are more efficient in forcing copulation and/or detecting females.

Evolution of mate guarding under the risk of intrasexual aggression in a mite with alternative mating tactics

Mate-guarding strategies are known to evolve in response to changes in the environment, but little is known about the genetic and plastic components of this source of variation. Here, we investigated how risk associated with aggression shapes postcopulatory association time between mates in the bulb mite, Rhizoglyphus robini, a species in which aggressive, armoured fighters often coexist with unarmoured scramblers. In some populations, scramblers have been reported to prevent females remating by remaining in copula for over 6 h. In this study, we investigated whether mate guarding by scramblers is affected by the presence of aggressive fighters in populations. We investigated whether guarding is riskier in the presence of fighters and found that guarding males were more likely to be attacked. Our data allowed us to determine whether the presence of fighters can affect mate-guarding duration, by comparing guarding duration between populations (both natural and artificially selected). We found that in both types of population, males guarded longer when fighters were absent. Comparisons between lines selected for the presence of fighters, lines selected for the presence of scramblers and their source populations indicated that scrambler morphs evolved prolonged guarding. We also investigated whether males show plasticity and shorten guarding in response to the presence of fighters in a social group. Surprisingly, we found that males in a mixed-morph context copulated significantly longer than males from single-morph groups. Our results demonstrate that mate guarding may evolve in response to the presence or absence of fighters in populations, but males are not able to adjust guarding behaviour to the risk of being attacked by fighters. The study provides insight into the role of genetics and plasticity in guarding strategies.

Effect of Fungal Species on the Development and Reproductive Traits of the Fungal-Feeding Mite Rhizoglyphus robini (Astigmata: Acaridae).

The bulb mite, Rhizoglyphus robini (Claparède; Astigmata: Acaridae), is a cosmopolitan pest with a broad host range, including commercially grown edible fungi in China. In this study, we recorded the development and reproductive traits of the bulb mite reared on four mushroom species: Agaricus bisporus Lange, Pleurotus ostreatus Kumm, Pleurotus pulmonarius (Fr.) Quél., and Lentinula edodes (Berk.) Pegler at six constant temperatures ranging from 15 to 31°C and 80% RH. Developmental time for the immature stages was significantly affected by fungal species, ranging from 9.45 ± 1.83 d (reared on L. edodes at 31°C) to 26.39 ± 2.10 d (reared on A. bisporus at 15°C). Edible fungi species significantly affected intrinsic rates of natural increase (rm) at 31°C (varied from 0.23 to 0.28) as did the mite's net reproductive rates (R0) (103.78, 90.43, 70.77, and 97.45, respectively). Longevity, fecundity and female lifespan were dependent on host fungi.

Unsuccessful dispersal affects life history characteristics of natal populations: The role of dispersal related variation in vital rates

Individuals that disperse from one habitat to another has consequences for individual fitness, population dynamics and gene flow. The fitness benefits accrued in the new habitat are traded off against costs associated with dispersal. Most studies focus on costs at settlement and effects on settlement populations; the influence of dispersal to natal populations is assessed by monitoring change in numbers due to emigration. However, the extent to which natal populations are affected when individuals that invest in dispersal fail to disperse/emigrate is unclear. Here, we use an Integral Projection Model (IPM) to assess how developing into a disperser affects natal population structure and growth. We do so using the bulb mite (Rhizoglyphus robini) as a study system. Bulb mites, in unfavourable environments, develop into a dispersal (deutonymph) stage during ontogeny; these individuals are called dispersers with individuals not developing into this stage called non-dispersers. We varied disperser expression and parameterised IPMs to describe three simulations of successful and unsuccessful dispersal: (i) ‘no dispersal’ - dispersal stage is excluded and demographic data are from non-disperser individuals; (ii) ‘false dispersal’ - dispersal stage included and demographic data from non-disperser individuals are used; (iii) ‘true dispersal’ - dispersal stage included and demographic data are from individuals that go through the dispersal stage and from non-disperser individuals. We found that the type of dispersal simulation (no dispersal<false dispersal<true dispersal) and disperser expression increases generation time and reduces lifetime reproductive success and population growth rate. Our findings show that disperser individuals that fail to leave, can change the structure and growth of natal populations.