Genetics of resistance against defences of the host plant Barbarea vulgaris in a Danish flea beetle population

One essential aspect of the study of the evolution of host-plant use by insects is (variation in) its genetic basis. The genetic basis of the ability of a flea beetle (Phyllotreta nemorum) to use the crucifer Barbarea vulgaris ssp. arcuata (G type) as a host plant was studied in a Danish population (Kvaerkeby) occurring naturally on this atypical host plant. Evidence was found that this ability was determined by a single, major, autosomal gene, although the presence of genes at additional loci at lower frequencies could not be excluded. No evidence was found for sex-linked inheritance, which was common in a second population in Denmark (Ejby) using Barbarea as a host plant. All beetles in the Kvaerkeby sample were homozygous 'resistant' to Barbarea defence. After crossing resistant F1 offspring from pairs consisting of a field-collected beetle and a susceptible one amongst each other, genotyping the F2 (reared on radish) showed a 1:2:1 ratio of homozygous resistant, heterozygous and susceptible beetles. No evidence was found for a reduction in the viability of beetles that were homozygous resistant at the autosomal locus, in contrast to what had been found earlier for two backcrossed lines founded by beetles from Ejby. The results show that there is variation in the genetic basis of host-plant use across local populations and imply that population structure should form part of the study of the interaction between P. nemorum and its host plants.     

Genetics of the ability of Phyllotreta nemorum larvae to survive in an atypical host plant, Barbarea vulgaris ssp. arcuata

A polymorphism in host plant exploitation has been discovered in the flea beetle, Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae) where one resistant population is able to use Barbarea vulgaris R.Br. ssp. arcuata (Opiz.) Simkovics (Brassicaceae) as a host plant while a susceptible population is not. Crosses (F1, F2, and backcrosses) between the two flea beetle populations were made, and survival of the progeny on B. v. ssp. arcuata was measured. The ability of P. nemorum larvae to survive in this plant species depended on the presence of major, dominant genes (R-genes). The two most abundant R-genes in the resistant flea beetle population were X- and Y-linked, respectively. The use of B. v. ssp. arcuata as a natural host plant by the resistant population of P. nemorum seems to be an extension of the host plant range of the species. The role of sex-linked genes in the evolution of host range is discussed.     

Genetic basis for established and novel host plant use in a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Genetic trade-offs in host plant use are thought to promote the evolution of host specificity. Experiments on a range of herbivorous insects, however, have found negative genetic correlation in host plant use in only a limited number of species. To account for the general lack of negative genetic correlation, recent hypotheses advocate that different stages in evolution of host use must be distinguished: initial performance on a novel host in comparison with the established host, and performance on both hosts after the insect population has interacted with both hosts for a long time. The hypotheses suggest that genetic correlation may not necessarily be negative at the initial stage. The present study examines growth performance on both the established and a novel host in a herbivorous ladybird beetle, Epilachna vigintioctomaculata Motschulsky (Coccinellidae, Epilachninae). The results show that traits of growth performance across hosts were positively or neutrally correlated, but there was no evidence of a negative genetic correlation. In addition, significant genetic variance of growth performance on each host was detected, suggesting that E. vigintioctomaculata can potentially respond to selection for increased performance on both plant species. These results and similar results from experiments on other herbivores suggest that host expansion may not be constrained genetically, at least at the initial stage of host range evolution.     

Suitability of some Lantana camara varieties as host plants for the root-feeding flea beetle, Longitarsus bethae

Compatibility between the root-feeding agent Longitarsus bethae Savini & Escalona and the varieties of the target weed Lantana camara L. could influence the chances of successful establishment and effectiveness of this agent as a biocontrol agent of lantana in South Africa. Laboratory experiments were conducted to determine the influence of major South African lantana varieties on feeding, colonization, oviposition preference and larval survival of L. bethae. Feeding preference and the number of adults per plant varied among the varieties. The most damaged and colonized varieties were 150 Orange, 009 Light Pink and 113 Dark Pink. Variety 018 Dark Pink was the least damaged, while 021 Total Pink and 029 White Pink were the least colonized. Females laid the highest number of eggs on variety 150 Orange. The lowest number of eggs was recorded on Australian Richmond Pink and the closely related species Lippia wilmsii. Larval survival differed significantly among the varieties. The highest survival was recorded on 150 Orange. Duration of development also varied significantly among the varieties, with the slowest rate of development recorded on variety 029 Light Pink and the quickest on variety 015 White Yellow. Although variations in female body size were less significant among varieties, males emerging from variety 150 Orange were slightly smaller than those emerged from other varieties. There was a significant tendency for females to select varieties that enhanced the survival of their offspring, which met the predictions of the preference-performance hypothesis. The data suggest that the chances of establishment and successful control of the weed are expected to be much greater on those varieties that were preferred for oviposition and provided better larval performance.     

Host plant use in sympatric closely related flea beetles

Studies on strategies of host plant use in sympatric-related species are significant to the theory of sympatric speciation. Altica fragariae Nakane and Altica koreana Ogloblin are sympatric closely related flea beetles found in Beijing, northern China. All their recorded host plants are in the subfamily Rosoideae of the Rosaceae, so we regard them as a model system to study interactions between herbivorous insects and plant-insect co-evolution. We conducted a set of experiments on the host preference and performance of these flea beetles to study whether these closely related species have the ability to use sympatric novel host plants and whether monophagous and oligophagous flea beetles use the same strategy in host plant use. Oviposition preference experiments showed that A. koreana, a monophagous flea beetle, displayed high host fidelity. However, A. fragariae, which is oligophagous, often made "oviposition mistakes," ovipositing on nonhost plants such as Potentilla chinensis, the host plant of A. koreana, although normal host plants were preferred over novel ones. Larval performance studies suggested that A. fragariae was able to develop successfully on P. chinensis. Feeding experiences of larvae had no effect on feeding preference, oviposition preference, and fecundity of adults. However, females were impaired in their reproductive ability when fed on nonhost plants. Therefore, A. fragariae finished their development of larval stages on P. chinensis and came back to their primary host plant, Duchesnea indica, for feeding and reproduction after eclosion.     

The evolution of larval foraging behaviour in response to host plant variation in a leaf beetle

The evolutionary causes of variation in host specialization among phytophagous insects are still not well understood and identifying them is a central task in insect–host plant biology. Here we examine host utilization of the chrysomelid beetle Oreina elongata that shows interpopulation variation in the degree of specialization. We focus on larval behaviour and on what selection pressures may favour the use of two different larval host plants ( Adenostyles alliariae and Cirsium spinosissimum ) in one population as opposed to specialization onto one of them as is seen in other populations. The results suggest that the degree of exploratory foraging behaviour is higher in larvae from the two-host population than in single host populations, and a field survey of the two-host population also indicated that larvae do move between host species. A field experiment indicated that predation rates on O. elongata larvae in the two-host population are higher on one of the host species, A. alliariae , than on the alternative C. spinosissimum . In combination with earlier results this finding suggest that larvae move between hosts to obtain better food on one host, and to get better protection from predators on the other. It appears that in this two-host situation a single plant species does not provide the most beneficial conditions in all parts of O. elongata life cycle and individuals may obtain different plant-specific benefits by moving between host species. This heterogeneous host situation appears to have selected for the explorative larval foraging strategy seen in the in the two-host population. In general, the results support the notion that to understand patterns of host plant use in insects it is often vital to consider a range of host related selection pressures whose relative importance may vary between life stages of the insect.     

No impact of a native beetle on exotic plant performance and competitive ability due to plant compensation

The novel associations between invasive plants and their natural enemies in the introduced range have recently received increasing attention; however, the effects of novel enemies on exotic plant performance and competition with native species remain poorly explored. Here, we tested the impact of herbivory by a native beetle, Cassida piperata, on the performance of the exotic species Alternanthera philoxeroides and competition with a native congener, Alternanthera sessilis, using common garden experiments in central China. We found A. philoxeroides was able to fully compensate for intense herbivory by C. piperata. Herbivory by C. piperata that released at the average density in this region had no impact on competition between the native and exotic plant species. Our results indicate that herbivory by novel enemies may not reduce exotic plant performance due to plant compensation. However, high tolerance to herbivory may not confer a competitive advantage for exotic species compared to less tolerant native competitors if the herbivore damage is below a certain threshold. Thus, it is necessary to assess the impact of novel enemies on exotic plant performance and competition with native plants along gradients of insect densities. This may lead to a better understanding of how best to exploit the role of native herbivores in facilitating or slowing plant invasions.     

Variation in defences of the plant Barbarea vulgaris and in counteradaptations by the flea beetle Phyllotreta nemorum

Several sorts of variation in the interaction between the insect, Phyllotreta nemorum L. (Coleoptera:Chrysomelidae:Alticinae), and the plant, Barbarea vulgaris R.Br. (Brassicaceae), have been discovered: 1) genetic differences in the levels of defences in the plant, 2) genetic differences in the ability of insects to cope with the plant defences, 3) seasonal variation in levels of defences in the plant, and 4) differences between leaf types in levels of defences.Two plant accessions were suitable for larval development throughout the season while the remaining nine accessions were more or less unsuitable for larvae from the susceptible T-population at least at certain times of the year. All accessions were suitable for the resistant E-population throughout the year. There was a seasonal variation in levels of defences in some accessions which were unsuitable for the T-population during the summer period when beetles were present, but not during autumn and spring when the beetle were hibernating. Upper (younger) cauline leaves of these accessions had higher levels of defences than lower (older) cauline leaves. The resistant E-population used B. vulgaris as a natural host plant while the susceptible T-population did not. The use of B. vulgaris as a natural host plant by the E-population of P. nemorum seems to be an extension of the host plant range of the species. Variation in plant defences may have facilitated the switch in host plant use by the resistant flea beetle population.     

Constraints on host use by a parasitic plant

Consumers do not always utilize all suitable hosts. Understanding why parasitic plants do not always parasitize potentially suitable hosts requires a better understanding of the constraints that limit host use by parasitic plants. In Texas salt marshes, the parasitic plant Cuscuta indecora rarely parasitizes three hosts that support vigorous growth in the greenhouse. We identified three constraints on host use by C. indecora. First, a mismatch between the phenology of C. indecora and some suitable hosts meant that these hosts were not abundant when C. indecora was growing most vigorously, and therefore were underutilized. Second, C. indecora preferentially parasitized tall plants versus short ones, causing relatively short species to be underutilized. Third, C. indecora overwinters in some perennial hosts but has to reinfect annual hosts each year, causing annuals and perennials that do not support overwintering to be underutilized. In combination, these constraints, which reflect the general lack of mobility of parasitic plants relative to herbivores, remove half of the potential host species from the actual diet of C. indecora, and therefore likely represent a major limitation on the success of this parasite. Similar constraints are likely to limit the realized host range of many parasitic plants and select for generalized diet preferences.     

Response to host plant odors and aggregation pheromone by larvae of the Colorado potato beetle on a servosphere

Using a servosphere (locomotion compensator), locomotory behavior of Colorado potato beetle Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae) larvae was measured in detail in response to pulsed and non-pulsed odors of hostplant and conspecific pheromone. Second instars showed decreased straightness of movement, and all larvae showed decreased speed, in response to potato odor. Change in straightness by 2nd instars was also significantly affected by the interaction of pheromone and pulsing treatments. Fourth instars showed increased positive anemotaxis in response to the combined hostplant and pheromone odors. To our knowledge, this is the first demonstration of larval behavioral responses to adult pheromone in a holometabolous insect.     

Masking of host plant odour in the olfactory orientation of the Colorado potato beetle

The odour of potato plants Solanum tuberosum L., elicits a true odour-conditioned positive anemotaxis in the Colorado potato beetle, Leptinotarsa decemlineata Say. Blending the odour of non-host plant species, namely wild tomatoes Lycopersicon hirsutum f. glabratum C.H. Mull or cabbage Brassica oleracea L. var. gemmifera DC., with the attractive host plant odour blocks the release of upwind responses in non-experienced as well as experienced females. The neutralization of the beetle's orientation responses is obtained without repellency. It is expected that masking of host plant odour occurs often in mixed cropping systems.

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Résumé Un certain nombre d'articles rapporte une diminution du nombre des insectes phytophages spécialistes comme une conséquence à la diversification des agroécosystèmes (Altieri & Letourneau, 1982; Cromartie, 1981; Kareiva, 1983; Risch et al., 1983). Ainsi, il a été supposé que la présence de plantes non-hôtes pouvait interférer avec celle de plantes hôtes en modifiant la nature des informations chimiques parvenant aux insectes. Les modifications comportementales qui peuvent en résulter lors de l'attraction à distance sont toutefois encore peu claires. Les expérimentations présentées ici analysent les réponses individuelles du doryphore à diverses stimulations olfactives issues de plantes entières. Des femalles ont été ainsi étudiées un jour après l'émergeance en présence d'air vierge et d'air chargé d'odeurs de Solanum tuberosum, Lycopersicon hirsutum f. glabratum, Brassica oleracea var. gemmifera, ainsi que des mélanges, S. tuberosum avec L. hirsutum et S. tuberosum avec B. oleracea. A partir de l'enregistrement continu de l'activité locomotrice dans chaque condition (à l'aide du compensateur de locomotion), nous avons déterminé que L. hirsutum et B. oleracea masquaient l'odeur de la plante hôte S. tuberosum en supprimant la réponse anémotactique positive conditionnée par l'odeur de cette dernière. Les mélanges d'odeurs ainsi obtenus sont considérés comme neutres pour l'orientation à longue distance du doryphore. L'expérience préalable par le doryphore de l'ingestion de feuillage de pomme de terre ne modifie pas l'effet du camouflage obtenu. Ces résultats laissent penser qu'il est possible de limiter la découverte de la plante hôte chez les insectes, et que les méthodes de camouflages olfactives pourraient prévenir les attaques de certains ravageurs.

     

Pleiotropic effects associated with an allele enabling the flea beetle <Emphasis Type="Italic">Phyllotreta nemorum </Emphasis>to use <Emphasis Type="Italic">Barbarea vulgaris</Emphasis> as a host plant

In the Danish region of Kværkeby, a mutation in an, as yet, unknown single autosomal gene has resulted in a dominant resistance (R-) allele in the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae). It enables the beetle to overcome the defences of Barbarea vulgaris ssp. arcuata (Opiz.) Simkovics G-type (Brassicaceae) and use it as a host plant. In this study, we investigated the pleiotropic effects associated with the presence of this particular R-allele in female P. nemorum. These females had the R-allele backcrossed into the genetic background of non-resistant beetles. The effects were investigated under both favourable and stressful conditions (cold shock). The presence of the R-allele in a non-resistant genetic background caused a very high mortality in resistant individuals during the early stages of development under both conditions, but it did not affect the adult life-history traits longevity, body size and fecundity, under both conditions. Regardless of temperature treatment, resistant females in general were found to lay significantly more eggs. Developmental stability, as measured by tibia length fluctuating asymmetry, was not correlated with overall developmental stress in this study.     

Divergent host plant specialization as the critical driving force in speciation between populations of a phytophagous ladybird beetle

Detecting the isolating barrier that arises earliest in speciation is critically important to understanding the mechanism of species formation. We tested isolating barriers between host races of a phytophagous ladybird beetle, Henosepilachna diekei (Coleoptera: Coccinellidae: Epilachnine), that occur sympatrically on distinct host plants. We conducted field surveys for the distribution of the beetles and host plants, rearing experiments to measure six potential isolating factors (adult host preference, adult and larval host performance, sexual isolation, egg hatchability, F(1) hybrid inviability, and sexual selection against F(1) hybrids), and molecular analyses of mitochondrial ND2 and the nuclear ITS2 sequences. We found significant genetic divergence between the host races, and extremely divergent host preference (i.e. habitat isolation) and host performance (i.e. immigrant inviability), but no other isolating barriers. The fidelity to particular host plants arises first and alone can prevent gene flow between differentiating populations of phytophagous specialists.     

Divergence in host use ability of a marine herbivore from two habitat types

We studied ecological divergence of host use ability in a generalist marine herbivore living in two distinct host plant assemblages. We collected Idotea balthica isopods from three populations dominated by the brown alga Fucus vesiculosus and three dominated by the seagrass Zostera marina. In two reciprocal common garden feeding experiments for adult and laboratory‐born juvenile isopods, we found that isopods from both assemblages performed better with their sympatric dominant host species than did isopods allopatric to this host. This indicates parallel divergence of populations according to the sympatric host plant assemblage. Furthermore, initial body size and body size‐dependent mortality differed between populations from the two assemblages. In nature, this may result in lower fitness of immigrants compared with that of residents and consequently reinforce divergence of the populations. Finally, we discuss how phenotypic plasticity and maternal and random effects may associate with the results.     

Geographic patterns in the flight ability of a monophagous beetle

Summary The migratory ability of a monophagous insect, Tetraopes tetraophthalmus (Coleoptera: Cerambycidae), was measured in ten populations in each of two regional types. In the first region, a river valley, populations were abundant and close together; in the second regional type, mountainous and peripheral areas, populations were uncommon and far apart. The data revealed that regional variation in migratory behavior occurred along two dimensions. Regions differed not only in the mean migratory ability of individuals (1st dimension) but in the amount of variation between populations (2nd dimension). Whereas river valley populations were found to be uniformly sedentary, mountainous and peripheral populations were found to be heterogeneous, some being very vagile, others being very sedentary. In the mountainous and peripheral areas, small and sparse populations were more vagile than large dense populations. The site to site variation in the mountainous and peripheral areas was due exclusively to females. In these areas, the mean migratory ability of females in a population was positively correlated with mean body size of the females, and inversely correlated with populations density. The data are consistent with the FLY hypothesis for this species (long distance migraters=Females, Large, and Young). Taken together, the geographical patterns support the founding hypothesis of migration. According to this theory, the inevitable selection against migration within populations is balanced by periodic extinctions of the populations and the founding of new populations by migrating individuals.     

植物提取物对黄曲条跳甲成虫毒作用的研究

研究24种植物提取物对黄曲条跳甲成虫的毒杀活性.结果表明处理后72h,供试植物提取物中的7种对黄曲条跳甲成虫有毒作用,但毒杀活性较低,其中烟碱对黄曲条跳甲成虫的毒杀效果最好,累积死亡率为3.33%;假莲翘Duranta repens 乙醇得取物对黄曲条跳甲成虫的毒杀效果最差,累积死亡率为1.12%;其余提取物对黄曲条跳甲成虫的毒杀效果居中,累积死亡率为1.67%-2.78%.因此,毒杀作用并不是供试植物提取物对黄曲条跳甲成虫的主要控制机理.     

Larval host plant affects fitness consequences of egg size variation in the seed beetle Stator limbatus

Egg size variation often has large effects on the fitness of progeny in insects. However, many studies have been unable to detect an advantage of developing from large eggs, suggesting that egg size variation has implications for offspring performance only under adverse conditions, such as during larval competition, periods of starvation, desiccation, or when larvae feed on low-quality resources. We test this hypothesis by examining the consequences of egg size variation for survivorship and development of a seed-feeding insect, Stator limbatus, on both a low-quality (Cercidium floridum) and a high-quality (Acacia greggii) host plant. Our results are consistent with the hypothesis. S. limbatus larval performance was affected by egg size only when developing on the poor-quality host (C. floridum); larvae from large eggs survived better on C. floridum than those from small eggs, while there was no evidence of an effect of egg size on progeny development time, body weight, or survivorship when larvae developed on A. greggii. These results indicate intense selection for large eggs within C. floridum-associated populations, but not in A. greggii-associated populations, so that egg size is predicted to vary among populations associated with different hosts. Our results also support this hypothesis; females from a C. floridum-associated population (Scottsdale) laid larger eggs than females from an A. greggii-associated population (Black Canyon City).