Phenotyping tree resistance to a bark‐chewing insect,the pine weevil Hylobius abietis

Breeding for resistance to forest pests and pathogens is emerging as a promising tool for minimising the impact of the increasing biotic threats that our forests are experiencing as a consequence of global change. Efficient phenotyping protocols of resistance are urgently needed. Here we present the results of two experiments aimed to determine whether the variation in resistance to the pine weevil Hylobius abietis, a harmful pest of European conifers, can be inferred by nondestructive bioassays using excised plant material collected in forest genetic trials. Weevil damage and amount of nonvolatile resin induced by weevil feeding were assessed in young trees and in branches of adult trees using several phenotyping procedures (bioassays using either living trees, excised plant material and cut stem twigs) on four pine species (Pinus pinaster, P. radiata, P. sylvestris and P. pinea). Half of the plants were previously induced with methyl jasmonate (MJ), a treatment that is known to affect resistance to the pine weevil. In Experiment 1, living and excised plants showed parallel results: MJ treatment significantly reduced weevil damage, and saplings responded to weevil damage locally increasing the nonvolatile resin (NVR) in the stems proportionally to the damage suffered. This response was, however, slightly lower in excised than in living saplings. On the contrary, patterns of weevil feeding on stem twigs completely departed from those observed in living and excised seedlings. Moreover, cut stem twigs were unable to respond to weevil feeding increasing NVR according to the weevil damage. In Experiment 2, assessment of weevil damage on excised branches explained around 50% of variation in damage on living branches. This relationship became much more pronounced (R² = 0.81) when explored at the mean treatment level; branch manipulation did not alter the patterns of variation in resistance across pine species or MJ treatments. Irrespective of the assessment procedure, MJ consistently decreased weevil damage in all pine species, with larger reduction in weevil damage in stone and maritime pine than in radiata and Scots pine. Radiata pine was the most resistant while Scots pine was the most susceptible to the pine weevil. Overall, results suggest that using excised plant material is an operative alternative for phenotyping weevil resistance whenever care is taken to maintain the functionality of the excised plant material. This will allow taking advantage of multiple available conifer genetic trials to deepen the ecological genetics of resistance to the pine weevil and to screen for resistance without compromising the long‐term utility of those genetic trials.     

松墨天牛在秦巴林区不同寄主上的危害规律

松墨天牛Monochamus alternatus是次期性蛀干害虫,也是松材线虫病的主要传播媒介,廓清该害虫在秦巴林区不同寄主上的危害程度和发生规律,对于控制松材线虫病的扩散蔓延具有重要意义.本研究对秦巴林区遭受松材线虫病严重危害的3种主要松树(油松Pinus tabuliformis、华山松P.armandii和马尾松P.massoniana)进行了冬季疫木野外解析,并在统计不同胸径、不同高度疫木树干上松墨天牛侵入孔和幼虫数量的基础上,采用聚集度指标分析方法,对该害虫在不同寄主上的危害差异进行了比较研究.结果表明,松墨天牛在3种寄主上的危害程度具有显著差异,以华山松受害最重,其次分别为马尾松和油松;越冬幼虫在油松上主要危害皮下1～2 cm的边材;在华山松和马尾松上危害比较多样,以髓心部位受害最重,其后依次为心材、边材和树皮;在油松和华山松上,松墨天牛主要在树干7m以下部分危害,而在马尾松上主要危害树干7m以上部分,且虫口数量均与寄主胸径呈显著正相关.松墨天牛侵入孔和越冬幼虫在不同寄主树干上均呈聚集性分布.本研究揭示了松墨天牛在秦巴林区3种主要寄主上的危害规律,对于进一步开展大尺度地理范围松墨天牛的寄主选择性及危害规律研究提供了基础信息,也为秦巴林区松墨天牛的有效防治和松材线虫病的蔓延控制提供了新的重要信息.     

Testing associational resistance against pine weevils mediated by Lasius ants attending conifer seedlings

Natural enemies attracted to plants may provide those plants with protection against herbivores but may also protect neighbouring plants, that is through associational resistance. Ant attendance may be an important mechanism for the occurrence of such effects because ants can reduce the damage caused by a wide variety of herbivorous insects. Ants have been shown, in a previous field experiment, to decrease the damage caused by the pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae), a pest species that causes high seedling mortality in forest regeneration areas. In this study, we specifically tested whether seedlings planted close to ant‐attended seedlings experience associational resistance. We did this under laboratory conditions using the ant species Lasius niger (L.) (Hymenoptera: Formicidae). The feeding damage by pine weevils was significantly reduced on seedlings attended by ants. The neighbouring seedlings, however, did not experience associational resistance. Nevertheless, some associational effects were observed as the number of weevils recorded on both ant‐attended and neighbouring seedlings was significantly lower compared with ant‐excluded seedlings.     

Plant age, communication, and resistance to herbivores: young sagebrush plants are better emitters and receivers

Plants progress through a series of distinct stages during development, although the role of plant ontogeny in their defenses against herbivores is poorly understood. Recent work indicates that many plants activate systemic induced resistance after herbivore attack, although the relationship between resistance and ontogeny has not been a focus of this work. In addition, for sagebrush and a few other species, individuals near neighbors that experience simulated herbivory become more resistant to subsequent attack. Volatile, airborne cues are required for both systemic induced resistance among branches and for communication among individuals. We conducted experiments in stands of sagebrush of mixed ages to determine effects of plant age on volatile signaling between branches and individuals. Young and old control plants did not differ in levels of chewing damage that they experienced. Systemic induced resistance among branches was only observed for young plants. Young plants showed strong evidence of systemic resistance only if airflow was permitted among branches; plants with only vascular connections showed no systemic resistance. We also found evidence for volatile communication between individuals. For airborne communication, young plants were more effective emitters of cues as well as more responsive receivers of volatile cues.     

Rangewide variation of the maritime pine bast scale matsucoccus feytaudi duc. (Homoptera: matsucoccidae) in relation to the genetic structure of its host

The bast scale Matsucoccus feytaudi is a specific pest of maritime pine, but the damage inflicted by the insect on the host trees is variable, ranging from no apparent effect to severe decline of the maritime pine stands. Rangewide variation of mitochondrial DNA among M. feytaudi populations was analysed by polymerase chain reaction-restriction fragment length-single-strand conformation polymorphism (PCR-RFLP-SSCP) analysis and the results compared with the genetic information already available for its host. Three main nonoverlapping lineages can be distinguished in M. feytaudi. The phylogeography of the pest population is clearly related to the history of its host. Most local associations could result from common evolution while others must be interpreted as intraspecific host shifts. Because the distribution of cultivated tree species is greatly influenced by humans, much may be learned concerning their genetic structure from the indirect study of their specific pests.     

不同林龄马尾松人工林优势种径级结构及物种多样性分析

采用空间代替时间的方法,以黔中山地孟关林场10a、20a、30a和36a的马尾松人工林为研究对象,通过设置12块20 m×20 m的典型样地,分析了马尾松人工林发育过程中径级结构与物种多样性的动态特征。结果表明:(1)马尾松人工林不同林龄阶段,优势种径级结构存在一定差异,10a林分呈单峰型结构,20a、30a、36a林分呈多峰型结构。(2)林下共有植物种类35科53属53种,其中灌木16科24属24种,草本20科29属29种,物种较丰富;不同林龄林分物种组成种类为:20a林分10a林分30a林分36a林分。(3)随林龄增加,马尾松人工林物种丰富度指数先增大后减小,Shannon-Wiener指数和Simpson指数以及Pielou均匀度指数和Alatalo均匀度指数在10a～30a之间变化趋势相反,30a～36a之间变化趋势一致。     

Testing the enemies hypothesis in forest stands: the important role of tree species composition

Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth (Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch–pine mixtures than in other stand types. In 2003, survival probability was lowest in birch–pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch–pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.     

不同松林马尾松毛虫蛹及其寄生天敌群子的空间格局分析

应用地学统计学的原理和方法研究了３种松柯马尾松毛虫越冬工蛹及其寄生天敌群子的窨结构和空间相关性。结果表明,湿地松－马尾松－火炬松混交松林中蛹的半变异函数曲线为球形,其空间格局为聚集型,受害程度高的马尾松纯林中,蛹的半变异函数曲线为指数形,空间格局为聚集型,受害程度低的纯马尾松林中,蛹的半变异函数曲线是一条截距为０．４２３,斜率为０．００３８１４的直线,表明为随机或均匀型的空间格局。３种松林中蛹的寄     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

Spatial Pattern of Attacks of the Invasive Woodwasp Sirex noctilio,at Landscape and Stand Scales

Invasive insect pests are responsible for important damage to native and plantation forests, when population outbreaks occur. Understanding the spatial pattern of attacks by forest pest populations is essential to improve our understanding of insect population dynamics and for predicting attack risk by invasives or planning pest management strategies. The woodwasp Sirex noctilio is an invasive woodwasp that has become probably the most important pest of pine plantations in the Southern Hemisphere. Our aim was to study the spatial dynamics of S. noctilio populations in Southern Argentina. Specifically we describe: (1) the spatial patterns of S. noctilio outbreaks and their relation with environmental factors at a landscape scale; and (2) characterize the spatial pattern of attacked trees at the stand scale. We surveyed the spatial distribution of S. noctilio outbreaks in three pine plantation landscapes, and we assessed potential associations with topographic variables, habitat characteristics, and distance to other outbreaks. We also looked at the spatial distribution of attacked trees in 20 stands with different levels of infestation, and assessed the relationship of attacks with stand composition and management. We found that the spatial pattern of pine stands with S. noctilio outbreaks at the landscape scale is influenced mainly by the host species present, slope aspect, and distance to other outbreaks. At a stand scale, there is strong aggregation of attacked trees in stands with intermediate infestation levels, and the degree of attacks is influenced by host species and plantation management. We conclude that the pattern of S. noctilio damage at different spatial scales is influenced by a combination of both inherent population dynamics and the underlying patterns of environmental factors. Our results have important implications for the understanding and management of invasive insect outbreaks in forest systems.     

Tree species identity and forest composition affect the number of oak processionary moth captured in pheromone traps and the intensity of larval defoliation

1. Pure forests are often seen as being more prone to damage by specialist pest insects than mixed forests, and particularly mixed forests associating host and nonhost species. We addressed the effect of tree diversity on oak colonization and defoliation by a major specialist pest, the oak processionary moth (OPM)
2. We quantified the number of male OPM moths captured and larval defoliation in pure stands of two oak host species (Quercus robur and Quercus petraea) and in mixed stands associating the two oak species or each oak species with another nonhost broadleaved species. We conducted two complementary studies to test the effect of host species and stand composition: (i) we used pheromone trapping to compare the number of males OPM captured throughout the distribution of oak hosts in France and (ii) we noted the presence of OPM nests and estimated defoliation in mature forests of north‐eastern France.
3. Oak species and stand composition significantly influenced the number of male OPM captured and defoliation by OPM larvae. Quercus petraea was consistently more attractive to and more defoliated by OPM than Q. robur. Both oak trees were attacked more in pure stands than in mixed stands, in particular mixed stands associating oaks with another (nonhost) broadleaved species.
4. The results of the present study support the view that mixed forests are more resistant to specialist pest insects than pure stands, and also indicate that this trend depends on forest composition. Our study provides new insights into OPM ecology and has potential implications for forest management, including the management of urban forests where OPM causes serious human health issues.

     

Performance of the potentially invasive Siberian moth Dendrolimus superans sibiricus on coniferous species in Europe

1 The native range of the Siberian moth extends from the Pacific Ocean (Russian Far East, Japan and Northern Korea) across Siberia, Northern China and Mongolia to the Ural Mountains. At the beginning of the 21st Century, this species was documented west of the Ural Mountains in the Republic of Mari El, indicating range extension toward the west.
2 The Siberian moth has recently been suggested for regulation as a quarantine pest for European and Mediterranean Plant Protection Organization member countries. However, no specific report on European host plants for this pest has been published so far.
3 In the present study, larval host plant choice and performance was tested for the first time on coniferous tree species that are widely distributed and of commercial value in Europe.
4 Based on dual-choice tests on neonates and mortality, developmental duration and relative growth rates of the first- to sixth-instar larvae, we found European larch Larix decidua to be the most suitable host for the moth larvae, whereas European black pine Pinus nigra and Scots pine Pinus sylvestris were the poorest hosts. The remaining conifer species tested, European silver fir Abies alba , Nordmann fir Abies nordmanniana , and Norway spruce Picea abies , were intermediate host plants. Douglas-fir Pseudotsuga menziesii , originating from North America, was chosen by the larvae to the same extend as European larch, and was also highly suitable for larval development.
5 If the moth is introduced to European countries, it will become damaging in stands of European larch and Douglas-fir, mixed stands of fir and spruce; however, it will be less damaging in forests dominated by two-needle pines.
6 We predict that Dendrolimus superans sibiricus will be able to survive and develop on the main European coniferous tree species, including non-native coniferous tree species, resulting in severe damage to large areas of forests.     

Genetic variation in resistance of Norway spruce seedlings to damage by the pine weevil <Emphasis Type="Italic">Hylobius abietis</Emphasis>

Regeneration of northern conifer forests is commonly performed by reforestation with genetically improved materials obtained from long-term breeding programs focused on productivity and timber quality. Sanitary threats can, however, compromise the realization of the expected genetic gain. Including pest resistance traits in the breeding programs may contribute to a sustainable protection. Here we quantified the variation in different components of resistance of Norway spruce to its main pest, the pine weevil Hylobius abietis. We followed insect damage in two large progeny trials (52 open-pollinated families with 100–200 individuals per family and trial) naturally infested by the pine weevil. Pine weevils damaged between 17 and 48% of the planted seedlings depending on the trial and year, and mortality due to weevil damage was up to 11.4%. The results indicate significant genetic variation in resistance to the pine weevil, and importantly, the variation was highly consistent across trials irrespective of contrasting incidence levels. Individual heritability estimates for the different components of seedling resistance were consistently low, but family heritabilities were moderate (0.53 to 0.81). While forward selections and breeding for higher resistance seem not feasible, backwards selections of the best parent trees emerge as a putative alternative to reduce weevil damage. A positive genetic correlation between early growth potential and probability of being attacked by the weevil was also observed, but the relationship was weak and appeared only in one of the trials. Overall, results presented here open the door to a new attractive way for reducing damage caused by this harmful pest.     

Larval parasitism of a forest pest, the common pine sawfly Diprion pini (L.) (Hym., Diprionidae), during an endemic density phase

Abstract: Species composition and impact of larval parasitoids during an endemic density phase of a pine forest pest, the common pine sawfly Diprion pini (L.), were investigated by exposure of laboratory-reared larvae of the sawfly to parasitoid attack in natural habitats. Colony-associated host larvae of different instars were exposed continuously from early June until end of September of the years 1994 and 1995 in two pine stands with a different history of pine sawfly outbreaks. Host larvae were exchanged after a defined time interval and those collected up were examined in the laboratory for parasitism. Parasitoid assemblages were shown to be very species poor at both pine stands. The dominant species were the ichneumonid Olesicampe macellator and the tachinid Drino gilva , which reached attack rates on the collected host larvae of 60 and 94%, respectively, during peak activity. The patterns of temporal attack, of multiparasitism, and of superparasitism of these dominant species were studied. In an additional experiment, the response of these species to different host-patch sizes (number of host larvae per colony) were investigated. No aggregative response to higher host-patch densities could be detected. The role and function of these larval parasitoids in the population dynamics of D. pini are discussed.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

Plant resistance and its effect on the peritrophic membrane of southwestern corn borer (Lepidoptera: Crambidae) larvae

The southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a serious pest of corn, Zea mays L., in the southern United States. Corn germplasm lines with conventional genetic leaf-feeding resistance to this pest, the fall armyworm, Spodoptera frugiperda (J.E. Smith), and other lepidopterans have been released to the public by USDA-ARS scientists located in Mississippi. Recent studies suggest the insect resistant lines disrupt the integrity of the peritrophic membrane of the fall armyworm. The objectives of this study were to investigate any morphological differences in the structure of the peritrophic membrane of southwestern corn borer larvae feeding on resistant and susceptible corn hybrids and to quantify the damage. Larvae were reared under field and laboratory conditions on three corn hybrids (two resistant and one susceptible). Scanning electron microscopy was used to examine the peritrophic membrane for abnormalities such as holes or tears and to count the holes or tears in the membrane. Differences in the degree of damage to peritrophic membrane of larvae fed on resistant and susceptible plants were not detected. Up to five distinct layers of the membrane were observed in each larva. Variation in the amounts of damage to the peritrophic membrane observed from larvae feeding on all plant material was high. Plant resistance adversely affects growth and development of southwestern corn borer larvae, and further investigations are needed to explain the role of plant resistance and its relation to peritrophic membrane in southwestern corn borer larvae.     

Tritrophic Choice Experiments with Bt Plants,the Diamondback Moth (Plutella xylostella) and the parasitoid Cotesia plutellae

Parasitoids are important natural enemies of many pest species and are used extensively in biological and integrated control programmes. Crop plants transformed to express toxin genes derived from Bacillus thuringiensis (Bt) provide high levels of resistance to certain pest species, which is likely to have consequent effects on parasitoids specialising on such pests. A better understanding of the interaction between transgenic plants, pests and parasitoids is important to limit disruption of biological control and to provide background knowledge essential for implementing measures for the conservation of parasitoid populations. It is also essential for investigations into the potential role of parasitoids in delaying the build-up of Bt-resistant pest populations. The diamondback moth (Plutella xylostella), a major pest of brassica crops, is normally highly susceptible to a range of Bt toxins. However, extensive use of microbial Bt sprays has led to the selection of resistance to Bt toxins in P. xylostella. Cotesia plutellae is an important endoparasitoid of P. xylostella larvae. Although unable to survive in Bt-susceptible P. xylostella larvae on highly resistant Bt oilseed rape plants due to premature host mortality, C. plutellae is able to complete its larval development in Bt-resistant P. xylostella larvae. Experiments of parasitoid flight and foraging behaviour presented in this paper showed that adult C. plutellae females do not distinguish between Bt and wildtype oilseed rape plants, and are more attracted to Bt plants damaged by Bt-resistant hosts than by susceptible hosts. This stronger attraction to Bt plants damaged by resistant hosts was due to more extensive feeding damage. Population scale experiments with mixtures of Bt and wildtype plants demonstrated that the parasitoid is as effective in controlling Bt-resistant P. xylostella larvae on Bt plants as on wildtype plants. In these experiments equal or higher numbers of parasitoid adults emerged per transgenic as per wildtype plant. The implications for integrated pest management and the evolution of resistance to Bt in P. xylostella are discussed.     

转抗虫基因植物生态安全性研究进展

转抗虫基因植物如Bt棉花等已在美国、中国和澳大利亚等国家大规模商业化种植 ,有关转抗虫基因植物潜在的生态风险已引起广泛的关注。该文综述了转抗虫基因植物研究应用现状与安全性研究进展。主要内容包括 :转抗虫基因植物的种类及其对靶标害虫的抗性 ,对非靶标害虫和天敌发生的影响 ,对农田生态系统生物多样性的影响 ,靶标昆虫的抗性治理及转抗虫基因植物的基因漂移等     

Vacuum infiltration transformation of non-heading Chinese cabbage (Brassica rapa L. ssp. chinensis) with the pinII gene and bioassay for diamondback moth resistance

Non-heading Chinese cabbage (Brassica rapa L. ssp. chinensis) is a popular vegetable in Asian countries. The diamondback moth (DBM), Plutella xylostella (L.), an insect with worldwide distribution, is a main pest of Brassicaceae crops and causes enormous crop losses. Transfer of the anti-insect gene into the plant genome by transgenic technology and subsequent breeding of insect-resistant varieties will be an effective approach to reducing the damage caused by this pest. We have produced transgenic non-heading Chinese cabbage plants expressing the potato proteinase inhibitor II gene (pinII) and tested the pest resistance of these transgenic plants. Non-heading Chinese cabbages grown for 45 days on which buds had formed were used as experimental materials for Agrobacterium-mediated vacuum infiltration transformation. Forty-one resistant plants were selected from 1166 g of seed harvested from the infiltrated plants based on the resistance of the young seedlings to the herbicide Basta. The transgenic traits were further confirmed by the Chlorophenol red test, PCR, and genomic Southern blotting. The results showed that the bar and pinII genes were co-integrated into the resistant plant genome. A bioassay of insect resistance in the second generation of individual lines of the transgenic plants showed that DBM larvae fed on transgenic leaves were severely stunted and had a higher mortality than those fed on the wild-type leaves.     

Plant neighbour identity and invasive pathogen infection affect associational resistance to an invasive gall wasp

Theory predicts that mixed forests are more resistant to native pests than pure forests (i.e. associational resistance) because of reduced host accessibility and increased top-down control by natural enemies. Yet, whether the same mechanisms also apply to invasive pests remains to be verified. We tested the hypothesis of associational resistance against the invasive Asian chestnut gall wasp (ACGW, Dryocosmus kuriphilus) by comparing ACGW infestation rates on chestnuts (Castanea sativa) in stands varying in species composition (chestnut alone or associated with oaks, pines or ashes). We investigated the effects of reduced chestnut density and frequency in mixed stands, as well as the effect of biotic interactions between ACGW, its parasitoids and the chestnut blight disease (caused by Cryphonectria parasitica). ACGW infestation rates were significantly lower in chestnut–oak and chestnut–ash mixtures than in pure chestnut stands and chestnut–pine mixtures. Infestation rate decreased with decreasing chestnut relative proportion. The composition of native parasitoid communities emerged from galls significantly differed between pure and mixed chestnut stands, but not the species richness or abundance of parasitoids. The abundance of the introduced parasitoid Torymus sinensis was not correlated with ACGW infestation rates and was independent of stand composition. Blight symptoms modified ACGW infestation rates with taller trees being preferred when they were asymptomatic but avoided when they presented blight disease damage. Our results suggest that conservation biological control based on tree species mixtures could contribute to reducing the damage of invasive forest pests.