Screening for environmental stress-tolerant entomopathogenic nematodes virulent against cotton bollworms

A total of 27 entomopathogenic nematode (EPN) strains originally isolated from different cotton fields were characterized in laboratory experiments for their virulence, reproductive potential and environmental tolerance. The EPN strain collections included 16 Steinernema carpocapsae (SC), three Steinernema siamkayai (SS), one Steinernema monticolum (SM), and seven Heterorhabditis bacteriophora (HB). Their virulence was tested against cotton bollworms such as the American bollworm Helicoverpa armigera, the spotted bollworm Earias vittella and the cotton leafworm Spodoptera litura. Larvae of H. armigera, E. vittella and S. litura were susceptible to all the tested EPN species/strains with significant differences among EPN species/strains. The most virulent strains were APKS2 (SC), TRYH1 (HB) KKMH1 (HB) and KKMH2 (HB) on H. armigera (91.9–93.5% mortality); KKMS1 (SC), APKS2 (SC), TRYH1 (HB), KKMH1 (HB), KKMH2 (HB) and APKH1 (HB) on E. vittella (92.7% mortality); and APKS2, TRYH1, KKMH1, KKMH2 and KKMH3 on S. litura (92.7% mortality). The results of the invasion rate assay indicated that the EPN strains more virulent against the target host had greater invasion rates. In the multiplication assay, KKMH1and OCMS1 (SC) produced a greater number of infective juveniles (IJs) (32.1–32.4 x 1000 IJs/ cadaver) in Carcyra cephalonica. Test for tolerance to heat at 40°C for 2 h revealed that KKMH1, TRYH1, KKMH2, KKMS1 and APKS2 were highly tolerant (>85% survival). IJs exposed to ambient room conditions (27–29°C; 65–70% r.h.) for 2 h showed that APKS2, OCMS1 and KKMS1 were more tolerant (68–69% survival) of rapid desiccation than others. APKS2, KKMS1 and KKMH1 showed better survival (70–73%) in slow desiccation assay when exposed to 25°C with 97% r.h. for 72 h, followed by 25°C with 93% r.h. for 48 h. The H. bacteriophora KKMH1 and S. carpocapsae APKS2 performed best in nine traits out of ten tested, followed by H. bacteriophora TRYH1, which performed best for six traits. It is suggested that the EPN strains KKMH1 and APKS2 could be deployed for a cotton bollworm management program after testing their performance under field conditions.     

Designing the ideal habitat for entomopathogen use in nursery production

BACKGROUND: Greenhouse and nursery producers use entomopathogens (nematodes and fungi) to control soil pests. Although it is known that the physical and chemical properties of mineral soil significantly impact upon soil pathogens, the influence of soilless media used for plant production on entomopathogen performance is poorly understood. RESULTS: Survival and foraging distance were differently affected by sand:peat, bark and sawdust media for entomopathogenic nematodes, but not for the immobile fungus Metarhizium anisopliae. Redwood sawdust medium consistently had a negative impact upon entomopathogenic nematodes. Dividing media into individual components supported the hypothesis that redwood sawdust reduced foraging and infection abilities of S. riobrave and H. bacteriophora. Physically altering the components by adding sand significantly improved foraging and infection success for S. riobrave in media not optimum for foraging. CONCLUSION: This study is the first to highlight the importance of selecting the appropriate soilless media and pathogen species combinations to increase efficacy of biological control. H. bacteriophora was able to find hosts in a wider diversity of medium components than S. riobrave, although both nematode species performed well in peat moss and recycled plant material. These results suggest that peat moss, recycled plant material and hardwood bark are components amenable to EPN biological control programs. Copyright © 2012 Society of Chemical Industry     

昆虫病原线虫生物防控草地贪夜蛾的室内及田间效果和影响因素

联合国粮食及农业组织2018年向全球发布草地贪夜蛾Spodoptera frugiperda是重要的预警害虫。据报道,2019年我国已有26个省（区、市）的作物受到草地贪夜蛾为害。目前已经发现草地贪夜蛾幼虫对某些化学农药产生抗性的现象,为缓解其抗药性产生,具有主动寻找寄主害虫的昆虫病原线虫为草地贪夜蛾的生物防治提供了应用潜能。该文从昆虫病原线虫高效侵染种（品系）及其浓度、共生菌、草地贪夜蛾幼虫易感龄期、田间致死力及影响防控效果的因素等方面进行综述,以期为田间有效利用昆虫病原线虫防治草地贪夜蛾提供参考。     

Biological control potential of Turkish entomopathogenic nematodes against the Mediterranean fruit fly <Emphasis Type="Italic">Ceratitis capitata</Emphasis>

The entomopathogenic nematodes Steinernema weiseri, S. feltiae, S. carpocapsae and two strains of Heterorhabditis bacteriophora, isolated from Turkish soils, were evaluated against larvae of the Mediterranean fruit fly (medfly) Ceratitis capitata in plastic cups under laboratory conditions with sandy loam soil and 10% moisture level. At a rate of 100 infective juveniles (IJs)/cm², the last instar larvae of C. capitata were susceptible to the entomopathogenic nematodes: the S. feltiae 09-31 strain recovered from Aydin provided 78% mortality, whereas S. weiseri and S. carpocapsae killed 50% and 56% of the larvae, respectively. Both strains of H. bacteriophora species caused less than 50% mortality. Except for S. feltiae, the majority of infected medflies died as prepupae or pupae within the puparia. More than 90% larval mortality was recorded at 200 and 400 IJs/cm² for S. feltiae. None of the nematode isolates infected the medfly pupae within the puparia. In pot experiments containing soil, S. feltiae caused 96% and 97% mortality at 100 and 200 IJs/cm², respectively. In pot experiments with grass present, more than 94% mortality was obtained in the presence of grass roots.     

Early timing and new combinations to increase the efficacy of neonicotinoid-entomopathogenic nematode (Rhabditida: Heterorhabditidae) combinations against white grubs (Coleoptera: Scarabaeidae)

BACKGROUND: An investigation was carried out to determine whether new neonicotinoid-nematode combinations and earlier applications against younger larval stages could increase the efficacy of synergistic neonicotinoid-entomopathogenic nematode combinations against white grubs. RESULTS: In the laboratory, combinations of the neonicotinoids imidacloprid and clothianidin and the nematodes Heterorhabditis bacteriophora Poinar and H. zealandica Poinar against third instars were compared. In Anomala orientalis (Waterhouse) and Popillia japonica Newman, H. bacteriophora-imidacloprid combinations showed the most consistent synergism but did not cause significantly higher mortality than H. zealandica-imidacloprid combinations. In Cyclocephala borealis Arrow, there was no clear trend as to which combinations caused the most consistent synergism, but H. zealandica-imidacloprid combinations tended to cause the highest mortalities. In the laboratory, imidacloprid-H. bacteriophora combinations provided more consistent synergism against third-instar than against second-instar A. orientalis, but mortality was higher in second instars. In field experiments, imidacloprid-H. bacteriophora combinations against A. orientalis and P. japonica provided more consistent synergism when applied in mid-September but more consistent control when applied in late August. CONCLUSIONS: Imidacloprid is a better synergist for entomopathogenic nematodes than clothianidin. Imidacloprid-nematode combinations are more effective against second instars than against third instars, allowing rate reductions of both agents to make this approach more competitive with full-rate applications of neonicotinoid alone.     

Steinernematid and heterorhabditid nematodes for biological control of the giant looper,Boarmia selenaria

The pathogenicity of steinernematid and heterorhabditid nematodes to caterpillars of the giant looper,Boarmia selenaria (Schiffermüller) (Lepidoptera: Geometridae), was determined under laboratory conditions. High mortality (over 80%) was achieved, in a petri dish bioassay, with a minimum of 20,000 infective juveniles (IJ) ofSteinernema carpocapsae (Filipjev) (=Neoaplectana carpocapsae Weiser), strain ‘All’. The LD₅₀ was 4250 IJ per caterpillar. Similar results were obtained withS. carpocapsae, strain ‘Mexican’,Steinernema sp., strain ‘CR’;Heterorhabditis bacteriophora, strain ‘HP88’; andHeterorhabditis sp., strain ‘IS’. The first and second instars were the most susceptible stages to nematode infection. A laboratory model assay indicated that 80–90% of the caterpillars were killed when they migrated to the soil to pupate; this occurred when IJ were applied to sandy and heavy soils at a concentration of 400 IJ/cm³ soil. A greenhouse test reconfirmed the effectiveness of utilizing the entomopathogenic nematodes in an “ambush” system against caterpillars migrating to the soil.     

The influence of herbicides on the viability of entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae)

Entomopathogenic nematodes (EPNs) are agents that can be used for the biological control of pests associated with pesticides in a tank mix. Compatibility studies need to be conducted to analyse which products are compatible with EPNs. The survival of infective juveniles (IJs) of four species of EPNs (Steinernema, Heterorhabditis) was determined after exposure to eight chemical herbicides. The effect of direct IJs exposure to herbicides for 1, 4 and 24 h was tested in a Petri dish at 15, 20 and 25 °C. The study showed that Steinernema kraussei was the most tolerant among the tested EPN species, while S. carpocapsae was the most sensitive to all tested herbicides. The lowest mortality of IJs was at 15 °C (19%). Our investigation showed, overall, the herbicides negatively affected EPN survival. The results confirmed that the compatibility is a species-specific characteristic, influenced by the temperature and time of exposure. Application of two different control ingredients (insecticide and herbicide) at the same time would reduce cost and time consumption in pest/weed control.     

Sensitivity of caterpillars of the pine tree lappet moth Dendrolimus pini to native isolates of entomopathogenic nematodes

Abstract

The pine tree lappet moth Dendrolimus pini (Lasiocampidae) is one of the most dangerous pests of pine Pinus sylvestris stands in Central Europe. To develop biological control method of this pest, the bioassay was made to assess the sensitivity of D. pini caterpillars to native EPMs strains representing two species: Steinernema feltiae and Heterorhabditis megidis in laboratory conditions. The results showed higher activity of S. feltiae strains which parasitized 86.7 - 100% of D. pini caterpillars, compared to the 20-100% of insects parasitized by H. megidis strain. Most nematodes were found in caterpillars treated with S. feltiae (in average 40 IJs/caterpillar), the least – in individuals parasitized by H. megidis (in average IJs/caterpillar). No effect of dose and temperature on parasitizm was found with S. feltiae strains while in H. megidis increased doses resulted in higher extensity of parasitism. The results indicate higher biological activity of S. feltiae strains and advisability of their further studies in field experiments.     

Evaluation of entomopathogenic fungi and a nematode against the soil-dwelling stages of the crane fly Tipula paludosa

BACKGROUND: Larvae of the crane fly Tipula paludosa (Diptera: Nematocera) are economically important pests of grasslands, nurseries and organically grown crops. This study was conducted to evaluate several entomopathogenic fungal strains, the entomopathogenic nematode Heterorhabditis bacteriophora and the organophosphate insecticide Dursban WG (75% chlorpyrifos) against larval stages of T. paludosa under laboratory and greenhouse conditions. RESULTS: Metarhizium robertsii strain V1005 was the most virulent, causing 100% larval mortality 4 weeks post-inoculation. Eight other M. robertsii strains and M. brunneum (ARSEF 3297) caused mortality ranging between 0 and 60%, whereas strains of Beauveria bassiana, Isaria fumosorosea and Lecanicillium longisporum were non-pathogenic to T. paludosa. In greenhouse tests, medium (2 × 10¹⁴ conidia ha⁻¹) and high (2 × 10¹⁶ conidia ha⁻¹) concentrations of V1005 gave higher larval mortalities (90 and 100%) than lower (2 × 10¹³ conidia ha⁻¹) concentrations (52 and 78%) at 4 and 8 weeks after treatment respectively. Dursban WG gave 100% larval mortality 4 weeks after treatment; H. bacteriophora UWS1 caused 28 and 65% mortality 4 and 8 weeks after treatment respectively. CONCLUSIONS: This study shows that M. robertsii V1005 has considerable potential for the control of T. paludosa, thereby reducing the use of chemical insecticides. Copyright © 2012 Society of Chemical Industry     

Toxicity of ddt and malathion to various larval stages of Mamestra brassicae L.

The toxicity of DDT and malathion to the larvae of Mamestra brassicae was determined following several methods of application. The toxicity (LD₅₀), expressed as μg insecticide per g of insect, did not change significantly between larval instars (a) when either insecticide was injected into fourth to sixth instars; (b) when DDT was applied in the food of fifth and sixth instars; or (c) when malathion was applied topically to second to sixth instars. Significant changes in toxicity were found between successive instars when DDT was applied topically, but there was no clear trend. When malathion was applied in the food, the fifth instars were more susceptible than the sixth instars; it was found that the former consumed a toxic dose of malathion at a greater rate, and that probably malathion was degraded in the gut at a slower rate. In a contact test, the first to third instars were far more susceptible than the later instars to malathion; with DDT this trend was much less marked. Uptake studies with [¹⁴C]malathion showed that differences in the contact toxicity of malathion between instars could be explained, at least partly, by the decline in uptake per unit weight with increasing larval size.     

Comparative efficacy of indigenous heterorhabditid nematodes from north western Himalaya and Heterorhabditis indica (Poinar,Karunakar & David) against the larvae of Helicoverpa armigera (Hubner)

The efficacy of three Heterorhabditis spp of entomopathogenic nematodes (EPNs) from north western Himalaya, India was studied against the larvae of pod borer, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), under the laboratory conditions. The larvae were exposed to 10, 20, 30 and 40 infective juveniles (IJs) of each nematode species for different time periods and they were found to be susceptible to all the EPNs tested. However, the susceptibility of larvae to nematode infection varied according to the concentrations of IJs and their exposure periods. The efficacy of these indigenous entomopathogenic nematodes was also compared against commercially available entomopathogenic nematode, H. indica. Appreciably good performance was achieved by H. bacteriophora (HRJ), which showed 73.3% mortality of insect larvae in 96 h exposure time against third instar larvae, while H. indica produced 80.0% mortality. However it was noticed that with the advancement of larval stage its mortality rate reduced and vice versa with the exposure period. All tested EPNs were also found to reproduce within the host and maximum production of IJs was recorded in H. bacteriophora (26.0 ± 3.76 × 10³ IJs/larva) at the concentration of 40 IJs/larva.     

Virulence and efficacy of different entomopathogenic nematode species against western flower thrips (Frankliniella occidentalis)

Virulence and efficacy of five species and strains of the entomopathogenic nematodes of the families Steinernematidae and Heterorhabditidae:Steinernema riobravis, Steinernema feltiae strains Ger. and UK, andHeterorhabditis bacteriophora strains HP88 and IS5, against the prepupal and pupal stages of the western flower thrips (WFT),Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), were investigated in the laboratory. Although all these nematodes controlled WFT to some extent, they differed in efficiency. The heterorhabditid nematodeH. bacteriophora strain HP88 was more specific to the soil-inhabiting WFT stages (36–49% thrips mortality). The steinernematid nematodesS. riobravis andS. feltiae strains Ger. and UK had only a slight effect (10% mortality) on prepupal and pupal populations of WFT, andH. bacteriophora strain IS5 had the least effect of all. A possible reason for such species variation is suggested and discussed.     

Effect of rapid and gradual increase of osmotic stress on survival of entomopathogenic nematodes

The effect of rapid and gradual exposure of entomopathogenic nematodes to osmotic stresses on the induction of a dormant state was determined with the nematodeSteinernema feltiae IS-6 infective juveniles (IJs). Rapid exposure of nematodes to glycerol at concentrations of 24% and 28% (w/w) caused the nematodes to enter a dormant state which was characterized by shrinking and impeded motility of all nematodes within 8 h. However, pre-exposure to gradually increasing glycerol concentrations of 5%, 10% and 18% at 4-h intervals resulted in dormancy after 4 h exposure to 24% glycerol. The total time of exposure to glycerol solution was 16 h in gradual osmotic stress. For nematodes exposed to 24% glycerol solution either rapidly or gradually, recovery occurred after 40 min in distilled water. Infectivity of osmotically stressedS. feltiae IJs was evaluated by two criteria, insect mortality and invasion rate. The assays indicated that infectivity of nematodes desiccated by rapid and gradual osmotic stresses was similar to that of fresh nematodes. Rapid exposure ofS. carpocapsae ‘All’,S. riobravis ‘Texas’,S. glaseri ‘NI’ andHeterorhabditis bacteriophora HP88 IJs to the 24% glycerol solution resulted in dormancy within 8 h. These treatments caused mortality of 48.4% and 11.7% amongS. glaseri Nl andH. bacteriophora HP88 IJs, respectively. Similar effects were observed when these nematode species were exposed to increasing osmotic stress of 5%, 10% and 18% at 6-h intervals. Under these same conditions, mortality ofH. bacteriophora HP88 andS. glaseri Nl IJs was 27.5% and 61.8%, respectively. http://www.phytoparasitica.org posting Sept. 29, 2004.     

Entomopathogenic fungi and nematodes against larvae of the chestnut weevil,Curculio elephas (Coleoptera: Curculionidae): a laboratory evaluation

Pest control in chestnut orchards is mainly achieved by chemicals, but there is growing focus on the potential use of biological control agents. The larval susceptibility of the chestnut weevil Curculio elephas L. (Coleoptera: Curculionidae) to different strains/species of entomopathogenic fungi and nematodes was evaluated in laboratory by soil cup bioassays. In the experiments with fungi, a wild strain of Metarhizium anisopliae caused the highest larval mortality (about 90%) compared with a commercial and a wild strain of Beauveria bassiana (80% and 77% respectively). Regarding nematodes, the commercial strain of Heterorhabditis bacteriophora was more effective in the control of chestnut larvae (77% insect mortality) than Steinernema carpocapsae (43%).     

PCR‐based identification of cacao black pod causal agents and identification of biological factors possibly contributing to Phytophthora megakarya's field dominance in West Africa

Among the Phytophthora species that cause black pod of cacao, P. megakarya is the most virulent, posing a serious threat to cacao production in Africa. Correct identification of the species causing the black pod and understanding the virulence factors involved are important for developing sustainable disease management strategies. A simple PCR‐based species identification method was developed using the species‐specific sequences in the ITS regions of the rRNA gene. A phylogenetic tree generated for 119 Phytophthora isolates, based on the 60S ribosomal protein L10 gene and rDNA sequence, verified the PCR‐based identification assay and showed high interspecific variation among the species causing black pod. Phytophthora megakarya isolates were uniformly virulent in an assay using susceptible cacao pod husks inoculated with zoospores, while the P. palmivora isolates showed greater divergence in virulence. The virulence of P. megakarya was associated with earlier production of sporangia and an accelerated induction of necrosis. While zoospore germ tubes of both species penetrated pods through stomata, only P. megakarya produced significant numbers of appressoria. A hypersensitive‐like response was observed when attached SCA‐6 pods were inoculated with P. palmivora. SCA‐6 pods became vulnerable to P. palmivora when wounded prior to zoospore inoculation. Phytophthora megakarya was more aggressive than P. palmivora on attached SCA‐6 pods, causing expanding necrotic lesions with or without wounding. Phytophthora megakarya is predominant in the Volta region of Ghana and it remains to be seen whether it can displace P. palmivora from cacao plantations of Ghana as it has in Nigeria and Cameroon.     

The potential of entomopathogens in biological control of white grubs

Abstract

White grubs are highly polyphagous and most destructive soil pests inflicting damage to a wide variety of crops. In India, more than 1000 species of white grubs are known of which over 40 species attack wide range of plants. White grubs are naturally infected by various entomopathogens which include fungi, bacteria and nematodes. Entomopathogenic fungi offer great potential and members of genera Beauveria and Metarhizium are widely used against white grubs. Several commercial products of entomopathogenic fungi like Bio Green, ORY-X, Grub X 10G, Betel, Biotrol FMA and Meta-Guard have been developed for the control of white grubs. In India, good control of white grubs in paddy, ginger and sugarcane has been achieved with different entomofungi. Among EPNs, Heterorhabditis bacteriophora is moderately effective against Popillia japonica and Rhizotrogus majalis. H. indica and H. bacteriophora are effective against potato white grubs in India. Paenibacillus popilliae cause milky disease in P. japonica grubs. The bacterium is pathogenic to Holotrichia consanguinea, H. serrata and Leucopholis lepidophora. In north-western Himalaya, B. cereus is highly toxic to the grubs of H. seticollis and Anomala dimidiata.     

Mycelial compatibility groups and pathogenic diversity in Sclerotium rolfsii populations from sugar beet crops in Mediterranean‐type climate regions

The population structure of Sclerotium rolfsii from autumn‐sown sugar beet crops in Mediterranean‐type climate regions of Chile, Italy, Portugal and Spain was determined by analyses of mycelial compatibility groups (MCGs) and pathogenicity to 11 economically important plant species. Twelve MCGs (i–xii) were identified among 459 S. rolfsii isolates. MCG iii was the most prevalent group in all countries except Italy. MCG i, the most abundant group (64·7% of isolates) was identified in Portugal and Spain. The remaining MCGs were restricted to various regions within one country (ii, vi, ix) or different countries (v), or to specific localities (iv, vii, viii, x, xi, xii). MCGs iv, vii and x each comprised one isolate. Fields extensively sampled in southern Spain were infected with one to three MCGs. Plant species differed in susceptibility to MCG tester isolates with a MCG by species interaction. Cluster analyses allowed selection into five MCG groupings and grouped plant species into species‐groups 1 (broccoli, chickpea, sunflower, tomato) and 2 (cotton, pepper, sugar beet, watermelon). MCG groupings 1 (i, ix), 2 (ii, iii, vi, viii) and 5 (x, xii) were moderately virulent to species‐group 1 and mildly virulent to species‐group 2. MCG groupings 3 (iv, v, xi) and 4 (vii) were mildly virulent to both species‐groups. Across MCG groups, species were rated highly susceptible (chickpea, sunflower), susceptible (cotton, pepper, tomato, watermelon), moderately resistant (broccoli, melon, sugar beet) and resistant (corn, wheat). Establishing the MCG population structure and virulence variability among S. rolfsii isolates should help in the management of sclerotium root rot diseases.     

Enhanced ambient storage stability of an entomopathogenic nematode through anhydrobiosis

Limited shelf‐life is a major obstacle to large‐scale use of entomopathogenic nematodes in biological control. Recently, the development of a water‐dispersible granular formulation (WG) containing entomopathogenic nematodes was described. This paper reports that storage stability of Steinernema carpocapsae Weiser infective juveniles at 25 °C is enhanced from three to six months in this WG. It is also demonstrated that the enhanced stability is achieved through the induction of partial anhydrobiosis that reduces oxygen and lipid reserve utilization by infective juveniles. Nematode infectivity is unaffected during storage under partial anhydrobiosis in the WG formulation. © 2000 Society of Chemical Industry     

Note: Molecular identification of three entomopathogenic nematodes from turkey by PCR-RFLP of the ITS regions

Molecular identification methods are widely used for the classification of organisms worldwide. Entomopathogenic nematodes are the most often isolated insect parasitic nematodes in the tropical and subtropical regions. In our investigation, PCR-RFLP (Polymerase Chain Reaction — Restriction Fragment Length Polymorphism) of the ITS region (Internal Transcribed Spacer) on the ribosomal (r) DNA of three entomopathogenic nematodes isolated from Ankara, Turkey, was analyzed for identification. The ITS region of rDNA was amplified by PCR and then digested with the following nine restriction enzymes: Alu I, Dde I, Hae III, Hha I, Hind III, Hinf I, Hpa II, Rsa I and Sau 3AI. The amplified and restricted sequences of the ITS regions were separated by agarose gel electrophoresis and the RFLP patterns of these three species were shown in this study. According to our results, these species were identified asSteinernema feltiae, Steinernema carpocapsae andHeterorhabditis bacteriophora. http://www.phytoparasitica.org posting Nov. 4, 2005.     

The reproductive potential of the root-knot nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> is affected by selection for virulence against major resistance genes from tomato and pepper

The emergence of virulent root-knot nematode populations, able to overcome the resistance conferred by some of the resistance genes (R-genes) in Solanaceous crops, i.e., Mi(s) in tomato, Me(s) in pepper, may constitute a severe limitation to their use in the field. Research has been conducted to evaluate the durability of these R-genes, by comparing the reproduction of several laboratory-selected and wild virulent Meloidogyne incognita isolates, on both susceptible and resistant tomatoes and peppers. We first show that the Me1 R-gene in pepper behaves as a robust R-gene controlling avirulent and virulent Me3, Me7 or Mi-1 isolates. Although the reproductive potential of the virulent isolates was highly variable on susceptible and resistant plants, we also confirm that virulence is highly specific to a determined R-gene on which selection has occurred. Another significant experimental result is the observation that a reproductive fitness cost is associated with nematode virulence against Mi-1 in tomato and Me3 and Me7 in pepper. The adaptative significance of trade-offs between selected characters and fitness-related traits, suggests that, although the resistance can be broken, it may be preserved in some conditions if the virulent nematodes are counter-selected in susceptible plants. All these results have important consequences for the management of plant resistance in the field.