Spider Mite Resistance Research Articles

The effects of water‐stress, temperature, and plant traits on the outbreak potential of a specialist and generalist spider mite species (Acari: Tetranychidae)

AbstractThe host‐generalist two‐spotted spider mite [Tetranychus urticae (Acari: Tetranychidae); TSM] and host‐specialist Banks grass mite [Oligonychus pratensis (Acari: Tetranychidae); BGM] are common pests of corn (Zea mays L.) in the arid western United States. Climate warming and decreased precipitation may promote conditions favoured by these spider mites. However, rapid evolution of spider mite resistance to commercially available acaricides is driving the need for alternative solutions for managing outbreaks. Planting of drought‐tolerant corn hybrids has been proposed to be a dual‐purpose strategy for mitigating water deficits for irrigation and reducing leaf conditions favourable for BGM outbreaks. However, understanding of the mechanisms responsible for reducing the BGM in the field is lacking and determining whether outbreaks of the TSM can also be averted using drought‐tolerant corn is a pressing concern. We conducted a 2‐year field study testing a drought‐tolerant corn hybrid and an analogous drought‐susceptible hybrid under water‐stress, with artificially infested spider mite populations. Drought‐tolerant corn had larger stem diameter, more massive cobs, and greater leaf water mass compared to the drought‐susceptible corn under water stress. We also found that the BGM populations were reduced on drought‐tolerant plants under water‐stress, as expected, but we found an opposite trend in the TSM. Lastly, water‐stressed leaves were warmer, transpired less, and had higher carbon concentration, which contributed to larger investment in eggs and growth in the BGM. We anticipate that further evaluation of irrigation and crop drought‐tolerance in management of agriculture systems for multiple pest species will be increasingly impactful in arid regions.

Effects of tomato inoculation with the entomopathogenic fungus Metarhizium brunneum on spider mite resistance and the rhizosphere microbial community.

Entomopathogenic fungi have been well exploited as biocontrol agents that can kill insects through direct contact. However, recent research has shown that they can also play an important role as plant endophytes, stimulating plant growth, and indirectly suppressing pest populations. In this study, we examined the indirect, plant-mediated, effects of a strain of entomopathogenic fungus, Metarhizium brunneum on plant growth and population growth of two-spotted spider mites (Tetranychus urticae) in tomato, using different inoculation methods (seed treatment, soil drenching and a combination of both). Furthermore, we investigated changes in tomato leaf metabolites (sugars and phenolics), and rhizosphere microbial communities in response to M. brunneum inoculation and spider mite feeding. A significant reduction in spider mite population growth was observed in response to M. brunneum inoculation. The reduction was strongest when the inoculum was supplied both as seed treatment and soil drench. This combination treatment also yielded the highest shoot and root biomass in both spider mite-infested and non-infested plants, while spider mite infestation increased shoot but reduced root biomass. Fungal treatments did not consistently affect leaf chlorogenic acid and rutin concentrations, but M. brunneum inoculation via a combination of seed treatment and soil drenching reinforced chlorogenic acid (CGA) induction in response to spider mites and under these conditions the strongest spider mite resistance was observed. However, it is unclear whether the M. brunneum-induced increase in CGA contributed to the observed spider mite resistance, as no general association between CGA levels and spider mite resistance was observed. Spider mite infestation resulted in up to two-fold increase in leaf sucrose concentrations and a three to five-fold increase in glucose and fructose concentrations, but these concentrations were not affected by fungal inoculation. Metarhizium, especially when applied as soil drench, impacted the fungal community composition but not the bacterial community composition which was only affected by the presence of spider mites. Our results suggest that in addition to directly killing spider mites, M. brunneum can indirectly suppress spider mite populations on tomato, although the underlying mechanism has not yet been resolved, and can also affect the composition of the soil microbial community.

Soybean (Glycine max L Merr) host-plant defenses and resistance to the two-spotted spider mite (Tetranychus urticae Koch).

In southern Ontario, Canada, the two-spotted spider mite (Tetranychus urticae) is an emerging pest of soybean (Glycine max) due to the increasing incidence of warmer, drier weather conditions. One key strategy to manage soybean pests is breeding resistant cultivars. Resistance to pathogens and herbivores in soybean has been associated with isoflavonoid phytoalexins, a group of specialized metabolites commonly associated with root, leaf and seed tissues. A survey of 18 Ontario soybean cultivars for spider mite resistance included evaluations of antibiosis and tolerance in relation to isoflavonoid and other metabolites detected in the leaves. Ten-day and 4-week trials beginning with early growth stage plants were used to compare survival, growth, fecundity as well as damage to leaves. Two-spotted spider mite (TSSM) counts were correlated with HPLC measurements of isoflavonoid concentration in the leaves and global metabolite profiling by high resolution LC-MS to identify other metabolites unique to the most resistant (R) and susceptible (S) cultivars. Within 10 days, no significant difference (P>0.05) in resistance to TSSM was determined between cultivars, but after 4 weeks, one cultivar, OAC Avatar, was revealed to have the lowest number of adult TSSMs and their eggs. Other cultivars showing partial resistance included OAC Wallace and OAC Lakeview, while Pagoda was the most tolerant to TSSM feeding. A low, positive correlation between isoflavonoid concentrations and TSSM counts and feeding damage indicated these compounds alone do not explain the range of resistance or tolerance observed. In contrast, other metabolite features were significantly different (P<0.05) in R versus S cultivars. In the presence of TSSM, the R cultivars had significantly greater (P<0.05) concentrations of the free amino acids Trp, Val, Thr, Glu, Asp and His relative to S cultivars. Furthermore, the R cultivar metabolites detected are viable targets for more in-depth analysis of their potential roles in TSSM defense.

Spider mite resistance to miticides in South Carolina strawberry and implications for improved integrated pest management.

Tetranychus urticae Koch (Acari: Tetranychidae), twospotted spider mite, is a major secondary pest of strawberry and can cause significant yield loss. Tetranychus urticae is typically controlled using miticides, which has led to rapid resistance development. In South Carolina (USA), extension agents and growers have reported field failures of miticides (inadequate pest suppression), but resistance has not been quantitatively determined. In 2018, we determined the level of miticide resistance of six T. urticae populations found on strawberry across South Carolina. We examined efficacy of all miticides registered for use on US strawberry by conducting an initial diagnostic bioassay at 20% of the maximum labeled field rate. Any population × active ingredient combination resulting in < 55% mortality was identified as 'potentially resistant' and concentration-response bioassays were then conducted to calculate LC50 values for an individual population. These values were compared with those of a known-susceptible laboratory population to calculate resistance ratios (RR). Our results indicate that examined South Carolina populations of T. urticae from strawberry were highly resistant to bifenthrin (RR = 100-60,000) and there was reduced susceptibility to fenbutatin-oxide (RR = 25-123). The 'Sardinia' population had decreased abamectin susceptibility (RR = 25). No resistance to hexythiazox, etoxazole, acequinocyl, bifenazate, fenpyroximate, spiromesifen, or cyflumetofen was found. Based on available data, it appears that miticide resistance is not a likely cause of field failures and issues related to application error and coverage should be investigated. Overall, this work supports the need to reduce the use of broad-spectrum pesticides and older products, in favor of newer miticide chemistries due to resistance issues.

Screening cultivated eggplant and wild relatives for resistance to sweetpotato whitefly (Bemisia tabaci) and to two-spotted spider mite (Tetranychus urticae)

Whiteflies and spider mites are amongst the most harmful eggplant (Solanum melongena) pests. Considering the need for reduction of chemical applications for whitefly and spider mite control, the exploitation of wild relatives of eggplant as sources of pest resistances represents an important strategy in order to improve cultivated eggplant. The objectives of this study were to evaluate 15 accessions from 11 species of eggplant wild relatives together with seven S. melongena accessions for resistance to sweet potato whitefly (Bemisia tabaci) and to two-spotted spider mite (Tetranychus urticae). Resistance to whitefly was evaluated based on number of eggs, nymph, puparium and whitefly adults in a choice bioassay, while for two-spotted spider mite it was based on leaf damage scores in the choice and no-choice bioassays. The results revealed significantly (P < 0.05) different levels of resistance to the two pests among the accessions evaluated. Considering all screening parameters in the whitefly choice bioassay, the highest levels of resistance in wild eggplant relatives were detected in Solanum dasyphyllum (DAS1) and S. pyracanthos (PYR1), although one of the cultivated S. melongena (MEL2) accessions also displayed similar resistance levels. In addition, S. campylacanthum (CAM8) and S. tomentosum TOM1 were also resistant to whitefly based on numbers of puparium and adult whiteflies. Two accessions of S. sisymbriifolium (SIS1 and SIS2) exhibited strong resistance to two-spotted spider mite based on the choice and no-choice bioassays. High levels of spider mite resistance were also detected in the no-choice assay in S. dasyphyllum (DAS1) and S. torvum (TOR2) accessions. These resistant accessions can be used in pre-breeding program aiming to breed pest-resistant cultivars in cultivated eggplant. Moreover, to our knowledge, this study represents the first report on potential sources of resistance to whitefly and two-spotted spider mite in wild relatives of eggplant.

Productivity, acylsugar concentrations and resistance to the two-spotted spider mite in genotypes of salad tomatoes

ABSTRACT The objective of this study was to characterize genotypes of the salad-type tomato with a determinate growth habit and select genotypes resistant to the two-spotted spider mite for higher productivity and fruit quality. The experiment was conducted at the Horticultural Experimental Station (18°42’43.19” S and 47°29’55.8” W, 873 m altitude) of the Universidade Federal de Uberlândia (UFU), Monte Carmelo campus, from January to June 2018. The experimental design was in randomized blocks containing 16 treatments [12 populations of the F3RC2 generation and four controls: two commercial cultivars (Santa Clara and Rio Grande), the recurrent parent (UFU-057) and the wild species Solanum pennellii (LA-716)] with three repetitions. The accessions UFU-057F3RC210#3.6.1 and UFU-057F3RC213#2.1.2 have high potential for genetic improvement regarding resistance to the two-spotted spider mite. The genotypes UFU-13#4.4.1, UFU-7#4.7.1, UFU-7#4.3.2, UFU-13#2.5.2 and UFU-13#2.1.3 present higher acylsugar concentrations than S. pennellii, in addition to higher productivity and number of fruits per plant. It is suggested to explore combinations between those genotypes previously described with UFU-8#2.4.1 to obtain hybrids with high acylsugar concentrations, two-spotted spider mite resistance, high productivity and increased fruit mass.

Sources of Resistance for Two-spotted Spider Mite (Tetranychus urticae) in Scarlet (Solanum aethiopicum L.) and Gboma (S. macrocarpon L.) Eggplant Germplasms

The two-spotted spider mite, Tetranychus urticae, is a serious global pest of eggplants and other vegetables in the tropics and subtropics. The scarlet (Solanum aethiopicum L.) and gboma (Solanum macrocarpon L.) are two cultivated eggplants indigenous to sub-Saharan Africa. In this study, 120 accessions of S. aethiopicum and five accessions of S. macrocarpon from the World Vegetable Center’s genebank collection were evaluated for spider mite resistance based on leaf damage scores and trichome types and densities. The highest levels of resistance were detected in S. macrocarpon accessions VI050393 and VI050444. In addition, two accessions of S. aethiopicum, VI042749 and VI042753, were moderately resistant. All other S. aethiopicum accessions were susceptible or highly susceptible. Spider mite resistance was significantly associated with the presence of type VII glandular trichome, but nonglandular stellate trichomes were not associated with resistance. The resistant S. macrocarpon accessions can be directly used as resistant cultivars or in prebreeding programs aimed at breeding resistant lines of the most commonly cultivated eggplant (S. melogena).

ЗИМОСТОЙКОСТЬ И БИОМЕТРИЧЕСКИЕ ПОКАЗАТЕЛИ ИНТРОДУЦИРОВАННЫХ СОРТОВ ЗЕМЛЯНИКИ В УСЛОВИЯХ ПЛЕНОЧНЫХ ТЕПЛИЦ В ЛЕНИНГРАДСКОЙ ОБЛАСТИ

Background. The intensive inflow of strawberry cultivars from abroad in recent years and a great number of cultivation practices make an accelerated comprehensive study of these cultivars in new soil and climate conditions a high priority for the Northwest of the Russian Federation. Materials and methods. In 2016–2017, 35 newly introduced strawberry cultivars were studied at St. Petersburg State Agrarian University according to the Program and Methodology of Variety Research on Fruit, Berry and Nut Crops. The primary study focused on the biometric characteristics of strawberry cultivars, their winter hardiness, performance of the full cycle of phenological stages in plant development, resistance to spider mite, and productivity. These are the main criteria for cultivars to be grown in plastic film greenhouses in Leningrad Province. Results and conclusion. The primary study of the introduced cultivars under plastic film greenhouse conditions in Leningrad Province and their evaluation for winter hardiness, productivity and spider mite resistance made it possible to identify the cultivars ‘Vechnaya Vesna’, ‘Vystavochnaya’, ‘Yubileynaya Govorovoy’ ‘Jive’, ‘Diamante’ and ‘Elianny’ as promising for cultivation. They require further research targeted at a set of economically important traits.

Multivariate techniques in the determination of genetic diversity in pest-resistant mini tomato genotypes

ABSTRACT The objective of this study was to compare methods of multivariate analysis on the evaluation of genetic diversity of mini tomato and to identify promising genotypes with resistance to pests. The experiment was conducted at the Vegetable Experiment Station of the Universidade Federal de Uberlândia, Monte Carmelo campus, from April 2013 to November 2016. The experimental design was a randomized complete block design with 16 treatments and four replications totaling 64 plots, and each plot represented by five plants. Sixteen genotypes were characterized, 12 from the F2RC1 generation, obtained through the interspecific crossing between the wild access LA-716 (Solanum pennellii) and pre-commercial lines of mini tomato (UFU-73 and UFU-2) (Solanum lycopersicum) and the UFU-2 lines. The content of acyl sugar, the amount of glandular trichomes (types I, IV, VI and VII), twospotted spider mite and whitefly resistance were evaluated. We concluded that there exist genetic variability between the genotypes. The number of groups formed by the canonical variated analysis was higher (four groups) than that obtained by the Tocher method (three groups) and UPGMA (three groups), demonstrating a greater discrimination power. The Tocher and UPGMA methods were consistent in the analysis of the genetic divergence in pest resistant germplasm of tomato, with the acyl sugar content being the most important variable. Genotype UFU-73-F2RC1 # 11 is resistant to pest attack, while the other studied lines have intermediate resistance.

Comparison of susceptibility of two populations of Tetranychus urticae Koch to two acaricides, abamectin and propargite

The spider mite, Tetranychus urticae Koch is a serious pest of economically important plants in closed and open area worldwide. The spider mite resistance to acaricide plays a major role in the failure of the chemical control method. Thus, the aim of the current study was to evaluate the efficacy of two acaricides, abamectin and propargite, against two populations (strains) of the spider mite. Results showed that LC50s of the abamectin against susceptible and resistant strains of the spider mite were 0.1 and 2730 ppm, respectively. Whilst LC50s of the propargite against susceptible and resistant strains of the spider mite were 55 and 7199 ppm, respectively. Resistance ratio (RR) calculated as the ratio of resistance LC50/susceptible LC50 showed that RR for abamectin and propargite was 20285 and 130, respectively. The enzyme assay results showed that three mechanisms of MFO, GST and EST are involved in the abamectin resistance of the spider mite. In gel assays, when α-naphthyl acetate was used as substrate, three bands appeared in the gel in which bands E2 and E3 were major bands and E1 was a minor band confirming that α-naphthyl acetate was a better substrate for general esterase activity in the spider mite whereas β-acetate when used for esterase activity, only two faint bands (E1 and E2) were observed. The order of their involvement in the abamectin resistance is EST > MFO > GST.

Genetic mapping of two QTL from the wild tomato Solanum pimpinellifolium L. controlling resistance against two-spotted spider mite (Tetranychus urticae Koch)

A novel source of resistance to two-spotted spider mite (Tetranychus urticae Koch) was found in Solanum pimpinellifolium L. accession TO-937 and thereby a potential source of desirable traits that could be introduced into new tomato varieties. This resistance was found to be controlled by a major locus modulated by minor loci of unknown location in the genome of this wild tomato. We first applied a bulked segregant analysis (BSA) approach in an F(4) population as a method for rapidly identifying a genomic region of 17 cM on chromosome 2, flanked by two simple sequence repeat markers, harboring Rtu2.1, one of the major QTL involved in the spider mite resistance. A population of 169 recombinant inbred lines was also evaluated for spider mite infestation and a highly saturated genetic map was developed from this population. QTL mapping corroborated that chromosome 2 harbored the Rtu2.1 QTL in the same region that our previous BSA findings pointed out, but an even more robust QTL was found in the telomeric region of this chromosome. This QTL, we termed Rtu2.2, had a LOD score of 15.43 and accounted for more than 30% of the variance of two-spotted spider mite resistance. Several candidate genes involved in trichome formation, synthesis of trichomes exudates and plant defense signaling have been sequenced. However, either the lack of polymorphisms between the parental lines or their map position, away from the QTL, led to their rejection as candidate genes responsible for the two-spotted spider mite resistance. The Rtu2 QTL not only serve as a valuable target for marker-assisted selection of new spider mite-resistant tomato varieties, but also as a starting point for a better understanding of the molecular genetic functions underlying the resistance to this pest.

Spider mite control and resistance management: does a genome help?

The complete genome of the two-spotted spider mite, Tetranychus urticae, has been reported. This is the first sequenced genome of a highly polyphagous and resistant agricultural pest. The question as to what the genome offers the community working on spider mite control is addressed.

Monitoring of spirodiclofen susceptibility in field populations of European red mites, Panonychus ulmi (Koch) (Acari: Tetranychidae), and the cross‐resistance pattern of a laboratory‐selected strain

Phytophagous mites such as the European red mite, Panonychus ulmi (Koch), are serious pests in European fruit tree orchards, and a number of acaricides are frequently used to control them. Spirodiclofen (Envidor(®)) has been a commonly used acaricide for several years. In the present study, European field populations collected in 2009 and 2010 were checked for their susceptibility to spirodiclofen by using discriminating dose and full dose response bioassays. In 2009 and 2010, a total of 63 field populations (including winter eggs) of European red mites were collected in different European countries, and in several populations from south-western Germany a shifting in susceptibility against spirodiclofen was observed. Full dose response bioassays on different developmental stages of field-collected strains suggested an age-dependent expression of resistance because eggs remain fully susceptible to spirodiclofen. Artificial selection with spirodiclofen of one of the field strains resulted in resistance ratios of > 7000. Synergism studies suggest a possible role of cytochrome-P450-dependent monooxygenases in spirodiclofen detoxification. Most of the other acaricides from different chemical classes displayed no or low cross-resistance in a spirodiclofen-selected strain. In order to preserve spirodiclofen as an important tool in spider mite resistance management, the efficacy situation should be continuously monitored, and it is suggested that spirodiclofen be alternated with acaricides coming from different mode-of-action classes. An observed age-specific expression of resistance revealed full susceptibility of eggs, so targeting spirodiclofen particularly against eggs is likely to reduce the selection pressures imposed on other life stages.

Varietal Resistance of Okra against Two Spotted Spider Mite, Tetranychus urticae

The present investigation was undertaken to evaluate the okra germplasm and identify the potential sources of two spotted spider mite resistance that can be exploited for the improvement of commercial okra. Based on the mean mite population and damage score six entries out of the 58 entries namely Kasturi bhendi, (R), Dharmapuri local (MR), Parbani Kranti (MR), Pusa Sawani (MR), COBhH1 (MR) and Mahyco 10 (S) with varied levels of resistance were identified.

Inheritance of glandular trichomes and two-spotted spider mite resistance in cross Lycopersicon esculentum ‘‘Nandi’’ and L. pennellii ‘‘LA2963’’

Parents and their F1 and F2 progenies were evaluated for density of type IV glandular trichomes and resistance to Tetranychus urticae Koch during three stages of the plant growth. Results indicated significant differences between density of type IV glandular trichomes on F1 and F2 populations at different sampling times. As age of plant increased, density of type IV glandular trichomes and resistance to two-spotted spider mite increased. Density of type IV glandular trichomes on F2 individuals showed a broad range of variability (0–42 trichomes/mm2). The estimated heritability was very high (>85%) for damage score and density of type IV glandular trichomes at different sampling times. Acylsugars content in Lycopersicon pennellii ‘‘LA2963’’ was more than two-folds high than those found in L. esculentum ‘‘Nandi’’, F1 and F2 populations, indicating that recessive gene(s) are responsible for the high acylsugar contents in L. pennellii ‘‘LA2963’’.

Role of cucurbitacin C in resistance to spider mite (Tetranychus urticae) in cucumber (Cucumis sativus L.).

Cucurbitacins are bitter triterpenoid compounds that are toxic to most organisms and occur widely in wild and cultivated Cucurbitaceae. The only cucurbitacin identified in Cucumis sativus is cucurbitacin C. The bitter taste of cucumber has been correlated with resistance to the spider mite Tetranychus urticae, but a quantitative relationship has not been established. We determined the spider mite resistance and cucurbitacin C content in the dihaploid progeny derived from the F1 generation of a cross between a bitter, spider-mite-resistant cucumber line and a bitter-free, spider-mite-susceptible line. The ratio of the number of bitter to bitter-free dihaploids conformed to the expected 1:1 ratio, based on a monogenic segregation pattern. Genetic analysis ascribed 69% of the variance of the difference in spider mite survival rate to the bitterness locus. Within the group of bitter dihaploids, cucurbitacin C content was significantly correlated with spider mite resistance. Thus, a quantitative relationship between cucurbitacin C content and spider mite resistance could be established.

Spider mite (atari: tetranychidae) resistance to acaricides

Spider mites (Acari: Tetranychidae) were known before the so-called chemical era of plant protection as a minor threat to what were then mostly low-productivity agro-ecosystems. Various predators kept spicier mite populations mostly below the control threshold. After World War Two. however, the growing use of organophosphorus insecticides (designed primarily to control key pest insects) provoked a rapid evolution of resistance to organophosphates and made spider mites an economically threatening pest species. The grow inn resistance to organophosphates and altered level of harmfulness of Tetranychidae have imposed the need for synthesizing specific acaricides i.e. compounds primarily intended for controlling that group of phytophagous mites. However, ever since 1950s resistance of Tetranvchus spp. and Panouxchus spp. populations/strain to all major acaricides has been recorded after such brief periods as several years. Serious failures in controlling them have increasingly induced research into the phenomenology, evolution and mechanisms of resistance of Tetranychidae to acaricides. The paper provides a brief survey of the history and characteristics of resistance and summarizes data on factors of evolution and molecular mechanisms of spider mite resistance to acaricides. emphasizing some elements of the programme of preventive activities.

Compatibility of Host Plant Resistance and Biological Control of the Two-Spotted Spider Mite Tetranychus urticae in the Ornamental Crop Gerbera

We investigated the compatibility of host plant resistance to the spider mite Tetranychus urticae Koch in the ornamental crop gerbera with the use of the predatory mite Phytoseiulus persimilis Athias-Henriot for biological control. We used four gerbera cultivars on which the intrinsic rate of population increase (rm) of T. urticae was previously found to differ largely. We performed life-history studies on those cultivars with P. persimilis and determined whether the developmental time, peak oviposition rate, and rm of the predator was influenced by host plant resistance. Because the first egg of P. persimilis is usually a male and therefore does not contribute to the population growth rate, we discarded the first egg of all predators from our analyses. The egg-to-second-egg developmental time of the predator was approximately 5.2 days. This developmental time was slightly but significantly shorter on gerbera cultivar Fame than on the other three cultivars. However, this shorter developmental time did not lead to a higher rm of the predators on this cultivar. No cultivar effect was found on the peak oviposition rate, which varied from 5.1 to 5.4 eggs/female/day. On the cultivars used, the rm of the spider mites was previously found to vary from 0.09 to 0.23 per day. We did not find any effect of spider mite resistance in gerbera on the rm of the predators, which ranged from 0.43 to 0.45 per day. Hence, the capacity for population increase of P. persimilis is not influenced by host plant resistance to its prey.

Gradients in Susceptibility and Resistance Mechanisms of Buddleia L. Taxa to the Two-spotted Spider Mite (Tetranychus urticae Koch)

Buddleia taxa were assessed for two-spotted spider mite (Tetranychus urticae Koch) resistance using a leaf disk bioassay, a novel shell vial bioassay and a field trial. Leaf pubescence and chemistry were examined for their role in two-spotted spider mite resistance. Results from bioassays and field sampling identified highly resistant taxa including B. fallowiana Balif. `Alba' and B. davidii × B. fallowiana Franch. `Cornwall Blue' as well as susceptible taxa including B. davidii Franch. `African Queen' and B. lindleyana Fort. ex Lindl. `Gloster'. The shell vial bioassay was an accurate predictor of field resistance to spider mite. Leaf pubescence was quantified by calculating the collective length of trichome branches per square millimeter of leaf surface area [effective branch length (EBL)]. EBL values ranged from 39 to 162 mm·mm-2 of leaf surface area among Buddleia taxa. Resistance was positively correlated with increased pubescence. Removal of pubescence by peeling resulted in increased oviposition of two-spotted spider mites. Exposing female two-spotted spider mites to a methylene chloride extract of B. davidii × B. fallowiana `Cornwall Blue' using a modified shell vial bioassay resulted in reduced oviposition and a methylene chloride extract of B. davidii `African Queen' resulted in no difference in oviposition when compared with a control. While pubescence is the best indicator of resistance to the two-spotted spider mite in Buddleia taxa, it is possible that defensive compounds are involved.

Variation of 2-Tridecanone level in tomato plant leaflets and resistance to two mite species (Tetranychus sp.)

The wild tomato accession Lycopersicon hirsutum f. glabratum ‘PI 134417’ is a promising source of resistance to arthropods in tomato. It is self compatible and intercrosses easily with the cultivated species Lycopersicon esculentum. It is resistant to several arthropods, including spider mites, and these resistances have been related to the presence of high contents of 2-tridecanone (2-TD) and other methyl ketones in the leaves. The objective of this study was to evaluate the contribution of 2-TD concentration in the leaflets of Lycopersicon for resistance to the spider mites Tetranychus urticae (Koch, 1836) and T. ludeni Zacher, 1913. Plants of two segregating generations (F2) were evaluated through the thumbtack bioassay (Weston & Snyder, 1990) in two different assays. Both experiments included as checks L. esculentum ‘TSWV-547’ (mite susceptible), L. hirsutum f. glabratum ‘PI 134417’ (resistant) and the interspecific hybrid F1 (TSWV-547 × PI 134417). In the first experiment, 10 genotypes selected from F2 (TSWV-547 × PI 134417), with contrasting 2-TD concentrations in the leaflets, were tested for resistance to T. urticae. In a second experiment, seven genotypes of the F2 (TSWV-547 × BPX-318p1#230), also with contrasting 2-TD concentrations, were used to evaluate the resistance to T. ludeni. BPX-318p1#230 is a single plant selected from the segregating population F2 (TSWV-547 × PI 134417) for its high 2-TD content. In both experiments, high negative correlations (r = − 0.88; − 0.91 and − 0.93 in the first experiment, and r = − 0.78, − 0.99 and − 0.96 in the second experiment) were found between the 2-TD content and average distance travelled by the spider mites after 60, 90 and 120 minutes, respectively. The level of resistance (repellency) to spider mites T. urticae and T. ludeni in the genus Lycopersicon appears to be mediated by the 2-TD levels present in the tomato leaflets. The similar results of both experiments showed that selection for high 2-TD content in leaflets can lead to genetic improvement in the direction of increased levels of spider mite resistance in tomato.