Preference of red mite Tetranychus ludeni Zacher ( Acari : Tetranychidae ) to sweet potato genotypes

Tetranychus ludeni damages the sweet potato. Pest development can vary between plant genotypes. The objective was to identify the preference of Tetranychus ludeni for Ipomoea batatas genotypes, from the germplasm bank at the Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM). Natural infestations of this mite were observed on 54 sweet potato genotypes in potted, in a greenhouse. Three mite-infested leafs of each genotype were collected and analyzed. The red mite showed different population density rate in genotypes. The BD 29 genotype was found to be highly susceptible, the BD 08, BD 57, BD 17 and Espanhola genotypes were moderately susceptible, and the others forty-nine genotypes showed low susceptibility to the mite.

Pest development varies with the plant genotypes (Hochwender et al., 2005), such as the decreased development of T. urticae on the rose cultivar Virginia than on Emma, Gala and Luna ones (Landeros Flores et al., 2013).The development period was longer and the survival and reproduction lower for T. urticae on the strawberry (Fragaria x ananassa) cultivars than on the Camarosa, Diamond and Seascape ones (Monteiro et al., 2014).Sweet potato genotypes show resistance to pests like Diabrotica spp.(Jackson and Bohac, 2007), Chaetocnema confinis Crotch (Abney and Kennedy, 2011) (Coleoptera: Chrysomelidae), and the root borer, Euscepes postfasciatus (Coleoptera: Curculionidae) (Okada et al., 2014).
The objective was to identify the preference of Tetranychus ludeni for Ipomoea batatas genotypes, from the germplasm bank at the Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM).
The sweet potato genotypes were planted in 10-liter pots in a greenhouse and watered daily.Three sweet potato branches were transplanted per pot with soil, fertilized and limed as recommended for the crop.The experimental design was completely randomized with 54 treatments (genotypes), three replications and three plants per pot to represent each experimental unit.
A natural T. ludeni outbreak was observed 90 days post transplantation of the sweet potato branches.Three infested leafs were collected per genotype.They were taken for analysis to the laboratory of Entomology (UFVJM).An area of 3.8 cm 2 covering the midrib of the abaxial surface of leafs (the most frequent point of mite occurrence), was photographed using a camera coupled with an optical microscope.The T. ludeni adults were counted on the leaf area with computer assistance.
Data were subjected to the analysis of variance and the means were grouped using the Scott-Knott test at 5% probability, employing the statistical analysis program SAEG.

Results and Discussion
Tetranychus ludeni showed lower population density rate on different I. batatas genotypes (F= 3.55; df= 53, 108; P< 0.0001).The genotypes were categorized under three groups: high, moderate or low susceptibility to this mite, indicating the resistance mechanisms in these plants.
The BD 29 genotype (72.33 ± 29.19) was the most infested and was therefore classified as highly susceptible (Table 1).Density, distribution, and damage by pests vary with genotype (Hochwender et al., 2005), being plant resistance correlated with their genetic characteristics (Leimu and Koricheva, 2006).Plant preferences can be based on chemical (Kos et al., 2014) and/or morphological mechanisms such as leaf toughness, trichomes, density, stomata size, and epicuticular wax content (Nair et al., 2012).The research and selection of resistant genotypes may contribute to integrated pest management (IPM) aimed at safely, economically, and ecologically methods to eliminate pests (Ehler, 2006), in a more sustainable manner over the long term (Soares et al., 2007;Soares et al., 2009).
The BD 08, BD 57, BD 17 and the Espanhola were moderately susceptible and the others showed low susceptibility to T. ludeni.The development of the mites may vary between the genotypes of the same species (Modarres Najafabadi et al., 2014) as reported for Amphitetranychus viennensis Zacher and Tetranychus urticae Koch (Acari: Tetranychidae) on apples (Kasap, 2003) and T. urticae on strawberry (Wold and Hutchison, 2003), cucumber (Park and Lee, 2007) and tomato (Keskin and Kumral, 2015).The total height, number of leafs, flowers and fruits and fruit weight were found to be lower on the eggplant (Solanum melongena L. Solanaceae) cultivar Panruti than on the other varieties of this plant (Reddy and Baskaran, 2006).The resistance of the Phaseolus vulgaris L. cultivars to T. urticae also varied.The Akhtar and GS11867 varieties are susceptible, while the KS41128 and Naz are resistant (Tahmasebi et al., 2014).Resistant cultivars can reduce pest population density and is a method compatible with other management tactics, such as biological control (Castro et al., 2014), an important aspect of IPM (Razmjou et al., 2009;Landeros Flores et al., 2013) and furthermore minimizes the application of chemicals (Zehnder et al., 2007) and consequently, the production costs.

Conclusions
The population density rate of T. ludeni was different in each of the sweet potato genotypes.The BD 29 was found to be highly susceptible, the BD 08, BD 57, BD 17 and Espanhola genotypes were moderately susceptible, while the others showed low susceptibility to T. ludeni.Means followed by the same letter were grouped using the Scott-Knott test at 5% probability.