No abstract

SUMMARYOne of the major pests of soybeans in Bulgaria is the spider mite Tetranychus turkestani Ug et Nik (Acari: Tetranychidae) and different results have been reported about the impact of water stress on its development and reproduction. Soybean plants exposed to natural infestation by spider mites, water deficit and treatment with imidacloprid were examined under greenhouse conditions at the Institute of Forage Crops, Pleven, Bulgaria, over the period 2011-2012. The development of mites and their eggs was faster when plants were exposed to water deficit, which created favorable conditions for high density and reproduction of T. turkestani. There was a vertical distribution of protonymphs, deutonymphs and adults as their numbers and egg counts were significantly higher under conditions of water deficit on the upper and middle plant parts, compared with their bottom parts, and imidacloprid treatment had a stronger and more prolonged activity against mites, compared to its influence on well-watered plants. Spider mites on water-stressed plants caused a 24.8% reduction in the contents of plastid pigments, and carotenoids, and 21.5% decrease on well-watered plants.

Bruchus pisorum (L.) is one of the most intractable pest problems of cultivated pea in Europe. Development of resistant cultivars is very important to environmental protection and would solve this problem to a great extent. Therefore, the resistance of five spring pea cultivars was studied to B. pisorum: Glyans, Modus; Kamerton and Svit and Pleven 4 based on the weevil damage and chemical composition of seeds. The seeds were classified as three types: healthy seeds (type one), damaged seeds with parasitoid emergence holes (type two) and damaged seeds with bruchid emergence holes (type three). From visibly damaged pea seeds by pea weevil B. pisorum was isolated the parasitoid Triaspis thoracica Curtis (Hymenoptera, Braconidae). Modus, followed by Glyans was outlined as resistant cultivars against the pea weevil. They had the lowest total damaged seed degree, loss in weight of damaged seeds (type two and type three) and values of susceptibility coefficients. A strong negative relationship (r = -0.838) between the weight of type one seeds and the proportion of type three seeds was found. Cultivars with lower protein and phosphorus (P) content had a lower level of damage. The crude protein, crude fiber and P content in damaged seeds significantly or no significantly were increased as compared with the healthy seeds due to weevil damage. The P content had the highest significant influence on pea weevil infestation. Use of chemical markers for resistance to the creation of new pea cultivars can be effective method for defense and control against B. pisorum.

A study was conducted to determine the response of five pea varieties (Pisum sativum L.) to damage degree of Bruchus pisorum: Glyans, Modus, Kamerton, and Svit (Ukrainian cultivars) and Pleven 4 (Bulgarian cultivar). The seeds were classified into three types: healthy seeds (type 1), damaged seeds with parasitoid emergence hole (type 2), and damaged seeds with bruchid emergence hole (type 3) and they were sown. It was found that the weight of 1000 seeds did not affect the field germination of the pea varieties. Healthy and damaged seeds with parasitoid emergence holes (first and second seed types) provide a very good opportunity for growth and development while plants from damaged seeds with bruchid emergence holes had poor germination and vigor and low productivity. These seeds cannot provide the creation of well-garnished seeding and stable crop yields. Among tested varieties, the Ukrainian variety Glyans had considerably higher seed weight, field germination, and index germination and weak egg-laying activity of B. pisorum compared to others. Use of spring pea cultivars that are weakly preferred by the pea weevil in breeding programs would reduce losses due to pea weevil and provide an environmentally safer option to its control.

In order to evaluate the allelopathic effect of Sorghum halepense extracts on germination and initial growth of six pea (Pisum sativum subsp. sativum, Pisum sativum subsp. arvense) cultivars and to identify tolerant cultivars, a laboratory experiment was conducted. The studied cultivars revealed different levels of susceptibility to allelopathic impact of root and aboveground biomass extracts of S. halepense. Root growth parameters (length and weight) of the pea cultivars exhibited greater susceptibility to weed extracts than stem parameters. The inhibitory effects of the extracts on germ length of P. sativum ranged from 1.4% (cultivar Mir) to 45.0% (Kamerton), on germ weight - from 3.5% (Pleven 4) to 42.9% (K-80), and on seed germination - from 11.8% (Mir) to 31.3% (K-80). Total inhibitory effect, i.e. the impact of S. halepense extracts on all studied parameters of P. sativum, revealed that the cultivars Mir and Pleven 4 were the most tolerant. Growing such cultivars may reduce weed damage. Low tolerance was manifested by the cultivar K-80, while Modus, Glyans and Kamerton ranked intermediate. The cultivars with large-size seeds or lower grain protein content were more affected by the depressing effect of S. halepense extracts.