Biotic factors are more important than abiotic factors in regulating the abundance of Plutella xylostella L., in Southern Brazil

ABSTRACT The serious economic loss caused Plutella xylostella L., 1758 in several regions of the world has prompted a demand for alternative management strategies. In this context, understanding the key factors governing the population dynamics of the pest is important for development of management strategies. This study aimed to identify the larval parasitoids associated with P. xylostella and investigate the biotic (crop subspecies, plant age and parasitism) and abiotic factors (minimum and maximum temperatures, rainfall, relative humidity and planting season) affecting the population dynamics of the pest in organic crops located in Southern Paraná State, Brazil. Despite the continuous and abundant availability of host plants throughout the year, P. xylostella occurred between June and November, and the largest peaks of abundance were observed between August and September, when low temperatures and rainfall were recorded. According to the stepwise regression analysis, P. xylostella was more abundant in broccoli during winter. Neither temperature, nor rainfall significantly influenced pest abundance. Four species of larval parasitoids were identified associated with the pest, of which Diadegma leontiniae (Brèthes) (Hymenoptera: Ichneumonidae), Apanteles piceotrichosus Blanchard (Hymenoptera: Braconidae) and Siphona sp. Meigen (Diptera: Tachinidae) were abundant, while Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae) was rarely found. Parasitism was the major factor influencing population dynamics of P. xylostella, contributing to 48% of the variation in pest abundance. These results show the importance of larval parasitoids complex in regulating P. xylostella population and that the temperature and rainfall recorded during field experiments did not influenced pest abundance.

Mortality of Plutella xylostella (Lepidoptera, Plutellidae) by parasitoids in the Province of Santa Fe, Argentina

Mortality of Plutella xylostella (Lepidoptera, Plutellidae) by parasitoids in the Province of Santa Fe, Argentina. Plutella xylostella (Linnaeus, 1758) (Lepidoptera, Plutellidae) larvae cause severe economic damage on cabbage, Brassica oleracea L. variety capitata (Brassicaceae), in the horticultural fields in the Province of Santa Fe, Argentina. Overuse of broad spectrum insecticides affects the action of natural enemies of this insect on cabbage. The objectives of this work were to identify the parasitoids of P. xylostella and to determine their influence on larva and pupa mortality. Weekly collections of larvae and pupae were randomly conducted in cabbage crops during spring 2006 and 2007. The immature forms collected were classified according to their developmental stage: L1 and L2 (Ls = small larvae), L3 (Lm = medium larvae), L4 (Ll = large larvae), pre-pupae and pupae (P). Each individual was observed daily in the laboratory until the adult pest or parasitoid emergence. We identified parasitoids, the number of instar and the percentage of mortality of P. xylostella for each species of parasitoid. Parasitoids recorded were: Diadegma insulare (Cresson, 1875) (Hymenoptera, Ichneumonidae), Oomyzus sokolowskii (Kurdjumov, 1912) (Hymenoptera, Eulophidae), Cotesia plutellae (Kurdjumov, 1912) (Hymenoptera, Braconidae) and an unidentified species of Chalcididae (Hymenoptera). Besides parasitoids, an unidentified entomopathogenic fungus was also recorded in 2006 and 2007. In 2006, the most successful parasitoids were D. insulare and O. sokolowskii, while in 2007 only D. insulare exerted a satisfactory control and it attacked the early instars of the pest.

Mortalidade de Plutella xylostella (Lepidoptera, Plutellidae) por parasitóides na Província de Santa Fé, Argentina. Plutella xylostella (Linnaeus, 1758) (Lepidoptera, Plutellidae) causa danos econômicos severos em repolho, Brassica oleracea variedade capitata L. (Brassicaceae), na área de horticultura localizada na Província de Santa Fé, Argentina. O uso excessivo de inseticidas de largo espectro afeta a ação dos inimigos naturais de P. xylostella em repolho. O presente trabalho teve como objetivo identificar os parasitóides de P. xylostella e determinar sua influência na mortalidade de larvas e pupas em Santa Fé, Argentina. Coletas semanais de larvas e pupas em culturas de repolho foram realizadas aleatoriamente na área durante a primavera de 2006 e 2007, registradas pelo estágio de desenvolvimento: L1 e L2 (Ls = larvas pequenas), (Lm = larvas médias) L3, L4 (Ll = larvas grandes) pré-pupa e pupa (P). Cada indivíduo foi observado diariamente no laboratório até a emergência dos adultos da praga ou dos parasitóides. Foram identificadas as espécies de parasitóides o número de instars e a porcentagem de mortalidade. Os parasitóides registrados foram: Diadegma insulare (Cresson, 1875) (Hymenoptera, Ichneumonidae), Oomyzus sokolowskii (Kurdjumov, 1912) (Hymenoptera, Eulophidae), Cotesia plutellae (Kurdjumov, 1912) (Hymenoptera, Braconidae) e uma espécie não identificada pertencente à família Chalcididae. Além dos parasitóides, um fungo entomopatogênico não identificado foi registrado. Em 2006, os parasitóides mais bem sucedidos foram D. insulare e O. Sokolowskii; enquanto que em 2007 apenas D. insulare exerceu um controle satisfatório, atacando os instares iniciais da praga.

Seletividade fisiológica de inseticidas aos inimigos naturais de Plutella xylostella (L. ) (Lepidoptera: Plutellidae) em brássicas / Insecticide physiological selectivity to natural enemies of Plutella xylostella (L. ) (Lepidoptera: Plutellidae) in Brassicae

A conservação de inimigos naturais é um componente fundamental no manejo integrado de pragas. Neste trabalho, estudou-se a seletividade de sete inseticidas usados no manejo de Plutella xylostella (L.) (Lepidoptera: Plutellidae) ao parasitóide Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae) e aos predadores Discodon sp. (Coleoptera: Cantharidae) e Lasiochilus sp. (Hemiptera: Anthocoridae). Os organofosforados metamidofós e paratiom metílico e o carbamato carbaril foram altamente tóxicos aos três inimigos naturais estudados. Dimetoato apresentou seletividade em favor do Discodon sp., na dose e subdose. O inseticida acefato apresentou alto impacto ao Discodon sp., no momento da aplicação e seu efeito foi reduzido com a metade da concentração do princípio ativo. Deltametrina apresentou seletividade à O. sokolowskii nas duas dosagens. Já permetrina foi seletiva a Lasiochilus sp., somente na subdose. Discodon sp. foi mais tolerante à dose e subdose do dimetoato e à subdose do acefato do que O. sokolowskii e Lasiochilus sp. O predador Lasiochilus sp. foi mais tolerante à dose e subdose da permetrina do que O. sokolowskii e Discodon sp. O parasitóide O. sokolowskii foi mais tolerante à dose e subdose da deltametrina do que Lasiochilus sp. e Discodon sp. Os resultados desta pesquisa foram obtidos em condições de exposição extrema, portanto indicam que os inseticidas seletivos possivelmente não serão prejudiciais aos inimigos naturais, em condições de campo.

The conservation of natural enemies is a fundamental component in the integrated pest management. In this work, one studied the selectivity of seven insecticides, used in the management of Plutella xylostella (L.) (Lepidoptera: Plutellidae), to the parasitoid Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae), and to the predators Discodon sp. (Coleoptera: Cantharidae) and Lasiochilus sp. (Hemiptera: Anthocoridae). The organophosphates methamidophos and methyl parathion, and the carbamate carbaryl showed high toxicity to the three natural enemies studied. Dimethoate showed selectivity in favor of Discodon sp. in the dose and sub-dose. The insecticide acephate showed a high impact on Discodon sp. at the time of the application, and its effect was reduced with half of the concentration of the active ingredient. Deltamethrin showed selectivity to O. sokolowskii in both dosages. Permethrin was selective to Lasiochilus sp. only in the sub-dose. Discodon sp. was more tolerant to the dose and sub-dose of dimethoate and to the sub-dose of acephate than O. sokolowskii and Lasiochilus sp. The predator Lasiochilus sp. was more tolerant to the dose and sub-dose of permethrin than O. sokolowskii and Discodon sp. The parasitoid O. sokolowskii was more tolerant to the dose and sub-dose of deltamethrin than Lasiochilus sp. and Discodon sp. The results of this research were obtained in conditions of extreme exposition. Thus, the results indicate that it is probable that the selective insecticides will not be harmful to the natural enemies in the field.