Distribuição espacial de nematóides fitoparasitos em áreas cultivadas com cana-de-açúcar

Nematóides são importantes parasitos da cana-de-açúcar no Brasil e, em função disso, muitos estudos foram conduzidos, nos últimos anos, para avaliar a eficiência de medidas de controle. Entretanto, não há estudos sobre a distribuição espacial de nematóides em canaviais e esses estudos são essenciais para o desenvolvimento de planos de amostragem, visando aplicação em programas de manejo integrado. A distribuição espacial de Meloidogyne javanica, Pratylenchus zeae e de populações mistas de P. zeae e P. brachyuurs foi estudada em seis campos comerciais, com áreas de 0,26 a 0,50 ha, nas quais procedeu-se à amostragem na época chuvosa do ano. As amostras, representadas por cerca de 50 g de raízes, foram coletadas em grid retangular de 7 × 12 m, 7 × 10 m ou 5 × 13 m. Dos dez semivariogramas calculados (quatro para populações de M. javanica e seis para populações de P. zeae ou mistura de P. zeae + P. brachyurus), quatro não se ajustaram a qualquer modelo, apresentando efeito pepita puro; o modelo esférico foi o que melhor se ajustou aos semivariogramas dos dados observados nas outras seis situações. Nesses casos, o alcance variou de 18 a 35 m, permitindo estimar a área de agregação dos nematóides em 2110 m², em média, sugerindo que, seriam necessários, em média, pelo menos cinco pontos de amostragem por hectare para obter uma estimativa confiável da população desses fitoparasitos em determinada área.Nematodes are important parasites of sugarcane in Brazil. Because of this, several studies have been conducted in recent years to evaluate the effectiveness of control methods. However, no studies have been reported on the spatial distribution of nematodes in sugarcane fields, and such studies are indispensable for the development of sampling plans, aimed at their application in integrated management programs. The spatial distribution of Meloidogyne javanica, Pratylenchus zeae, and mixed populations of P. zeae and P. brachyurus was studied in six commercial fields, with areas ranging from 0.26 to 0.50 ha; samples were obtained during the rainy season. The samples, represented by about 50 g roots, were collected within a rectangular grid measuring 7 × 12 m, 7 × 10 m or 5 × 13 m. Among the ten calculated semivariograms (four for M. javanica populations and six for P. zeae populations or the mixture between P. zeae + P. brachyurus), four could not be fitted to any model and presented a pure nugget effect; the spherical model showed the best fit to the semivariograms of data observed in the other six conditions. In those cases, values of range in semivariogram varied from 18 to 35 m, allowing the nematode aggregation area to be estimated at 2,110 m², on average, suggesting that at least five sampling points per hectare would be necessary, on average to obtain a reliable estimate for the population of these plant parasites in a given area

Economic injury level for sugarcane caused by the spittlebug Mahanarva fimbriolata (Stål) (Hemiptera:Cercopidae)

A cigarrinha-das-raízes, Mahanarva fimbriolata (Stål), é uma das mais importantes pragas da cana-de-açúcar na região Centro-Sul do Brasil. Apesar disso, avanços na implantação de programas de manejo estão sendo limitados pela falta de informações sobre o nível de dano econômico. Neste trabalho, o nível de dano econômico de M. fimbriolata foi estimado em experimento em campo, em áreas colhidas em setembro, aplicando thiamethoxam na dose de 200 g a.i. ha-1 ou imidacloprid na dose de 720 g a.i. ha-1. Em um dos ensaios, as aplicações de inseticidas foram feitas quando as infestações da praga eram de 4.2 (11/12/04), 7.1 (01/11/05) ou 16.3 (01/18/05) insetos m-1 e no ensaio 2, quando as populações da praga eram de 5.6 (11/12/04), 8.5 (01/11/05) ou 15.3 (01/11/05) insetos m-1. Parcelas testemunhas sem inseticida foram mantidas. Após as aplicações, as infestações de cigarrinha foram estimadas mensalmente e os ensaios foram colhidos em setembro de 2005. O controle de cigarrinha pela aplicação de inseticidas resultou incrementos de produtividade de colmos e de açúcar, em relação à testemunha, em ambos os ensaios. Aplicações feitas sob infestações menores resultaram em maiores produtividades do que aplicações feitas com populações mais elevadas. Não houve diferenças entre os inseticidas em um dos ensaios, mas no outro, thiamethoxan contribuiu para maiores incrementos de produtividade do que imidacloprid. Aplicações de inseticidas feitas sob infestações mais baixas resultaram em maiores lucros. Análises de regressão permitiram estimar o nível de dano econômico da praga, nas condições do presente ensaio, entre 2 e 3 insetos m-1.The sugarcane spittlebug, Mahanarva fimbriolata (Stål), is currently one of the most important pests of the sugarcane crop in Brazil. In spite of its economic importance, advances in the management of this pest have been limited by the lack of information on the economic injury level. In this study, the economic injury level for M. fimbriolata was estimated in a field experiment, over areas harvested in September, applying thiamethoxam at 200 g a.i. ha-1 or imidacloprid at 720 g a.i. ha-1. In one of the experiments, insecticide applications were made at pest infestation values of 4.2 (11/12/04), 7.1 (01/11/05), or 16.3 (01/18/05) insects m-1, and in experiment 2 when pest populations were 5.6 (11/12/04), 8.5 (01/11/05), or 15.3 (01/11/05) insects m-1. Control plots without insecticide were maintained. After the applications, spittlebug infestations were estimated monthly, and the experiments were harvested in September 2005. Spittlebug control with the application of insecticides resulted in stalk and sugar yield increases in relation to the control, for both experiments. Applications performed under smaller infestations resulted in higher yields than applications made under higher populations. There were no differences between insecticides in one of the experiments; in the other, however, thiamethoxan contributed to greater yield increases than imidacloprid. Insecticide applications made under lower infestations resulted in greater profits. Regression analyses allowed the estimation of the pest economic injury level to be between 2 to 3 insects m-1 for the conditions of this experiment

Resistance mechanisms of sugarcane cultivars to spittlebug Mahanarva fimbriolata

The spittlebug Mahanarva fimbriolata (Stål) (Hemiptera: Cercopidae) is one of the most important pests of sugarcane in Brazil. Measures for population control are currently restricted to the use of chemical insecticides and fungus Metarhizium anisopliae, in part because very little information exists regarding resistance of sugarcane cultivars. Therefore, the aim of this study was to evaluate resistance mechanisms of 12 different sugarcane cultivars to M. fimbriolata to provide information for growers for pest management. Isolated buds of each cultivar were planted in pots and kept in a greenhouse for approximately three months. The pots were then moved to climate test chambers (26 ± 1 ºC; 70 ± 10 % relative humidity; 12 h photoperiod) to carry out laboratory tests to evaluate adult feeding and female oviposition preferences (using both free-choice and no-choice tests) as well as the effects of cultivars on nymph development and cultivars tolerance to pest attack. The least attractive cultivars for adult feeding were IACSP96-2042 and IAC91-1099. Cultivars IACSP96-2042, IACSP96-3060 and IACSP94-2101 received the fewest eggs in the free-choice and no-choice oviposition tests and exhibited some level of antixenosis resistance. Cultivar IAC91-1099 showed the highest level of antibiosis resistance with a nymph survival rate in the roots of only 20 %. Finally, cultivar IACSP94-2094 appears to be tolerant to M. fimbriolata, as it did not show significant reductions in aboveground biomass weight, despite showing reduced leaf chlorophyll levels following pest attack

Resistance of sugarcane cultivars to Diatraea saccharalis

O objetivo deste trabalho foi avaliar a preferência de oviposição de Diatraea saccharalis e o efeito de dez cultivares de cana‑de‑açúcar no desenvolvimento larval. A preferência para oviposição foi avaliada em casa de vegetação, por meio de três liberações de casais da praga, com posterior contagem de posturas e de ovos em cada planta. Para avaliar o efeito das cultivares sobre o desenvolvimento da fase larval, cada planta foi infestada com cerca de 150 ovos e, após 29 dias, foram avaliados: número de entrenós totais, número de entrenós brocados, número de formas biológicas encontradas, peso e comprimento das lagartas e das pupas, e largura da cápsula cefálica das lagartas. As cultivares IACSP94‑2101 e IACSP96‑2042, as menos preferidas por D. saccharalis para oviposição, e IACSP94‑2094, a mais desfavorável para a entrada e o desenvolvimento das lagartas no interior dos colmos, apresentam resistência ao inseto.The objective of this work was to evaluate the oviposition preference of Diatraea saccharalis and the effect of ten sugarcane cultivars on larval development. Oviposition preference was assessed under greenhouse conditions by three releases of couples of moths, with subsequent counting of egg masses and eggs per plant. In order to evaluate the effect of the cultivars on larval development, each plant was infected with about 150 eggs, and, 29 days later, the total number of internodes, number of bored internodes, number of life forms found, larval and pupal weight and length, and the width of larval head capsule were evaluated. The cultivars IACSP94‑2101 and IACSP96‑2042, the least preferred by D. saccharalis for oviposition, and IACSP94‑2094, the most unfavorable for larvae entrance and development, show resistance to the pest

Sugarcane straw and the populations of pests and nematodes

The green cane harvesting represented a significant change in sugarcane ecosystem due to the presence of straw left on the soil and to the absence of fire. These two factors may affect the populations of pests and their natural enemies. Among the pests benefit from the green cane harvesting stand out the spittlebug, Mahanarva fimbriolata, the curculionid Sphenophorus levis and sugarcane borer, Diatraea saccharalis. In areas of green cane harvesting, the population of these species grew faster than in areas of burnt cane. On the other hand, there are virtually no records of attacks by lesser cornstalk borers in areas of green cane harvesting. Populations of plant parasitic nematodes and the beetles Migdolus fryanus, very important pests of sugarcane, were apparently not affected by the green cane harvesting. Despite the absence of more consistent information, it appears that populations of ants and the giant borer Telchin licus can increase in green cane areas, due primarily to the difficulty of pest control. The partial or total removal of straw from the field represents an additional change to the ecosystem that could alter the status of pests and nematodes. It is likely that spittlebug, the curculionid S. levis and sugarcane borer populations decrease if a portion of the straw is removed from the field. However, the pest populations in areas where the straw is collected will not return to their original conditions at the time of burnt cane harvesting because the absence of fire will be maintained