Distinctive seed dispersal and seed bank patterns of invasive African grasses favour their invasion in a neotropical savanna.

High propagule availability compared to native species is often critical to invasion success, but it is unclear if this has contributed to invasions by African grasses in Neotropical savannas. We compared patterns of occurrence in the vegetation, seed rain and seed bank among African and native grasses in Cerrado sites in southeastern Brazil. In grasslands and savannas, we obtained the abundance of grasses in the vegetation, in the seed rain (monthly for one year) and in the seed bank (rainy and dry season), and assessed seed limitation and relationships among compartments. Invasive grasses showed low abundance in all compartments and high seed limitation in grasslands, where the seed bank and seed rain were dominated by small-seeded native grasses, but were at least as abundant as the natives in the seed bank and seed rain in savannas, mostly due to high abundance of Melinis minutiflora at these compartments. Native grasses dispersal occurred in the rainy season, whereas invasive grass dispersal occurred from mid rainy to mid dry season (Urochloa decumbens) and in the dry season (M. minutiflora). Melinis minutiflora showed a more persistent seed bank than U. decumbens and natives in savannas. Abundance of invasive and most of the native grasses in the vegetation was positively related to their abundance in the seed rain. Differences in seed production, the timing of seed dispersal and seed bank persistence compared to native grasses seem to favour invasive African grasses in the Cerrado, but this role may differ between grasslands and savannas.

Abundance of invasive grasses is dependent on fire regime and climatic conditions in tropical savannas.

Invasive grasses are a threat to some tropical savannas, but despite being fire-prone ecosystems, little is known about the relationships between fire season, climatic conditions and invasive species on these systems. We evaluated the response of the perennial invasive grasses Melinis minutiflora and Urochloa brizantha to three fire seasons in an open tropical savanna in South America: Early-Dry (May), Mid-Dry (July) and Late-Dry (October) in relation to unburned Controls. Moreover, we investigated how these responses were influenced by precipitation and extreme air temperatures. We hypothesized that biomass of both species would be reduced by fires during their reproductive period and that climatic conditions would affect them equally. We conducted prescribed burns on 15 × 15 m plots (4 plots x 4 treatment x 2 invasive species = 32 plots) in 2014. We sampled the biomass before the burn experiments and for the next two years (five 0.25 m2 samples/plot). Our experiments revealed that the fire season did not influence the abundance of either species. However, the two species responded differently to fire occurrence: M. minutiflora decreased whereas U. brizantha was not affected by fires. Early-Dry and Late-Dry fire treatments enhanced the replacement of M. minutiflora by U. brizantha. We found that the influence of precipitation depended on the species: it reduced M. minutiflora but increased U. brizantha abundance. Lower monthly minimum temperatures decreased the abundance of both species. It directly reduced live M. minutiflora and increased dead U. brizantha biomass. Monthly maximum temperatures affected the invasive grasses by reducing live M. minutiflora. Since tropical savannas are predicted to face climatic instability and that climate influences the differential response of invasive species, the management of invaders should consider both the identity of the target species and the possible interactions with other invasive species. Moreover, it is essential to keep an adaptive management approach to face the uncertainties that climate change may pose to biodiversity conservation.

Molasses Grass Induces Direct and Indirect Defense Responses in Neighbouring Maize Plants.

Plants have evolved intricate defence strategies against herbivore attack which can include activation of defence in response to stress-related volatile organic compounds (VOCs) emitted by neighbouring plants. VOCs released by intact molasses grass (Melinis minutiflora), have been shown to repel stemborer, Chilo partellus (Swinhoe), from maize and enhance parasitism by Cotesia sesamiae (Cameron). In this study, we tested whether the molasses grass VOCs have a role in plant-plant communication by exposing different maize cultivars to molasses grass for a 3-week induction period and then observing insect responses to the exposed plants. In bioassays, C. partellus preferred non-exposed maize landrace plants for egg deposition to those exposed to molasses grass. Conversely, C. sesamiae parasitoid wasps preferred volatiles from molasses grass exposed maize landraces compared to volatiles from unexposed control plants. Interestingly, the molasses grass induced defence responses were not observed on hybrid maize varieties tested, suggesting that the effect was not simply due to absorption and re-emission of VOCs. Chemical and electrophysiological analyses revealed strong induction of bioactive compounds such as (R)-linalool, (E)-4,8-dimethyl-1,3,7-nonatriene and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene from maize landraces exposed to molasses grass volatiles. Our results suggest that constitutively emitted molasses grass VOCs can induce direct and indirect defence responses in neighbouring maize landraces. Plants activating defences by VOC exposure alone could realize enhanced levels of resistance and fitness compared to those that launch defence responses upon herbivore attack. Opportunities for exploiting plant-plant signalling to develop ecologically sustainable crop protection strategies against devastating insect pests such as stemborer C. partellus are discussed.

Insecticidal activity of a Melinis minutiflora grass extract on Stomoxys calcitrans flies / Actividad insecticida de un extracto del pasto Melinis minutiflora sobre moscas Stomoxys calcitrans / Atividade inseticida do extrato do pasto Melinis minutiflora sobre a mosca Stomoxys calcitrans

Several extraction methods were utilized to study the insecticidal activity of a Melinis minutiflora grass extract. Four concentrations (10, 20, 40, 80%) of the product which had the highest yield (i.e., a wax-free hexane extract) were tested to establish its insecticidal activity on S. calcitrans flies (adults and larvae). It is hypothesized that an 80% plant extract has a lethal effect similar to or higher than cypermethrin on flies and larvae. Mortality rates were assessed by survival analysis comparing the Kaplan-Meier mortality curves against the standard pyrethroid (cypermethrin) or against the solvent (ethyl acetate). The median mortality (time to kill 50% of the population) of adult flies exposed to cypermethrin over an eight-hour period was one hour, while it was three hours for the highest concentration of the extract tested. Both medians were significantly lower than those of the other treatments. The median mortality of larvae exposed to cypermethrin for a 30 minute period was four minutes, while it was eight minutes for the highest concentration (80%) of the plant extract. These values were significantly lower than the medians obtained for the other concentrations of the extract. These data suggest that extracts obtained from Melinis minutiflora grass could be a safe substitute for chemical insecticides.

Después de evaluar tres métodos para obtener un extracto del pasto Melinis minutiflora, cuatro concentraciones (10, 20, 40, 80%) del producto que tuvo el mayor rendimiento –un extracto hexanoico libre de ceras– fueron comparadas para establecer su actividad insecticida sobre moscas Stomoxys calcitrans (adultos y larvas). Se plantea como hipótesis de investigación, que una concentración de 80% del extracto, tiene un efecto letal similar o superior al de la cipermetrina. Las tasas de mortalidad se evaluaron mediante análisis de supervivencia comparando las curvas de mortalidad de Kaplan-Meier para un piretroide (cipermetrina) y el solvente (acetato de etilo). La mediana de la mortalidad (o sea el tiempo requerido para matar el 50% de la población) de las moscas adultas expuestas a cipermetrina durante un período de ocho horas fue de una hora, mientras que fue de tres horas para el extracto al 80%. Ambas medianas fueron significativamente menores que las de los otros tratamientos. La mediana de la mortalidad de larvas expuestas a cipermetrina para un período de 30 minutos fue de cuatro minutos y de ocho minutos para la concentración más alta (80%) del extracto del pasto. Estos valores fueron significativamente menores que las medianas obtenidas para las otras concentraciones del extracto.

Neste trabalho foram testados vários métodos de extração química para obter diferentes extratos do pasto Melinis minutiflora para estabelecer com qual obtém-se maior quantidade e maior atividade inseticida. O extrato obtido em maior quantidade foi o extrato hexânico livre de ceras em 0.60%, o qual determinou-se sua ação inseticida sobre larvas e adultos de S. calcitrans reproduzidas no laboratório. A mortalidade que produzem quatro concentrações (10, 20, 40, 80%) do extrato livre de ceras, percebe-se mediante a análise estatística de sobrevivência comparando as curvas de mortalidade de Kaplan-Meier frente ao padrão de referencia mosquicida cipermetrina (piretroide), e solvente acetato de etilo. As análise estatística da atividade inseticida das concentrações do extrato sobre S. calcitrans adultos em um período de oito horas, expressa pela mediana (tempo no qual se morrem o 50% da população), do efeito inseticida da cipermetrina foi de 1 hora, por enquanto na mediana do efeito inseticida do extrato à concentração de 80% sobre moscas adultas foi de 3.0 horas. Ambas as médias foram significativamente inferiores quando comparados com os dos demais tratamentos. Quanto à atividade inseticida de mortalidade, 50% das larvas expostas à cipermetrina ou acetato de etila por um prazo de 30 minutos a 4 minutos e a eficácia inseticida do extrato a 80% das larvas foi de 8 minutos, valores significativamente inferior ao de outras concentrações.

Allelopathic evidence in Brachiaria decumbens and its potential to invade the Brazilian Cerrados

The aim of this study was to look for evidence of allelochemicals in B. decumbens, in parts of the plant from where they could easily be released to the environment. The germination inhibition of Phalaris canariensis, Lactuca sativa (standard species) and Melinis minutiflora, another invasive African grass, was tested using B. decumbens germinating seeds and aqueous leachates of the roots, green and senescent leaves, at 5, 10 and 20% w/v. Both the germinating seeds and the aqueous leachates of B. decumbens reduced the germination of the species tested; the effectiveness of the aqueous leachates increased according to concentration. Apparently, the competitive advantage of B. decumbens in the cerrados could be amplified via allelopathy.

Invasão biológica é uma das maiores causas atuais da perda de biodiversidade. Várias espécies que se tornam invasoras produzem substâncias fitotóxicas que aumentam sua capacidade competitiva; assim, a alelopatia é uma estratégia capaz de potencializar o sucesso de invasão. Brachiaria decumbens, uma gramínea africana, invadiu os cerrados brasileiros e representa, atualmente, uma séria ameaça à biota regional. Neste ensaio, verificamos a presença de aleloquímicos em partes de B. decumbens das quais poderiam ser facilmente liberados para o ambiente. Testamos a inibição da germinação de Phalaris canariensis, Lactuca sativa (espécies padrão) e Melinis minutiflora (outra gramínea africana invasora) usando sementes de B. decumbens e soluções aquosas lixiviadas de suas raízes, folhas verdes e folhas senescentes, a 5, 10 e 20% de peso do material por volume de água. Tanto as sementes como as soluções lixiviadas de decumbens reduziram a germinação das outras espécies; a eficiência dos lixiviados aumentou de acordo com a concentração da solução. Acreditamos que a grande vantagem competitiva de B. decumbens no cerrado possa ser amplificada pela alelopatia.