ABSTRACT
Abstract Chrysodeixis includens (Walker, [1858]) is currently the main plusiine pest in the Americas, not only because of the damage caused to soybean, but also with several crop species in a broad geographical range. However, its population dynamics is still poorly understood, despite outbreak records that are common across different locations in the Americas. The current study aimed at identifying phenological patterns of C. includens emphasizing its differences among the three years of sampling effort in an intercropping area of the Brazilian Savanna. Thereafter, we tested whether the El Niño size effect, meteorological factors, or soybean, corn and wheat cycles, are better predictors of its monthly abundance. The insects were collected with a light trap during five consecutive nights (repetitions) during 35 new moons. In total, 2026 specimens were collected in all months of the year although not consecutively. Across each year, monthly abundance of C. includens was non-uniform, characterized by sharp population peaks concentrated in the rainy season. These peaks varied from January until March, depending on the year sampled. We found that the local soybean cycle and El Niño effect to significantly influence the species abundance across the entire period of study. These results aid in understanding the species population dynamics and its status as a pest, providing evidence of factors that determine its phenological patterns. Although it presents a very defined phenology, the population dynamics of C. includens varies significantly between years and locations, which demonstrates the importance and need to monitor local populations of larvae and adults for its management.
ABSTRACT
A cigarrinha das pastagens Deois flavopicta Stal (Hom.: Cercopidae) ocorre naturalmente em gramíneas nativas no Brasil Central em baixas densidades populacionais. Após a introdução de gramíneas africanas, principalmente do gênero Brachiaria, D. flavopicta passou a produzir explosões populacionais e tornou-se a principal praga das pastagens na região. Estudos visando comparar os efeitos da planta hospedeira nativa e da exótica sobre a fecundidade do inseto foram desenvolvidos. Fêmeas de D. flavopicta, mantidas durante a fase adulta em Brachiaria ruziziensis, produziram mais ovos e tiveram maior longevidade média que aquelas mantidas em Axonopus marginatus, independente da planta hospedeira em que foram criadas durante a fase ninfal. Devido aos danos causados pelos adultos na planta hospedeira, o efeito da densidade de indivíduos em sua própria fecundidade foi avaliado em gaiolas de oviposição contendo plantas de B. ruziziensis padronizadas no comprimento e quantidade de perfilhos. Densidades de um, dois, três, quatro e seis casais com fêmeas virgens foram avaliadas. A densidade populacional de três casais, equivalente a 150 adultos/m², ou maior que esta, reduziu a fecundidade das fêmeas. Estes estudos contribuíram para o entendimento dos mecanismos que influem para as baixas populações de cigarrinha observadas em gramíneas nativas e que promovem explosões populacionais em pastagens cultivadas com plantas introduzidas.
The spittlebug Deois flavopicta Stal (Hom.: Cercopidae) occurs naturally on native grasses in Central Brazil in low population densities. After the introduction of African grasses, mainly of the genus Brachiaria, for cattle raising, D. flavopicta began to produce population outbreaks and became a pest. Two studies were conducted, aiming to estimate the effects of a native and an exotic host plant on the fecundity of this insect. Females of D. flavopicta maintained during the adult stage on Brachiaria ruziziensis produced more eggs and lived longer than those maintained on Axonopus marginatus (a native grass widely distributed in Brazil), independently of the host plant on which the nymphs were reared. Due to the severe damage produced by adult D. flavopicta on the host plant, the effect of insect density on its own reproductive capacity was evaluated in oviposition cages containing plants of B. ruziziensis, standardized in height and stem number. Densities of one, two, three, four and six couples with virgin females were evaluated. Population densities of three couples, equivalent to 150 adults/m², or higher decreased insect's fecundity. These results contributed to the understanding of the mechanisms that determine low levels of spittlebug populations in the native grasses and promote population outbreaks in introduced ones.