Effects of cocoa tree phenology and canopy microclimate on the performance of the mirid bug Sahlbergella singularis

Abstract

Populations of the cocoa mirid, Sahlbergella singularis Haglund (Hemiptera: Miridae), on cocoa, Theobroma cacao L. (Malvaceae), display sharp seasonal fluctuations in population size. To better understand factors and mechanisms involved in seasonal variation, we studied the life history traits of 10 successive generations of a caged population under field conditions. Nymphal survival and development were assessed on pods stored under cocoa canopy, whereas female survival and fecundity were evaluated on sleeved pods in cocoa trees. At the same time, cocoa phenology and cocoa canopy microclimate were characterized. A life table analysis was conducted with survival and fecundity temporal data. Reproductive parameters showed far more variation than nymphal development parameters, both within and among generations. We obtained low intrinsic rates of natural increase (about 0.04), the variation of which was mainly linked to the decrease in fecundity. Fecundity as well as life table parameters varied in accordance with natural population fluctuations, suggesting that S. singularis seasonal variation might be mainly linked to variation in female fecundity. A multiple regression analysis showed that variation in female fecundity was first correlated with the presence of young fruits on cocoa trees. Fecundity was also weakly affected by daily minimum r.h. and daily mean temperature. The usefulness of our results for modeling of S. singularis population dynamics and for the improvement of IPM strategies is discussed.

Models for Miridae, a cocoa insect pest. Application in control strategies

“…Mirid S. singularis has a very long life cycle (eggs to adults). It is on average 40 days with a minimum of 36 days and a maximum of 50 days . The percentage of hatching eggs is globally 96.53% as the eggs are protected in the pods cortex.…”

“…Indeed, after this mating period females are considered as mature. On average 72.1% of immature females become mature females . After the emergence, adults (males and females) fly from one cocoa tree to another ensuring the spatial dispersion of the individuals and causing the spatial distribution of the damage in the plots.…”

“…In fact, mirids dynamics vary greatly during the year; density of populations is likely to be influenced by pods avail ability on the trees. In Babin et al, it is admitted that lower mirid populations observed in the plots during a certain period of the year is due to the declining fertility of females and increasing mortality of individuals. Thus, it seems that development parameters (longevity, fecundity, and mortality) of mirids vary depending on season.…”

“…We use these data to estimate several parameters like mortality of eggs, nymphs, and adults ( μ E , μ L , and μ A ), fecundity of adults female ( b ), the mean duration of immature females stage (1/ ν F ) and the mean necessary time for development of eggs. For some parameters (the sex ratio r , the mean duration of egg stage (1/ ν E ) and the mean duration of nymph stage (1/ ν L )), we used data from Babin et al because it was the last experiment on S. singularis life cycle realized in Cameroon. The most difficult data to obtain were the daily evolution of pods number in the plot.…”

Isolation and Characterization of Twelve Polymorphic Microsatellite Loci for the Cocoa Mirid Bug Sahlbergella Singularis

“…Since evidence was produced that individuals on a cocoa tree may be related [2], we collected a single individual per tree. PCR amplification was performed with a thermocycler TC-512 (Techne) following a Touchdown procedure [14] with an annealing temperature decrease of 0.5 °C per cycle during the first 10 cycles of the PCR (from 60 °C to 55 °C).…”