Both prey density and developmental stage of pests and natural enemies are known to influence the effectiveness of biological control. However, little is known about the interaction between prey density and population structure on predation and fecundity of generalist predatory mites. Here, we evaluated the functional response (number of prey eaten by predator in relation to prey density) of adult females and nymphs of the generalist predatory mite Euseius concordis to densities of different developmental stages of the cassava green mite Mononychellus tanajoa, as well as the fecundity of adult females of the predator. We further assessed the instantaneous rate of increase, based on fecundity and mortality, of E. concordis fed on eggs, immatures and adults of M. tanajoa. Overall, nymphs and adults of E. concordis feeding on eggs, immatures and females of M. tanajoa had a type III functional response curve suggesting that the predator increased prey consumption rate as prey density increased. Both nymphs and adult females of the predator consumed more eggs than immatures of M. tanajoa from the density of 20 items per leaf disc onwards, revealing an interaction between prey density and developmental stage in the predatory activity of E. concordis. In addition, population growth rate was higher when the predator fed on eggs and immatures in comparison with females. Altogether our results suggest that E. concordis may be a good candidate for the biological control of M. tanajoa populations. However, the efficiency of E. concordis as a biological control agent of M. tanajoa is contingent on prey density and population structure.
The cassava green mite, Mononychellus tanajoa, is an important pest of cassava, Manihot esculenta (Euphorbiaceae), in the northeastern state of Maranhão, Brazil. Predatory mites of the family Phytoseiidae are key natural enemies of pest mites and are found inhabiting cassava plants. We compared some biological aspects and the growth rate of M. tanajoa and the generalist phytoseiid Euseius ho the most abundant predatory mite inhabiting cassava plants in the study region. All experiments were conducted in the laboratory on leaf discs of cassava leaves. The predatory mite was fed in all developmental stages with M. tanajoa. Euseius ho had lower periods of egg, larva, protonymph and deutonymph developmental periods, as well as the period from egg to adult compared to M. tanajoa. Furthermore, the predatory mite E. ho had a high instantaneous rate of increase (ri), yet lower than that observed for its prey, the cassava green mite M. tanajoa. The laboratory results suggest that the predatory mite E. ho may contribute to regulate populations of M. tanajoa in the field.
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