Bio-Ecological Investigations onExochomusnigromaculatus(Coleoptera: Coccinellidae) as a Predator of Phenacoccussolenopsis in Khartoum Area- Sudan

Abstract

The cotton mealybug (CMB) {Phenacoccussolenopsis (Tinsley); HemipteraPseudococcidae} is an accidentally introduced pest during the last decade in Sudan. Soon after its appearance, the pest managed to multiply rapidly in population and spread to different parts of the country causing severe infestation and serious damage to various cultivated and wildplant species.Therefore, the current study was anattempt to evaluate the status of CMB in Khartoum area, largely through some bio-ecological investigations. This included field survey of CMB and its associated coccinellid predators on some main host plants during January March 2019, where population levels of the main detected coccinellid species (Exochomusnigromaculatus) were assessed on selected hosts and non-hosts (i.e., aphid infested) of CMB in the area. Moreover, studies dealt with some parameters on developmental durations of pre-imaginal stages, life tables data and feeding rates of E.nigromaculatus, were tracked when the predator reared on P. solenopsisfed on sprouting potato (Solanumtuberosum L.) tubers. Such parameters were performed during March – May 2019, under laboratory conditions (24.26 ± 0.98 ºC and 44.94 ± 1.45% R.H), at theEnvironment, Natural Resources and Desertification Research Institute, National Centre for Research, Khartoum. Results of field survey revealed that there was a declining population trend ofP. solenopsis in the study area, with just localized niches on few hidden host plants.This suggesting the presence of certain stresses (a biotic and biotic), the important of which wastheoccurrence of potent indigenous natural enemies, particularly predators and parasitoids, which invite further research to evaluate their actual role in regulating the population of the pest. Four coccinellid predators were detected in association with the CMB in the field, with E. nigromaculatusbeing the major species found. The level of E. nigromaculatusseemed to follow the population density of its prey host, P. solenopsis. However, regarding plants infested with three species of aphids but non-hostingCMB, the predator showed higher levels on hosts ofMelanaphissacchariand Aphis neriithan on hosts of Rhopalosiphummaidis. For laboratory results, the mean incubation, total larval and total pre-imaginal periods of E. nigromaculatuswere, 6.85 ± 0.13, 16.79 ± 0.32 and 29.24 ± 0.30 days, respectively. There was a progressive increase in larval duration, starting from the second instar (2.88 ± 0.15 days) up to the fourth instar (7.47 ± 0.26 days). Oviposition period was 13.50 ± 1.06days, and the mean fecundity was (82.63 ± 12.13) eggswhen the predator preyed onP. solenopsis. The mentioned bioagent conveyed high survival rates during the fourth larval instar (100%) and pupal stage (97.14%), and medium survival during the other pre-adult stages (50.00 – 65.42%). The calculated major life tables’ parameters indicated that the net reproduction rate (Ro), mean generation time (T), intrinsic rate of increase (rm) and doubling time (DT) computed were, 12.08, 34.85, 0.07 and 9.69, respectively. The weaknessesobserved in fecundity and some critical life tables data could be attributed to climatic conditions and some other uncontrollable technical factors during the study period, which necessitate re-evaluation of these biological aspects in future research. Nevertheless, the current predator was thoughtto be important, because of several characteristics that believed to add to its potentiality, including its wide prevalence and dominance on all CMB infested plants, besidesits good survival rate and potential ability to devourappreciable amounts of P. solenopsisinsects during both larval and adult durations. The results of a non-choice testconducted showed that the total larval instars killed37.13±2.11CMB nymphs during their lifetime, while adult male and female consumed respectively as high as 11.00 ±2.12and 12.50 ±1.69nymphs merely per one day. The overall results concluded that there is a possibility of management of the CMB in the country via proper assessment of all abiotic and biotic factors affecting population buildup and spreading of this pest, to formulate a sound program for that goal.