Exochomus troberti Mulsant (Coleoptera: Coccinellidae): a predator of cassava mealybug, Phenacoccus manihoti Mat-Ferr (Homoptera: Pseudococcidae) in southeastern Nigeria

The developmental periods of the immature stages of Exochomus troberti Mulsant were studied in the laboratory at the prevailing temperature range of 26-30 degree C. that of the second instar larva was the shortest (2.1 days) followed by the third instar larva (3.1 days). The longest developmental period (approximately 5 days) was recorded for the egg, first- and fourth-instar larvae and pupa. Oviposition commenced after 1 week of adult life and peaked in the 5th week with 37.2 female eggs/female. All ovipositing females survived the first 9 weeks of adult life when 82% of the total eggs were laid. By the 13th week, which was the effective end of reproduction (98% fecundity) only 40% of the females had died. Both capacity for increase (r(c)) and intrinsic rate of increase (r(m)) were calculated for E. troberti and found to be 0.092/day and 0.105/day, respectively. Mean daily predation rates in non-choice experiments on the second instar nymphs of P. manihoti by the fourth, third and second instar larvae of E. troberti were 28.3, 15.6 and 8.0, respectively. On the third instar nymphs and fourth instar young adults of P. manihoti the mean daily predation rates by the fourth, third and second instar larvae of E. troberti were 17.6,8.3 and 4.5; and 4.0,3.0 and 1.0, respectively. The populations of E. troberti, other coccinellid predators and P. manihoti were monitored in two small local farms for three seasons. E. troberti was found to be the dominant coccinellid predator in terms of numbers of individuals found in the field during the years of this experiment. The cyclical population oscillations of P. manihoti were similar in all the experimental years but differed only in the amplitude of oscillations. Those of E. troberti and the other coccinellid predators were lower than those of P. manihoti.