Biology and control of Planococcus Citri (Risso) (Hemiptera: Pseudococcidae) on five yam varieties in storage

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March 2016
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The Citrus mealybug, Planococcus citri (Risso) seriously infests yams in storage. However, there is little information on the biology and control of P. citri on stored yam locally and the West African sub-region. Incubation and developmental periods, adult longevity, total number of eggs laid and life span of P. citri on stored yam (Disocorea species) were studied on five yam varieties stored in the laboratory with an ambient temperature of 26.0 - 30.0 0C and relative humidity of 70.0 -75.0 %. The mean life span values of female P. citri on the Dioscorea rotundata varieties Pona, Labreko and Muchumudu and the D. alata variety Matches and Dioscorea rotundata var. Dente were 62.4, 63.5, 66.8, 67.3 and 69.8 days, respectively and those of the male were 23.8, 25.3, 29.6, 30.2 and 33.5 days, respectively. Generally, the adult female lived longer than the adult male. The citrus mealybug was found to prefer D. rotundata var. Pona to the rest of the varieties because it developed faster and laid more eggs (497.0 eggs) on it. The least number of eggs (257.0 eggs) was laid on Dioscorea rotundata var. Dente and the mealybug development on it was the slowest. It should therefore be the preferred yam for long-term storage. Six chemicals were evaluated for their effect on the third instar and adult P. citri in both the laboratory and in improved yam barn storage conditions. In the laboratory, the topical method was used and in the barns, yams were sprayed with a small hand-held sprayer. In the bioassay test, observations on mortality were recorded at 12, 24 and 48 h and subjected to Probit analysis to obtain LD50 and LD90 values. It was observed that in both stages of the insect pest, the LD50 and LD90 decreased with exposure period. At the same exposure period, the lethal dosage for the third instar was lower than that of the adult insect. In all the experiments, cypermethrin recorded the lowest LD50 and LD90 values for both the third instar and adult stages with key soap iii solution recording the highest LD50 and LD90 values. In the improved yam barns 0.05 % cypermethrin, 0.07 % imidacloprid, 1.0 % groundnut oil emulsion, 1.2 %, soyabean oil emulsion, 1.9 % sunlight dishwashing detergent, 2.5 % key soap solution as well as water (control) were sprayed four times (seven days apart) onto the mealybug infested yams. The results showed that a second application of cypermethrin, imidacloprid and groundnut oil to the yams caused over 95 % mortality and was significantly (P < 0.05) higher than mortality caused by soyabean oil, sunlight detergent and key soap solution. Therefore, for rapid control of mealybug infestation of yam setts in storage, cypermethrin is recommended and it should be applied at the concentration of 0.049 %. However, the following chemicals, groundnut oil emulsion 0.963 %, soyabean oil emulsion 1.231 %, sunlight dishwashing detergent 1.948 % and key soap solution 2.475 % could be used to effectively manage P. citri on ware yam.
A thesis submitted to the Board of Graduate Studies Kwame Nkrumah University of Science and Technology, Kumasi, Ghana in partial fulfillment of the requirement for the award of Doctor of Philosophy Degree (Entomology),