Studies on the Contribution of Weeds and their Management to the Prevalence of Pineapple Mealybugs

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Weeds pose a serious problem to the production of pineapple causing losses of up to 83% on the farm. They compete for nutrients, water, light and other resources as well as harbouring mealybugs and their tending ants which greatly affect production. This research was thus conducted first to catalogue the prominent weeds found in three major pineapple growing Districts in Ghana: Mfantsiman, Gomoa East and Akuapim South Districts. It further sought to determine which of these weed species harboured the pineapple mealybugs and their tending ants and to determine the effect of four weed management methods on the prevalence of pineapple mealybugs and the growth and yield of pineapples. The research was carried out in three phases: the first phase consisted of a survey to identify and quantify the common weeds found on pineapple farms in the three Districts. Cluster sampling method was employed to determine the weed species with the aid of a 1m2 quadrat on 15 farms in the three districts. The second phase involved the identification of the pineapple mealybugs and their tending ants, interviews with pineapple farmers on various weed management practices and alternative host of the mealybugs, and a scout for the mealybugs on the weeds within and adjacent the 15 pineapple fields. The last phase was a field experiment to evaluate the various methods of weed management employed by pineapple farmers. The experiment was a Latin square design with 5 treatments and 5 replications. A total of 43 weed species from16 families were recorded from the three districts with Mfantsiman recording the least number of species (29), followed by Gomoa East (34), and Akuapim South (40). Only Dysmicoccus brevipes (Homoptera: Pseudococcidae) was found to infest pineapples. Four ant genera (Crematogaster, Camponotus Pheidole and Solenopsis) were found to attend the mealybugs. No weed was identified as an alternative host to the D. iii brevipes. Six weed species were rather found to be positively associated with the tending ants of the mealybugs. The Mfantsiman District recorded the highest mealybug infestation with 78 ± 5.2 mealybugs per plant, and this was significantly different from Akuapim South and the Gomoa East Districts. The populations of tending ants and the density of grasses adjacent to field were found to be positively correlated to the population of the pineapple mealybugs. Of the four weed management methods evaluated, plastic mulch + synthetic herbicide was found to be most efficient in weed control (90.6% over weedy check). This was followed by the plastic mulch + manual weeding (80.2% over weedy check), synthetic herbicide only (73.4 % over weedy check), then manual weeding only (69.2% over weedy check) in that order. The same trend was observed in the ability of the weed management method to promote growth and yield, with plastic mulch + synthetic herbicide recording an average fruit weight of 1.95 kg, plastic mulch + manual weeding recording 1.82 kg, synthetic herbicide only, 1.61 kg, manual weeding, 1.56 kg and 1.40 kg for the weedy check. The weed management methods significantly reduced the populations of pineapple mealybugs on pineapple fruits but not on the roots.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the Degree of Master of Science, 2012