THE DISTRIBUTION OF CURCULIONID WEEVILS, BIOCONTROL MEASURES AND THEIR EFFECTS ON TEA YIELDS IN EASTERN REGIONS OF KENYA

Abstract

Two curculionid weevils namely: Entypotrachelus meyeri [Micans/Kolbe] (Kangaita weevil) and Sphrigodes mixtus [Hustache] (Nyambene weevil) defoliate tea bushes and affect tea production. Studies were conducted to determine the pests on prevalence, distribution and relation to cultural practices, yield loss, action threshold i.e the number of weevils per bush before any action is done and two control strategies. In this regard, a modeling exercise was undertaken to determine weevil distribution and determine potentially risky areas using Geographical Information System (GIS) with reference to rainfall, temperature and agro-ecological zones (AEZ) as parameters which influence weevil incidence. In addition, interrelations of weevil distribution to agricultural practices were gathered through a questionnaire. three management options were initiated utilizing two local Beauveria bassiana (balsom) Vulliemin isolates at a concentration of 1013 conidia /ha, Karate 2.5 WG, Water Dispersible Granules which contains Lambda Cyhalothrin 25g/Kg as the active ingredient, a chemical insecticide and push -pull technology based on two shade trees (Fraxinus spp. Milletia dura), avocado and two tea clones (TRFK 306 -purple tea and TRFK 31/8). The trials were laid out in complete random block designs with three replications were used in the field experiment to determine the efficacy of the fungi isolates. Leaves of shade trees or clones were placed in small pots in mixtures or alone and were arranged in a complete random design in cages and replicated three times to determine preference of weevils to plant leaves in the push –pull technology. The risk assessment made to determine whether the weevil species can enter and establish in new areas indicated that based on agro-ecological zone I and climatic conditions, temperature (13.5-23.5oC), rainfall (1000-2700mm), 266,399 ha of tea were at risk to weevils infestation in Eastern tea growing region of Kenya(East of Rift) . The study found that the weevil species distribution was site specific for E. meyeri while S. mixtus was not. The most prevalent weevil was the S. mixtus found in an estimate of 5,135 ha against E. meyeri in 1,231 ha. The study also showed that mixed clones of tea, excessive application (High use rate) of nitrogenous fertilizers, intercropping with indigenous trees i.e avocado and eucalyptus had high incidences of weevils. Two isolates of B. bassiana (BbGi7a and BbKe6a) were more efficacious ((P=0.05) compared to other isolates at concentration of 1 x 103 conidia/ml. The two isolates caused > 50% weevil mortality on day 9 and 14 after inoculation respectively. The tea weevils lowered productivity of tea significantly (P≤0.05) with reduction in yield of between 30-33%. The performance of B. bassiana isolates, applied in form of a solid substrate or sprayed as conidia on the foliage, was comparable to that of Karate at the rate of 2L/ha. The study found a positive correlations between weevil population around the bush and the destroyed leaves. (y=21.127x -11.019, R2=0. 83333 for Giciaro farm and y=0.2569x+1.3689 R2=0.5296 for Mununga). The economic threshold level was found to be 2-3 weevils per bush where the destroyed leaves through defoliation was more than 40% to 54% of leaves. The study identified one tea clone purple tea and avocado to be potential for use as a pull crop each.The weevils significantly (P<0.05) prefer the purple tea leaves with resultant defoliated area of 90mm2 and the avocado leaves (120 mm2) while the shade trees Milletia dura (area consumed 23.1 mm2) and Fraxinus spp. (11.8mm2/ ten weevils/ two days) were less preferred. Therefore, purple tea and avocado are potential "pull crops".