EFFICACY OF LANTANA CAMARA L. AND TEPHROSIA VOGELII HOOK AGAINST SITOPHILUS ZEAMAIS (COLEOPTERA: CURCULIONIDAE) IN STORED MAIZE GRAINS

The toxic and repellent effects of two tropical plants, Lantana camara L. and Tephrosia vogelii Hook were evaluated against Sitophilus zeamais Motschulsky in stored maize grains. Five rates (1.0, 2.5, 5.0, 7.5 and 10.0% w/w) of each powdered plant material, an untreated control and a synthetic insecticide (Actellic SuperTM 2% dust) were used to investigate treatment efficacy on mortality of the adult insect (five to eight days old), F1 progeny emergence and repellence against S. zeamais adults. After 21 days, L. camara and T. vogelii caused 82.7-90.0% and 85.0-93.7% insect mortality, respectively. The mean lethal exposure times (LT50) to achieve 50% mortality varied from five to six days (7.5-10.0% w/w) to seven to eight days (2.5-5.0% w/w) for both plants. Probit regression analysis showed a significant relationship between plant powder concentration and insect mortality. The plant powders and synthetic insecticide reduced adult F1 insects by more than 75% compared to the untreated control. T. vogelii was most repellent to S. zeamais at 7.5-10.0% (w/w), showing that 87.5% of insects were repelled, followed by T. vogelii at 2.5% w/w and L. camara at 10% w/w which showed that 65.0% and 62.5% of insects were repelled, respectively. The implications of these results are discussed in the context of small-scale farmer usage of these plants for stored product protection and implications on food security and poverty alleviation. INTRODUCTION The maize weevil, Sitophilus zeamais Motschulsky, is the most serious pest of stored maize in the tropics. Although it can be effectively controlled by synthetic insecticides (Pierce and Schmidt, 1992; Bekele et al., 1996), the majority of farmers in Africa are resource-poor and have neither the means nor the skills to obtain and handle pesticides appropriately (Saxena et al., 1990). The prohibitive costs of commercial synthetics, the increasing development of insect resistance to pesticides, toxicity concerns and the often erratic supply of insecticides have given impetus to the search for alternative insect control methods (Tembo and Murfitt, 1995). Most of the grain produced in sub-Saharan Africa comes from small-scale farmers, many of whom use different kinds of plant products for pest control (Bekele et al., 1996; Poswal and Akpa, 1991). In recent years, research has focused on the bioactivity, application methods and sustainable use of botanical pesticides against insect pests (Fatope et al., 1995; Talukder and Howse, 1994). Lantana camara L. (Verbenaceae) has been used as a tonic and stimulant in folk medicine (Masinde, 1996; Siddiqui et al., 1995) and is reported to have anti-insect properties against certain field and storage insect pests (Deka et al., 1998; Facknath, 1994). Tephrosia vogelii Hook (Fabaceae) has been used as a source of contact insecticides, fish and arrow poisons (Lambert et al., 1993; Soko, 1999). The objective of the current study was to investigate the potential of these two plant species as sources of sustainable alternative protectants to synthetic insecticides for use in stored product protection using methods compatible with small-scale farmer practices in Africa. Proc. WOCMAP III, Vol. 5: Quality, Efficacy, Safety, Processing & Trade in MAPs Eds. E. Brovelli, S. Chansakaow, D. Farias, T. Hongratanaworakit, M. Botero Omary, S. Vejabhikul, L.E. Craker and Z.E. Gardner Acta Hort. 679, ISHS 2005 138 MATERIALS AND METHODS Preparation of Plant Materials and Culturing of S. zeamais Fresh samples (mixture of leaves, succulent stems, inflorescence and fruits) of L. camara and T. vogelii were collected from the Kasipul division of Rachuonyo district in Kenya during September 1999. The plant materials were dried under shade at ambient temperatures of 27-30°C for 3 days and further oven-dried at 35°C for 48 hours. The dried plants were ground to a fine powder using an electric mill. S. zeamais were reared on maize (Zea mays L.) under laboratory conditions (1728°C, 38-69% RH, 12:12 light:dark). The maize had been previously disinfested in an oven at 40°C for 4hr (Bekele et al., 1996). Two hundred unsexed adults were placed in one litre plastic jars containing 500 g of maize. The top of each jar was covered with nylon mesh fastened tightly with elastic bands. The insects were allowed a seven-day egg laying period before all adults were removed. After 28 days, the emerging adults were removed daily and kept in separate jars according to age for experimental use. Toxicity Bioassay Ground powders of L. camara and T. vogelii were admixed with 250 g of maize in plastic jars at five rates (1.0, 2.5, 5.0, 7.5 and 10.0% w/w). Maize grains treated with Actellic Super 2% dust at 0.05% w/w and an untreated control was included. Forty unsexed 5-8 day-old S. zeamais adults were placed into the jars arranged in a completely randomised design (CRD) with four replicates. The top of each jar was covered with nylon mesh held tightly with elastic bands. A 2 cm wide band of fluon was smeared around the inside near the top of the jar to deter the insects from climbing out. The dead insects in each jar were counted every day for the first ten days, then on the 14 and 21 day. The percent (%) mortality was calculated by expressing the dead as a % of the total number of adult insects introduced into the jar at the start of the experiment. All adult insects were removed from the jars after 21 days. Cumulative counts of emerging F1 insects commenced on the 28 day (Bekele et al., 1996) and continued up to the 56 day. Repellence Bioassay The repellence of L. camara and T. vogelii against S. zeamais was evaluated in a choice bioassay consisting of a circular flat-bottomed plastic basin (45 cm diam. x 30 cm high) whose base was divided into four equal portions. Alternate treated and untreated maize (100 g) was placed equidistant from the centre of the circular base. This was repeated for all the treatments including a no-choice control with all untreated maize. A 2 cm fluon band was smeared at 15 cm height all around the basin. The treatments were arranged in a CRD with four replicates. Forty unsexed 5-8 day-old adult S. zeamais were placed at the centre of each basin, and the top of the basin was covered with nylon mesh. The total number of insects that settled on the control and the treated grain was recorded after 3, 4 and 24 hours of exposure. The percent repulsion (PR) was calculated and assigned to repellence classes (0-V) according to Talukder and Howse (1994) as follows: PR = 2 x (C-50) where C = the % of insects that settled on the untreated grain and trials showing a positive (+) PR value demonstrate repellence. Data from toxicity and repellence bioassays were log-transformed to correct for heterogeneity of treatment variances (Gomez and Gomez, 1984) before being subjected to ANOVA and repeated measures analysis. The transformed means were separated by Tukeys studentized (HSD) test (Gomez and Gomez, 1984; Scheiner and Guvevitch, 1993). The relationship between the plant concentration applied and insect mortality was determined using probit regression analysis of log-transformed concentrations. The regression lines for log10 concentration and insect mortality were then computed (Talukder and Howse, 1994). Any two regression lines were considered significantly different if their standard errors did not overlap (Talukder and Howse, 1994).