EVALUATION OF BEAUVERIA BASSIANA (Hyphomycetes) ISOLATES AS POTENTIAL AGENTS FOR CONTROL OF TEA WEEVILS (ENTYPOTRACHELUS MEYERI)

Abstract

Tea (Camellia sinensisKuntze) is an important cash crop in Kenya. Several insect pests, however, have been recorded on tea affecting production; prominent among which is the tea weevil. Common tea weevils reported to occur in Kenya are Kangaita/Kimari weevil (Entypotrachelusmeyeri) [micans/kolbe] Nyambene weevil (Sphrigodesmixtous) and Systatesweevil. Adult weevils damage tea by defoliating nursery, newly established and mature tea orchards. In addition, occasionally, they girdle roots, the stem just above ground level and feed on the bark of twigs and branches of tea. All these damages lead to decline in productivity. Presently, management strategy against tea weevils is only based on hand picking when they are in low population and spraying with synthetic insecticides. To avoid reliance on synthetic insecticides that have been reported to cause pests resurgence, environmental concerns and ill health effects on consumers, there is need for alternative method. Beauveriabassiana has proved to be competitive option to chemical insecticides for protection of forests and farms against pests. However, as far as we know it has not been used on tea weevils. Therefore, this research was designed to evaluate the potential of Beauveriabassianaas a biocontrol against the tea weevils. Twenty-two local isolates of Beauveriabassiana were characterized by their radial growth, sporulation and germination and assayed for insecticidal activities against adult tea weevils (Entypotrachelusmeyeri). The isolates were coded according to the different places from which they were isolated. The isolates included Bb Ke 1-5 recently isolated from Kericho county, TRFK timbilil tea fields and isolates Bb Ke 6a and Bb Ke 6b isolated in 2006 from within Kericho county, TRFK timbilil tea estate fields, Bb Gi7 series which were isolated from soils and assorted weevils from Giciaro farm in Nyambene, Meru county, Bb ch8 and Bb Ch c (1-5) isolated from Chepkoilel soils of University of Eldoret, UasinGishu county and Bb Mu 9 series which was isolated from Mununga, Kirinyaga county. The growth characteristics were determined by measuring the radial diameter (mm) of the isolates growing on Potato Dextrose Agar (PDA) for seven days, sporulation was determined after 2 weeks using a NeubauerHeamacytometer under a light microscope (400x) and germination of conidia after 16 hours of inoculation on freshly prepared PDA plates. A few representatives of the isolates were chosen to determine their efficacy against the tea weevils (Entypotrachelusmeyeri) by dipping the weevils in conidia concentrations of 105, 104 and 103 conidia/ml. Four B. bassiana isolates that caused mortality of 90 % or more within two days were subjected to dose-response mortality bioassays. The lethal concentration of B. bassiana isolates that caused 50% mortality (LC50) ranged between 106 - 108 conidia/ml. The lethal time to 50 % mortality (LT50) values was 4–10 days. The radial growth, germination and sporulation varied in the different isolates. Bb Ke 4 (51mm) and Bb Ke 5 (31.33mm) exhibited higher radial growth, Bb ch c4 (17216.70) had high sporulation whereas Bb Ke 6a (75.67%), Bb Ch c3 (70%) and Bb Ch c4 (68%) showed high number of germinating conidia. Isolate Bb Gi 7a showed higher insecticidal activity against the tea weevils compared to the other few selected isolates. High negative correlation was noted between some variables like radial growth and spore count. There was positive correlation between the colony forming units (cfu) and germinating conidial count, and between spore count and germinating conidia. The weevils ate the leaves irrespective of whether they were treated with the fungus or not. The eaten leaf area did not differ significantly between the treatments (p <0.05). The concentrations were found to be significantly different (f= 2.53, p (0=0.5), while the four isolates selected were also shown to be significantly different (f=2.83, p=0.5). The isolate BbGi7a gave the highest weevil mortality of 66.1 % with lethal time LT50 of 4.5days. The concentration of 107conidia per ml-1 had the highest weevil mortality of 73.3% in isolate Bb Gi7a and least in BbKe 6a with a weevil mortality of 28.8%. Therefore, the isolate BbGi7a is the best potential biocontrol for the tea weevils (Entypotrachelusmeyeri).