The biology, ecology and some aspects of economic importance of pyrethrum thrips thrips tabaci lind and thrips nigropilosus uzel in Kenya

Abstract

The biology of Thrips nigropilosus was studied using Pyrethrum leaves as the rearing medium in a growth chamber in a laboratory. Its life history is divided into a series of stages: eggs, larvae, pupae and adults with three instars between the stages. The average durations of development egg to adult was 25.7 (+ 0. 43) days at 20°C and 70°& RH. The sex ratio for adults were established under this laboratory conditions as 9:1 female-male suggesting the presence of pathogenetic type of reproduction. The longevity period for adult stood at between 1 to 21 days. Other than the pupal stage that moults in soil, all other developmental stages occur on the Pyrethrum plant tissues giving room therefore for an effective chemical spray to be directed on the soil surface to reduce emerging adult numbers, hence, an effective and economical method of the thrips control. At high temperature 30°C the average duration of development shortened to 14.2 (- 0.18) days and lengthened to 50.2(- 0.67) days at 12°C, thus expressing the significance played by temperature on the thrips development. The temperature range (12-30)°C xi ’ Xll however, were found to be within the condusive range for the thrips development. Higher temperatures 25-30°C induced faster rate of development which can also be significantly responsible for faster rates of thrips reproduction generations in a given season. The aims of the ecological studies were to help understand the importance of weather factors on the Thrips in Pyrethrum fields, compare thrips populations on commercially grown Pyrethrum plants, and to evaluate their alternative host plants for which series of experiments were conducted to assess. From the studies it was observed that high rainfall above 40 mm per monthVreduced drastically the thrips population in the Pyrethrum fields. Similarly the thrips population achieved importance after 2 months of continuous drought rising to about 20 per plant when chemical control becomes a necessity. Among the six pyrethrum clones/variety tested for thrips population density, clone Mo/70/1013 and variety P4 with less well developed leaves and floral structures to harbour the thrips and secondly Xlll with higher pyrethrin content of 1.990 and more were least preferred by the thrips compared to other tested clones: 4331,SB/66/107, 0/64/219 and Ma/63/1889. The general level of population density of T. tabaci in flowers influenced the total populations per cultivar. T. tabaci infested mainly open flowers and T. nigropilosus occupied the leaves and vegetative growth. And studies on alternative host plant suggested that Thrips tabaci is polyphagous, and T. nigropilosus oligophagous. The reason for the low population of T. nigropilosus compared to T. t ab a c i is the result of limited geneflow as they possess fewer alternative hosts. Frequent weeding is believed to be one agronomic practice that would further limit the population of these Thrips. Finally, the effect of Pyrethrum thrips (Thrips tabaci and Thrips nigropilosus) on the crops development was studied in a green house using pyrethrum clone 4331 exposed to varying thrips populations. There was a definite‘pattern of the flower bud deaths with plants exposed to high thrips population of 100 to 500 dying first and the trend shifting to plants exposed to low (10) thrips population 2 to 3 months XIV later. Flower number and weights were similarly faster reduced after a month’s exposure at high thrips number (500) compared to two months at moderate number (100) per plant. Plants exposed to low thrips number (10) realised low flower number and weight after about 3 months. Field observations showed that the harvestable flowers from exposed plants thrips were generally smaller, spiralled and hence the reason for the low yield. High thrips population caused immediate yield reduction and eventual deaths hence the need for their control on routine sprays.