Effect Of Different Pesticide Management Options On The Population Dynamics Of Aphids And Their Natural Enemies On Cabbage

Abstract

Cabbage is a popular vegetable grown in Ghana and serves as an important source of livelihood for small-scale farmers, especially in urban areas. However, its cultivation encounters several constraints such as attack by insect pests. A key insect pest is aphid that cause significant to the crops as they serve as vectors of plant diseases. Farmers thus resort to indiscriminate use of insecticides to reduce the damage caused by insect pests. However, insecticides are associated with adverse environmental and health implications. The current study sought to investigate the effect of commonly used management options on the population dynamics of aphids and their natural enemies. Cabbage seedlings were planted during the major and minor seasons of 2015 in 3 x 3 m plots. Treatments comprised, two (2) synthetic insecticides (chlorpyrifos and lambda-cyhalothrin) three (3) botanicals (hot pepper fruit extract, neem seed extract, solution of local soap (‘alata samina’) and water. Ten cabbage leaves per treated plot were randomly sampled weekly into 70% alcohol to obtain counts of aphids and their natural enemies. Weekly field observations were carried out to determine the number of other natural enemies and insect pests per treatment plot. Incidence and severity of a ‘suspected’ disease transmitted by aphids were monitored and scored twice for each season per sample. The least number of Lipaphis erysimi and Myzus persicae were recorded for the neem-treated plots, while lambda-cyhalothrin-treated plots recorded the highest number of aphids. The control and bio-pesticide-treated plots recorded the highest numbers of the natural enemies (hoverflies, ladybirds and spiders). Other insect pests observed in the field were Plutella xylostella, Hellula undalis and Bemisia tabaci. High population of P. xylostella was recorded in the chemical-treated plots compared to the biopesticide-treated plots, with neemtreated plot recording the least number. Helula undalis was highest in the control plots while B. tabaci was lowest in the neem-treated plot compared to the others. Disease incidence and severity of ‘suspected’ viral disease was highest in the control and pepper-treated plots, followed by alata samina and lambda-cyhalothrin-treated plots. The highest marketable yield was recorded for the neem-treated plots for both seasons. The yield and marketability of cabbages obtained from plots sprayed with alata samina and pepper were also higher than that obtained from control, lambdacyhalothrin and chlorpyrifos-treated plots, with the insecticide treated plots recording the least number of marketable heads. It was concluded that, neem effectively controlled the aphids, with mild effect on their natural enemies and also improved the yield and marketability of cabbage, followed by alata samina and pepper. The current findings suggest neem seed extract, local soap alata samina and pepper, as effective and safe options for managing aphids on cabbage and other insect pests. Effect of temperature, percentage relative humidity and rainfall on the population of aphids was also investigated. Results showed that, temperature and rainfall had a significant negative correlation for the major and minor seasons on the aphid populations except relative humidity that had a positive correlation on the population.