Phytochemical Profile and Insecticidal Potential of Leaf Essential Oil of Psidium guajava Growing in North Central Nigeria

Loss of stored maize grains via infestation by Sitophylus zeamais contributes to food insecurity. This necessitates the use of synthetic chemicals by farmers to curtail the pest. Unfortunately, the pesticides are toxic to humans and unsafe for the environment. Interestingly, some plants possess phytochemicals that exhibit insecticidal activity without these drawbacks. This activity is linked to the type of phytochemicals, whose presence depends on environmental conditions that change with time of collection of plant samples. It is on this basis that we investigated how time of harvest affects the phytochemicals and insecticidal activity of P. guajava leaf essential oils. To accomplish these, pulverized leaves (500 g) from 7.00am and 1.00pm harvests were hydrodistilled individually for three hours using a Clevenger set up. GC–MS technique was used to characterize the oils while contact toxicity bioassay was used to assess the insecticidal activity of the oils. The yields of essential oils obtained from the leaves were 0.27±0.015 and 0.24±0.018 (w/w)%. β-Caryophyllene (14.7 and 18.2%), α-guaiene (13.7 and 10.6%), α-selinene (10.9 and 12.9%), globulol (9.5 and 8.1%), caryophyllene oxide (7.8 and 7.0 %) and eucalyptol (5.6 and 5.8 %) existed in higher quantities in the GC-MS results. Both oils were active against S. zeamais with LT50 of 59.23 and 121.09 hours and the highest activity was recorded for the oil of afternoon harvest. The oil from afternoon harvest can therefore serves as a cheaper and more innocuous substitute to synthetic insecticide for S. zeamais management in stored maize.