Effect of sowing dates within a season on incidence and abundance of insect pests of canola crops

This study describes the population difference of some insect pests on canola crops when sown at three dates i.e.,mid october,early november and mid november in two years field trials during 2008-2009 and 2009-2010 at bahawalpur. crops sown at all three dates were invaded by similar insect pests viz.,cabbage aphid,brevicoryne brassicae linnaeus (hemiptera: aphididae); turnip aphid,lipaphis erysimi (kaltenbach) (hemiptera: aphididae); peach aphid myzus persicae sulzer (hemiptera: aphididae),whitefly,bemisia tabaci gennadius (hemiptera: aleyrodidae); armyworm,spodoptera exigua (hübner) (lepidoptera: noctuidae) and pod borer,helicoverpa armigera (hübner) (lepidoptera: noctuidae). mean seasonal population per plant of these aphids during two years in these three crops (1st,2nd and 3rd sowing dates) was recorded for b. brassicae as 18.73,20.81,26.92 aphids; for l. erysimi as 14.09,16.82 and 18.32 aphids; and for m. persicae as 0.24,0.23 and 0.27 aphids,respectively. other insects with low numbers per plant were observed as s. exigua with 0.10,0.17 and 0.14 larvae; h. armigera as 0.14,0.16 and 0.18 larvae per plant and b. tabaci as 0.32,0.17 and 0.34 flies per plant in crops grown on 1st,2nd,and 3rd sowing dates,respectively. result indicated that there was no difference in time of incidence of insect pests in crops in relation to different sowing dates however a clear difference was noted in per plant population (densities or abundance) of these insect pests. m. persicae,s. exigua,h. armigera and b. tabaci in all three crops were not significantly different,whereas densities of two species of aphids (b. brassicae and l. erysimi) differed significantly among three sowing dates. these two aphid species made dense colonies during mid to late season. among the three treatments,mid november sown crop was found the most susceptible to these two aphids when compared to crops sown during mid october and early november. © 2014 zoological society of pakistan.