Effects of the nectariless cotton trait on insect pests, parasites and predators : with special reference to the effects on the reproductive characteristics of Heliothis SPP

Abstract

Data are presented on the abundance and seasonal incidence of Heliothis spp. and 11 other species of insect pests of cotton on unsprayed nectaried and nectariless cotton varieties in the field over a four year period. The distribution pattern of each species of insect pest on the nectaried and nectariless cotton varieties was similar except that in all cases the nectaried had significantly higher populations than the nectariless cotton except in the case of Anomis flava F. , Dichocrocis punctiforalis (Guenee), Oxycarenus spp., Dysdercus sidae Montrouzier and Teotoooris diophthalmus Thunberg, where no detectable reductions in populations on the nectariless cotton was observed. Reductions in Heliothis spp. oviposition in the field on the nectariless cotton ranged from 28.5-45.2% and that in larval populations from 34.9-55.6%. Nocturnal studies indicated a 64.3% reduction in the moth population in the nectaried cotton, which implied that the higher oviposition and the subsequently greater larval populations in the nectaried cotton was partly due to the correspondingly higher moth population. Moth flight activity in the nectariless cotton was found to be four times that in the nectaried cotton. The implications of these results are discussed.Small plot sizes reduced the effectiveness of the nectariless cotton in suppressing Heliothis spp. populations and the importance of this in commercial cultivation of cotton is discussed.Greenhouse studies were undertaken to explain the observed field reductions in Heliothis spp. populations. The results indicated the nectariless trait significantly reduced oviposition of Heliothis punetigera Wallengren and Heliothis armigera Hubner by approximately 55%. There was 80% less mating in an H.punctigera population released on the nectariless cotton.The absence of extrafloral nectar on the nectariless cotton caused reductions in fecundity in H.punctigera and H.armigera ranging from 45.8-54.2% and in fertility ranging from 27.4-31.7%. The observed field and greenhouse reductions in oviposition by Heliothis spp. on the nectariless cotton is discussed in terms of the observed effects of extrafloral nectar on mating, fecundity and fertility. The nectariless trait had no effect on egg, larval and pupal periods, larval and pupal viability or pupal size and weight. Greenhouse studies showed that oviposition of H.punctigera was greatly influenced by the stage of development of nectaried cotton, with two well defined peaks of oviposition. The nectariless cotton received one-third as many eggs as the nectaried cotton.The pattern of extrafloral nectar production by nectaried cotton was elucidated and it was shown that peak oviposition by H.punctigera coincided with peak nectar production in the greenhouse. The data suggest that fecundity of Heliothis spp. is a function of nectar availability, and that the timing of oviposition in the field is largely determined by the phenology of nectar production by the cotton plant.Extrafloral nectar was produced in sub-bracteal, mainstem leaf, sympodial leaf and monopodial leaf nectaries, with the first two sources being most important. The main pattern of nectar production was a shift from leaf (vegetative) to sub- racteal (fruit) nectaries as the season progressed. Most nectar was located on the upper third of the cotton plant throughout its development. The relationship between extrafloral nectar location on the plant and oviposition sites of Heliothis spp. on the plant is discussed. Further, the abundance of other insects on the cotton plant is discussed in relation to the location of nectar on the plant. ........