Abstract:
Field and laboratory studies were conducted on the bioecology and economic injury levels (EIL) of Dysdercus volkeri Fab. (Pyrrhocoridae: Heteroptera) on sunflower in Samaru Nigeria. The objectives of the studies were to determine the biology of D. volkeri, susceptible stage of sunflower for damage by D. volkeri, the economic injury levels of D. volkeri, the influence of weather factors on D. volkeri and to screen different accessions of sunflower for resistance to D. volkeri damage. The result of the studies indicated that growth stages R7 (when the back of the head has started to turn a pale yellow colour), R8 (when the back of the head is yellow but the bracts remain green) and R9 (when the bracts become yellow and brown and the stage is regarded as physiological maturity) took about 10, 14, 14 days respectively. The result of the biological studies showed a marked difference between the life cycle of D. volkeri in the field and that in the laboratory. Eggs laid by adult in the laboratory took three days to hatch compared to 3-14 days in the field. D. volkeri appeared on sunflower at R7 growth stage in both 2008 and 2009. Nymphs started appearing at R8 growth stage in both years. A total of 28–42 days was required from egg laying to adult emergence in the field compared to 24-28 days in the laboratory. There was significant difference (P= 0.05) regarding the number of insects and the damaged seeds on the nine accessions of sunflower tested. On mortality and longevity, there was also a significant difference between treatments. Predators of D. volkeri were observed including the phytophagous Pyrrhocorid, Odontopus sexpunctatus L. that had not been reported attacking D. volkeri before. The result also showed that the number of predators was not commensurate with the number of D. volkeri to warrant effective control. The D. volkeri sex ratio indicated that it was 1:1 male-female and the best criterion for sex determination was the size based method. The result showed R8 as most susceptible and 2-3 insects/head of sunflower crop constituted the economic injury level. The gain threshold was 1254 kg/ha and 1255 kg/ha in 2008 and 2009 respectively. Accessions PI 298632, Funtua, Ames 3236 and PI 176974 showed resistance to D. volkeri damage. On lipid value, the result indicated significant difference among the treatments. Accessions Ames 3236, PI 176974, PI 193775 and Funtua recorded the highest lipid value. Similarly, the accession that recorded the highest fatty acid value was Ames 1838 with 0.97mg/100ml. On yield, the result showed significant difference P < 0.05 among accessions. Funtua and PI 298632 appeared to have the highest yield of 1240 and 1239 kg/ha respectively.
BIOECOLOGY AND ECONOMIC INJURY LEVELS (EIL) OF Dysdercus volkeri F (HETEROPTERA: PYRRHOCORIDAE) ON SUNFLOWER (Helianthus annuus L.) IN SAMARU ZARIA, NIGERIA.
