STUDIES ON VARIETAL RESISTANCE OF WHEAT CULTIVARS TO APHIDS (APHIDIDAE : HOMOPTERA)

Field and laboratory studies were carried out for four years (1989-93) on varietal resistance of wheat aphids. Aphids phenology studies indicated that four aphid species were found colonizing wheat during course of study. The aphid species were, greenbug, Schizaphis graminum (Rondani), corn leaf aphid, Rhopalosiphum maidis (Fitch), rice root aphid Rhopalosiphum rufiabdominals (Sasaki) and rusty plum aphid Hvsteroneura setariae, (Thomas). All aphids appeared on the crop in the month of January. There was positive correlation of population growth of greenbug with crop age. Studies on population growth of greenbug on 10 wheat cultivars under field conditions revealed that mean seasonal population per 5 tillers were highest on Kohinoor (47.6) followed by ZA-77 (46.3), Mehran-89 (44.4), Lira (41.4), Pavan (40.0), Blue silver (39.4), TJ-83 (36.6), Pak-70 (30.9), Sarsabz (21.8) and Anmol (18.9). Regression equations on population buildup of greenbug in different cultivars showed that reproduction rate was highest in Blue silver (0.129) followed by Mehran-89 (0.124), lira (0.097), Anmol (0.085) and Sarsabz (0.081). Laboratory studies on life tables parameters of green bug on three varieties, viz Kohinoor Sarsabz and Mehran-89 indicated that numbe2 of days to reproduction (x2 )=0.8) mean generation time (T) (x2)=0.11) on three wheat cultivars were non-significant different. The longevity (x =5.18), total off springs (x2 =14.36) and innate capacity of increase (r) (x2 = 6.04) were significantly different. The adult longevity was highest on Mehran-89 (41) followed by Kohinoor (31) and Sarsabz (22). The total number of off springs per female was lowest on sarsabz (2.37) and highest in Mehran-89 (23.49). The innate capacity of increase value was lowest for Sarsabz (0.062) and highest for Mehran-89 (0.132). The mortality value (qx) on all varieties was high initially and later decreased gradually. Within plant distribution of aphids results showed that greenbug Schizaphis raminum (Rondani) was found predominant among cereal aphid on wheat in Sindh. Percent vertical distribution was 4.673% on leaf 1 (top most leaf), 10.403% on leaf 2, 17.09% on leaf 3, 28.36% on leaf 4, and 39.43% on leaf 5 (bottom leaf). The top most leaf had minimum aphid numbers where as bottom leaf harbored maximum aphids. Results on spatial distribution of aphids on wheat indicated that the index of dispersion was >1 for both greenbug and corn leaf aphid and was less aggregated at low density levels. The over all average values for clumping (K) were higher (0.212) for greenbug and lowest (0.0315) for corn leaf aphid. Lloyd's crowing index was positively correlated with population mean for greenbug. In case of corn leaf aphid slope of line was steep (4.539). Parameters from regression of In (mean density) on In (-In Po) men also used to develop functional relationship between mean density and proportion of empty sample units. Field comparison of horizontal water-pan traps of four different colours with each other, and with suction trap were done in winter 1991 to assess their effectiveness in measuring -the landing rates of aphids on wheat. Aphids were monitored at weekly interval. four species namely, greenbug (Schizaphis raminum Rond.), corn leaf aphid (Rhopalosiphum maidis Fitch), rice root aphid (rufiabdominalis) and rusty plum aphid (Hysteroneura setariae Thomas) were collected. maidis was predominantly attracted towards yellow pan traps whereas raminum catches were maximum on suction trap. Yellow pan traps attracted the greatest number of aphids and met the criteria for the ideal trap. The level of aphid catch per unit of surface area decreased with the increased size of a trap. The effect of different rates of fertilizers and irrigations schedules on greenbug population indicated that fertilizer treatments increased greenbug population. The F-values showed significant difference in greenbug population for fertilizer treatments for all varieties except Sarsabz. Greenbug populations were high at 21 days interval irrigatior. schedule. Aphid predation studies indicated that 4 species of predators were found on wheat crop viz., Coccinella septempunctata Linn. , Coccinella undecimpunctata, Chrysopa'§"p". and Syrphus'§"p". Among predators Coccinella septempunctata was most predominant Chrysopa and Syrphus sp. were recorded during early stage of crop whereas coccinella septempunctata and Coccinella undecimpunctata were present throughout the wheat season. Laboratory studies on efficacy of coccinella septempunctata showed that, one adult beetle consumed 83.7 aphids per day and is potential predator to suppress aphid population below economic threshold in wheat agro-ecosystem. Varietal resistance studies on 22 cultivars and 6 allied cultivars of wheat indicated that among allied cultivars Triticum monococcum had maximum infestation of 2.569 aphids per tiller. Triticum durum was highly protected cultivar against aphid attack. The F-statistic for black aphid was 110.44 and at 0.05 level 7 groups of susceptibility resistance were obtained through LSD. The highly resistant cultivar was M-233. the F-statistic for corn leaf aphid was 124.04 and 8 group of cultivars were found with LSD test and M-233 found most resistant cultivar. The F-statistic for greenbug was 200.93. The most resistant variety was M-233. The regression equation gave negative but non-significant correlation between total amino acid content and black aphid or corn leaf aphid. The regression equation showed positive relationship between total amino acids content and greenbug population with significant r-value (0.60) step wise regression models showed negative correlation between corn leaf aphid at with methionine. Greenbug had positive dependence with good (r=0.56) on phenylalanine. Significant correlation r=(0.64) between alanine content and total aphid population. Similarly I better equation obtained with alanine plus glutamic acid, I parameter on the yield, duration and aphid population were used '1 to run hierarchical cluster analysis. The results are presented f in the form of icicle plots, agglomeration schedule and dendrogram.