Sorghum Midge Research Articles

Effect of sorghum resistant to Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae) on oviposition and development of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae)

Abstract Helicoverpa armigera oviposition preference for, and larval development on sorghum hybrids with differing resistance to sorghum midge, Stenodiplosis sorghicola, were investigated. When H. armigera larvae were fed seed of resistant and susceptible hybrids in the laboratory there were no differences in larval and pupal sizes or the rate of development. The same result was recorded when larvae fed on panicles on plants in a glasshouse. On some sampling occasions, significantly more eggs were laid on panicles of resistant hybrids in the field. This occurred when plants were in plots and also in a mixed planting. Midge‐resistance status did not affect levels of egg parasitism. In a field study using recombinant inbred lines between a midge‐resistant and a midge‐susceptible line, no relationship was found between level of resistance and oviposition of H. armigera. We conclude that, although midge‐resistant hybrids are sometimes preferred for oviposition by H. armigera, the resistance per se does not determine this preference. Egg survival, larval survival, development and resultant damage are not significantly affected by the midge‐resistance status of the host.

Environmental factors influence the expression of resistance to sorghum midge, Stenodiplosis sorghicola

Host plant resistance is an effective means of controlling sorghum midge (Stenodiplosis sorghicola). We studied the influence of environmental factors on expression of resistance to sorghum midge in three midge-resistant and two midge-susceptible genotypes. Midge-resistant lines AF 28, ICSV 197, and TAM 2566 suffered 8.8 to 17.3% damage across seven so wings compared to 25.6%damage in ICSV 112, and 69.4% damage in CSH 5. Susceptibility of the midge-resistant lines (AF 28, ICSV 197, and TAM 2566) decreased with an increase in open pan evaporation, maximum and minimum temperatures, and solar radiation; while the midge-susceptible lines (ICSV 112 and CSH 5) showed a poor interaction with these factors. Midge damage in ICSV 197 showed a negative correlation with minimum temperature and relative humidity and positive correlation with sunshine hours,while the reverse was true for CSH 5. Grain growth rate between 0 and 3 days after anthesis was lower in crops sown on 1st October, when AF 28 and ICSV 197 suffered maximum midge damage. Maximum and minimum temperatures and maximum relative humidity influenced the moisture content of the grain, grain growth rate, and sorghum midge damage. There was considerable variation in genotype × environment interaction for expression of resistance to sorghum midge,and the implications of these results have been discussed in relation to development of sorghum cultivars with resistance to this insect.

DNA sequence variation in the ITS‐1 rDNA subunit and host relationships in sorghum midge, Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae), in Australia

AbstractSequence variation in the internal transcribed spacer (ITS‐1) ribosomal DNA subunit was examined for sorghum midge obtained from introduced and native hosts in south‐eastern and central Queensland. No variation was observed relative to host plant or geographical distance for midges collected from two introduced hosts, grain sorghum (Sorghum bicolor) and Johnson grass (S. halepense); however, sequence differences were observed between midges from introduced and native hosts and among midges from a single native host, slender bluegrass (Dichanthium affine). No evidence was observed of introduced midges on native hosts, or vice versa. These results agree with previously hypothesised host distributions for native and introduced midges in Australia, and expand the sample of introduced hosts to include Johnson grass. They suggest that Stenodiplosis sorghicola, the principal midge infesting grain sorghum, is also the most common species on Johnson grass. This confirms that Johnson grass plays a role in the population dynamics of S. sorghicola and suggests that midges originating from Johnson grass may influence levels of infestation in grain sorghum.

Compensation in grain weight and volume in sorghum is associated with expression of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge, Stenodiplosis sorghicola (Coquillett) is one of the most important pests of grain sorghum worldwide. We studied the inheritance of resistance to sorghum midge and compensation in grain weight and volume in panicles of sorghum hybrids and their parents under uniform infestation (40 midges per panicle for two consecutive days). Sorghum midge damage ranged from 8.2 to 82.4% in the maintainer lines (B-lines) of the females parents (A-lines), and 9.0 to 67% in the male parents (restorer lines). Hybrids involving resistant × resistant parents were highly resistant, while those involving resistant ×susceptible and susceptible × resistant parents showed moderate susceptibility. Susceptible × susceptible hybrids were susceptible. Compensation in (percentage increase) grain weight and volume in midge-infested panicles of midge-resistant parents and their F1 hybrids was greater than in midge-susceptible parents and hybrids. General combining ability effects for midge damage, and grain weight and volume were significant and negative for the midge-resistant females (ICSA 88019 and ICSA 88020), whereas those for the midge-susceptible females (ICSA 42 and 296A) were significant and positive. However, the reverse was true in case of compensation in grain weight and volume. Inheritance of compensation in grain weight and volume and resistance to sorghum midge is controlled by quantitative gene action with some cytoplasmic effects. Resistance is needed in both parents to realize full potential of midge-resistant hybrids.

Flowering events in sorghum in relation to expression of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge (Stenodiplosis sorghicola Coquillett) is an important pest of grain sorghum, and host plant resistance is one of the most effective means of controlling this pest. Several studies have shown that resistance to sorghum midge is associated with short and tight glumes, faster rate of grain development, and tannins. However, some recent studies suggested that time of flowering is the principal component of resistance to sorghum midge. Therefore, we conducted a series of experiments under laboratory and field conditions on the flowering behaviour of a diverse array of midge-resistant and midge-susceptible genotypes to quantify the contribution of time of flowering in genotypic resistance to sorghum midge. Time of flowering under field and laboratory conditions did not show any differences between midge-resistant and midge-susceptible genotypes. Time to maximum flowering varied considerably between the rainy and the post-rainy seasons. Under field conditions, most of the spikelets in the midge-resistant lines opened between 0000 and 0400 h, while most of the spikelets in the susceptible check, CSH 1, opened at 0200 h. Under light and dark conditions in the laboratory, most of the spikelets opened at 0300 h in all genotypes, irrespective of their level of resistance to the sorghum midge. However, flowering events continued for a longer period under dark conditions outside the laboratory. There were no differences in oviposition in panicles on the midge-resistant genotype, ICSV 745, infested between 0200 and 0600 h, and significantly more eggs were laid in spikelets of the susceptible check, CSH 1, than in the midge-resistant, ICSV 745, at all infestation times. The peak oviposition was recorded at 0945 h irrespective of the level of resistance to sorghum midge, and significantly more eggs were laid in the spikelets of CSH 1 and Swarna than in ICSV 745 and ICSV 197. There was no evidence of change in the susceptibility of sorghum midge-resistant genotypes when infested at different times in relation to time of flowering. Therefore, flowering behaviour of sorghum genotypes seems to play little role in genotypic susceptibility to sorghum midge.

Mechanisms and diversity of resistance to sorghum midge, Stenodiplosis sorghicola in Sorghum bicolor

Sorghum midge, Stenodiplosis sorghicola (Coquillett) is the most important pest of grain sorghum worldwide, and plant resistance is an important component for the control of this pest. To identify sorghum genotypes with diverse mechanisms of resistance to sorghum midge, we studied oviposition, larval survival, and midge damage in 27 sorghum midge-resistant genotypes, and a susceptible check under greenhouse conditions. Observations were also recorded on floral characteristics and compensation in grain mass. Of the 28 sorghum genotypes tested, 19 showed high levels of antixenosis to oviposition as a component of resistance, and had 80% midge damaged spikelets in QL 12 - the susceptible check. Genotypes showing resistance to sorghum midge have smaller glumes than the susceptible check, QL 12. However, IS 7005, IS 18653, and ICSV 745 have relatively large sized glumes, but suffered 20% compensation in grain mass when the panicles were reduced to 250 spikelets and infested with 10 or 25 midges per panicle. There is considerable diversity in sorghum genotypes showing resistance to sorghum midge. Genotypes with diverse combination of characteristics associated with resistance to sorghum midge can be used in breeding programs to broaden the genetic base and increase the levels of resistance to this insect.

Artificial infestation of sorghum spikelets with eggs of Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae) by water injection

AbstractA technique for artificially infesting sorghum spikelets with eggs of sorghum midge is described and compared with natural oviposition achieved in a glasshouse cage trial. This technique was developed to facilitate antibiosis resistance related studies of midge biology, and to overcome the reduced and inconsistent oviposition achieved in sorghum lines that contain ovipositional‐antixenosis resistance. Injecting an aqueous suspension of exactly two midge eggs between the glumes of individual sorghum spikelets using a micropipette produced consistent, low egg densities across five lines of varying resistance (0.8–1.2 eggs per spikelet; 50–70% infestation), while water injection of four to six eggs per spikelet in the same lines produced high and consistent egg densities (two to three eggs per spikelet; > 80% infestation). In contrast to both of the water‐injection treatments, natural oviposition within the same five lines produced inconsistent egg densities, even when midge densities were adjusted to account for the variable levels of ovipositional antixenosis present in each line (one to four eggs per spikelet; 40–80% infestation). A bioassay was also conducted to determine the effect of suspending midge eggs in water on egg hatch, neonate survival and fitness. Aqueous suspensions of midge eggs stored for 4 h at room temperature produced 79% egg hatch. However, aqueous egg suspensions refrigerated at 4°C for 1–7 days reduced egg hatch (41–64%), lowered larval longevity and reduced maximum movement of neonate larvae. No eggs hatched after 14 days of refrigerated storage.

Incidence and distribution of the sorghum head bug, Eurystylus oldi Poppius (Heteroptera: Miridae) and other panicle pests of sorghum in West and Central Africa

L'incidence et la distribution de la punaise du sorgho, Eurystylus oldi Poppius (Heteroptera Miridae), et des autres ravageurs du panicule du sorgho ont ete evaluees de 1985 a 1994 en stations de recherches et en parcelles paysannes, en Afrique de l'Ouest et Centrale (AOC). Labondance maximale du miride a ete observee pendant le stade pâteux. L'abondance du miride etait plus importante sur les cultivars ameliores que sur les sorghos guineens locaux a longues glumes, aussi bien en stations de recherches qu'en parcelles paysannes. Les degâts sur graines etaient moderes a severes bien que les fermiers ignoraient la plupart du temps ces insectes et leurs degâts potentiels car ils restent caches dans les panicules. Les degâts provoques par le. moucheron du Sorgho (Stenodiplosis sorghicola Coq.) etaient tres importants dans les parcelles paysannes au Burkina Faso, au Niger et au Nigeria. La mouche du sorgho Atherigona soccata Rond., le foreur des tiges Busseola fusca Fuller, les criquets, le cercopide Locris rubens Erichson, Campylomma spp., Creontiades pallidus Ramb., les chenilles de la noctuelle Helicoverpa armigera Hub., Pyroderces simplex Wlsm. et Eublemma gayneri Roths etaient egalement des ravageurs responsables de degâts sur sorgho dans la zone prospectee. On suggere que les futures investigations pour l'amelioration de la culture du Sorgho en OAC pourraient porter sur le developpement de cultivars resistants au miride du Sorgho. (Resume d'auteur)

Host‐plant preference and oviposition responses of the sorghum midge, Stenodiplosis sorghicola (Coquillett) (Dipt., Cecidomyiidae) towards wild relatives of sorghum

Sorghum midge, Stenodiplosis (Contarinia) sorghicola (Coquillett) is an important pest of grain sorghum world‐wide. Considerable progress has been made in screening and breeding for resistance to sorghum midge. However, some of the sources of resistance have become susceptible to sorghum midge in Kenya, in eastern Africa. Therefore, the wild relatives of Sorghum bicolor were studied as a possible source of new genes conferring resistance to sorghum midge. Midge females did not lay eggs in the spikelets of Sorghum amplum, Sorghum bulbosum, and Sorghum angustum compared to 30% spikelets with eggs in Sorghum halepense when infested with five midge females per panicle under no‐choice conditions. However, one egg was laid in S. amplum when infested with 50 midges per panicle. A larger number of midges were attracted to the odours from the panicles of S. halepense than to the panicles of Sorghum stipoideum, Sorghum brachypodum, S.angustum, Sorghum macrospermum, Sorghum nitidium, Sorghum laxiflorum, and S. amplum in dual‐choice olfactometer tests. The differences in midge response to the odours from S. halepense and Sorghum intrans were not significant. Under multi‐choice conditions, when the females were also allowed a contact with the host, more sorghum midge females were attracted to the panicles of S. bicolor compared with S. amplum, S. angustum, and S. halepense. In another test, numerically more midges responded to the panicles of IS 10712 compared with S. halepense, whereas the differences in midge response to the panicles of ICSV 197 (S. bicolor) and S. halepense were not apparent, indicating that S. halepense is as attractive to sorghum midge females as S. bicolor. The wild relatives of sorghum (except S. halepense) were not preferred for oviposition, and they were also less attractive to the sorghum midge females. Thus, wild relatives of sorghum can prove to be an alternative source of genes for resistance to sorghum midge.

Cytoplasmic male-sterility and source of pollen influence the expression of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge, Stenodiplosis (Contarinia) sorghicola (Coquillett), is an important pest of grain sorghum, and host plant resistance is an important aspect of control of this pest. This research investigated how cytoplasmic male-sterility and source of pollen influence the expression of resistance to sorghum midge. Sorghum midge emergence was significantly lower in panicles of midge-resistant and midge-susceptible cytoplasmic male-sterile lines when pollinated with AF 28 - a midge-resistant restorer line, than those pollinated with Swarna - a midge susceptible restorer line, indicating the presence of xenia effects. Maintainer lines (B-lines) of midge-resistant parents had significantly lower numbers of eggs and larvae than the B-lines of midge-susceptible parents. Male-sterile lines of the both midge-resistant and midge-susceptible lines were equally susceptible, indicating that resistance to sorghum midge is influenced by factors in the cytoplasm of the B-line. These findings will have an important bearing on the production of hybrids with resistance to insects.

Evaluation of planting date, sorghum hybrid, and insecticide treatment on sorghum midge (Diptera: Cecidomyiidae) management in northeast Louisiana.

The combined effect of planting date, insecticide treatment, and host-plant resistance was studied in northeast Louisiana for management of the sorghum midge, Stenodiplosis sorghicola (Coquillett), during 1994 and 1995. Significantly higher numbers of sorghum midges were observed visiting flowering spikelets of the midge-susceptible sorghum hybrid (Delta and Pine Land 'DP1552') than those of the midge-resistant sorghum hybrid (DeKalb 'DK-60'). Numbers of midges averaged 1.2 and 0.6 per flowering panicle in the susceptible and resistant sorghum hybrids, respectively, in 1994 and 1.8 and 1.0, respectively, in 1995. Midge densities increased significantly as the sorghum flowering season progressed. Sorghum midge reached peak densities during the first half of August in 1994 and 1995. The length of the flowering period in the early-planted (mid-March) sorghum was significantly longer compared with the flowering periods in the mid-April, mid-May, or mid-June planted sorghums. This resulted in prolonged exposure of flowering panicles to ovipositing midges and increased midge damage in the early-planted (mid-March) sorghum. Damage by sorghum midge was significantly higher in the early-planted (mid-March) sorghum hybrids than in the late-planted (mid-June) sorghum hybrids. The midge-susceptible hybrid produced highest yields when planted in mid-April and mid-May (optimum period) and lower yields when planted very early (i.e., mid-March) or late (i.e., mid-June). No significant differences were observed in yields for the resistant hybrid at any planting date in 1994. However, in 1995, significantly lower yields were recorded in resistant sorghum planted in mid-June. Levels of sorghum midge damage and sorghum seed yields in the untreated resistant hybrid were not significantly different than those observed in the insecticide-treated susceptible hybrid. Numbers of adult midges captured on sticky traps were positively correlated to numbers of visual estimates of ovipositing midge females visiting flowering spikelets.

Midge Resistance and Hydrocyanic Acid Content of Sorghum bicolor

Sorghum midge (Stenodiplosis sorghicola [Coquillett]) is found in most countries where sorghum (Sorghum bicolor L. Moench) is grown (Doggett 1988). A number of sorghum lines resistant to the sorghum midge have been identified (Teetes 1985). Various mechanisms have been proposed for midge resistance, including cleistogamy, nonpreference, antibiosis, tannin content, and glume size and tightness (Doggett 1988 and Teetes 1985). Dhurrin[p-hydroxy-(S)-mandelonitrile-$-D-glucoside] is the cyanogen of sorghum (Gorz et al. 1977) that is enzymatically hydrolyzed to cyanide and p-hydroxybenzaldehyde (p-HB) when cellular integrity is interrupted (Wajant et al. 1994). Gorz et al. (1977) reviewed studies that showed colorimetric methods can be used to determine cyanogen content or hydrocyanic acid potential (HCN-p) in plant tissue. In an unrelated study in search for cyclic hydroxamic acids (e.g., DIMBOA), the second author (M. D. Richardson) found that midge resistant sorghum genotypes, Huerin and Tift MR88, produced 0.52 and 0.78 mg p-HB while midge intermediate (genotype 1821 cm) and midge susceptible (Tx623B) genotypes produced 0.18 and 0.15 mg p-HB per g fresh weight [LSD(0.05) = 0.10], respectively (unpublished data). The objective of this study was to determine whether resistance to sorghum midge may be related to HCN levels. In this study, three replications (pots) of 10 genotypes (Table 1) of sorghum with varying levels of midge resistance were planted in 20 cm wide plastic pots in the greenhouse on 17 Nov 1993 in a randomized complete block design. Plants were thinned to six plants per pot at ten days after planting. Response of the genotypes to midge (Table 1) were determined in a previous unpublished study, where a rating of 0 = resistant and 9 = susceptible. These entries were part of a larger experiment arranged in a randomized complete block with four replications where genotypes were rated for midge resistance under natural midge infestation at Tifton, Georgia. All genotypes in this study were inbred lines except DeKalb E57, Hyperformer, AgraTech 712G and Pioneer 8333 which were F, hybrids. Plants were grown under uniform temperature (28 to 30E C) and moisture conditions in the greenhouse. Plants were fertilized with a complete water-soluble fertilizer to maintain healthy growth. HCN contents were analyzed by analysis of variance procedures (SAS Institute, Inc., 1985) to evaluate genotype effects. Differences between genotype means were separated by the least significance difference (LSD) test at the 0.05 probability level. Leaf discs from non-expanded leaves of the six plants (50 days old) in a pot were cut with a #3 cork cutter and placed in 10 ml test tubes. Leaf tissue from each genotype in each replication weighed 0.07 g. No significant differences in dry weights of the leaf tissue were observed among replications and genotypes. Four drops of chloroform were added to each test tube. We used the colorimetric technique and standards described by Hogg and Ahlgren (1942). HCN content of the solutions were determined using a Milton Roy Company Spectronic 501 at 540 nm with Mr. Sipper.

Breakdown of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge (Stenodiplosis sorghicola Coquillett) is an important pest of grain sorghum worldwide. Several sources of resistance to sorghum midge have been identified in the world sorghum germplasm collection, of which some lines show a susceptible reaction in Kenya. Therefore, we studied the insect density damage relationships for a diverse array of midge-resistant and midge-susceptible sorghum genotypes, and variation in association of glume and grain characteristics with expression of resistance to sorghum midge. AF 28 and IS 8891 showed resistance to sorghum midge both in India and Kenya; DJ 6514 and ICSV 197, which are highly resistant to sorghum midge in India, showed a susceptible reaction at Alupe, Kenya. Sorghum midge damage in general was greater in Kenya than that observed in India at the same level of midge density suggesting that the breakdown of resistance in Kenya is due to factors other than insect density. Glume length, glume breadth, and glume area were positively associated with susceptibility to sorghum midge at both locations. However, under natural infestation, the correlation coefficients were stronger in India than in Kenya. Grain mass at 3 and 6 days after anthesis was positively associated with susceptibility to midge in India, but did not show any association with midge damage in Kenya. Grain growth rate between 3 and 6 days after anthesis was more strongly correlated with susceptibility to midge in Kenya than in India. Variation in the reaction of sorghum genotypes across locations may be partly due to the influence of environment on association between glume and grain characteristics with susceptibility to sorghum midge, in addition to the possible differences in midge populations in different geographical regions.

Identification of stable sources of resistance in sorghum to midge and their reaction to leaf diseases

Sorghum midge ( Stenodiplosis (= Contarinia) sorghicola Coquillett) is an important pest of grain sorghum worldwide, and several sources of resistance have been identified in the world sorghum germplasm collection. DJ 6514 and the breading lines derived from it become susceptible to sorghum midge in Kenya. Therefore, we evaluated a diverse array of midge-resistant and -susceptible lines at Alupe, Kenya; and ICRISTAT Center, India, to identify lines with stable resistance across seasons and locations. The test material was also evaluated for resistance to leaf diseases at Alupe, Kenya; to identify lines with multiple resistance to sorghum midge and leaf diseases. Across seasons and locations; IS 3461, IS 8884, IS 8887, IS 8891, IS 19476, IS 22806, and AF 28 showed high to moderate levels of resistance to midge, and these lines will be useful for use in resistance breeding programs. Thirty-nine lines showed resistance to midge both under natural infestation and no-choice headcage screening at ICRISTAT Center, India, over four seasons, of which IS 18696, IS 22806, ICSV, 197, ICSV 745, ICSV 88032, PM 20710-2, DJ 6514, and AF 28 were highly resistant. Genotypes IS 3461, IS 8884, IS 8887, IS 8589, IS 19476, IS 22806, ICSV 736, ICSV 90003, and AF 28 showed moderate levels of resistance to both midge and leaf diseases at Alupe, Kenya; and these lines can be used as sources of multiple resistance to these pests. Lines IS 2766, IS 7148, IS 8733, and IS 8589, showed high levels of resistance to leaf diseases in Kenya. Resistance to midge breaks down in some lines at Alupe, Kenya; possibly because of the influence of environment on the expression of resistance or the possible differences in midge populations at different geographic locations.

Spikelet Flowering Time: Cause of Sorghum Resistance to Sorghum Midge (Diptera: Cecidomyiidae)

Resistance of sorghum, Sorghum bicolor (L.) Moench, to sorghum midge, Stenodiplosis sorghicola (Coquillett), has not been well understood. In this study, possible causes of resistance were examined by comparing time of oviposition with times spikelets of resistant and susceptible sorghums flowered and thus were vulnerable to damage. Spikelets of resistant genotypes began flowering during the night, at 0100 hours (CDT), and most were closed and no longer vulnerable by the time sorghum midges began to oviposit in the field at 0800 hours. Peak flowering of resistant sorghums was between 0300 and 0700 hours. Most spikelets of susceptible sorghums, however, were flowering after daylight, at 0800 hours, when sorghum midge abundance began to increase. At 1000 hours, the time most sorghum midges (2.5 per panicle) were in the field, percentages of spikelets penetrable by a pin the diameter of a sorghum midge ovipositor were 32 and 62% for resistant and susceptible sorghums, respectively. Damage to panicles of resistant sorghums averaged 29.6%, whereas susceptible sorghums were more than twice as damaged (68.2%). Delaying for 5 h the daily flowering period of a resistant hybrid to coincide with time of peak sorghum midge abundance in the field resulted in 3.2 times as many sorghum midges emerging and 2.2 times as much damage as to panicles of the resistant hybrid that flowered at the normal time. Damage and numbers of sorghum midges emerging were not significantly different between normal and changed panicles of a susceptible hybrid. Most spikelets of resistant sorghums flowered and closed early and thus evaded damage before ovipositing sorghum midges were in the field. These results support the hypothesis that resistance is caused by asynchrony between time of sorghum spikelet flowering and presence of sorghum midge.

Tritrophic interactions in sorghum, midge ( Stenodiplosis sorghicola) and its parasitoid ( Aprostocetus spp.)

Studies on tritrophic interactions involving sorghum genotypes, midge ( Stenodiplosis sorghicola) and the predominant parasitoids ( Aprostocetus spp.) at ICRISAT Asia Center were conducted using three midge resistant (ICSV 745, ICSV 89058 and IS 10712) and three susceptible (Swarna, CSH 9 and ICSV 112) genotypes during the post-rainy (1992/93) and rainy (1993) seasons. A. coimhatorensis, the predominant parasitoid during post-rainy season, preferred mid-late midge larvae for parasitization, while A. gala, which was predominant during the rainy season, preferred early-mid larval stages. Variations in the preference of A. coimbatorensis and A. gala for the developmental stage of their host larvae indicate good prospects for the biological control of sorghum midge populations. There were significant differences in parasitization level of midge by Aprostocetus spp. between resistant and susceptible sorghum genotypes, and season. Higher parasitization was observed on susceptible genotypes than on resistant ones during both post-rainy and rainy seasons. However, the level of parasitization was greater in post-rainy than in rainy seasons. There was also evidence of higher midge infestation in rainy than in post-rainy seasons. Susceptible genotypes attracted more parasitoids because of high levels of midge infestations. Low parasitoid density in midge resistant sorghum under glasshouse and field conditions was associated with low midge infestations in these genotypes. However, parasitoids were always associated with their host in spite of low midge densities in resistant genotypes. The present study revealed that the interaction between host plant resistance and parasitoids of sorghum midge would thus be synergistic and complementary and could result in successful integration of these two important pest management tactics.

Evaluation of Sorghum Hybrids and Insecticide Application Timings for Management of Sorghum Midge in Grain Sorghum, 1997

Abstract A commercial sorghum midge resistant hybrid (DeKalb DK-60) and a commercial sorghum midge susceptible hybrid (Pioneer 8333) were evaluated in combination with an application of a pyrethroid insecticide at different sorghum development stages to suppress sorghum midge in grain sorghum. Experiments were conducted at the Macon Ridge location of the Northeast Research Station near Winnsboro, LA. Plots consisted of 4 rows (40-inch centers) X 45 ft and were planted in a Gigger-Gilbert silt loam. Treatments were arranged in a RCB design with 4 replications. The experiment was repeated three times during the growing season. Sorghum hybrids were planted on 17 Apr, 6 May, and 3 Jun in a conventional tillage system using a John Deere 7300 planter. Karate 1 EC at 0.02 lb (AI)/acre was applied with a high clearance sprayer equipped with a CO2 charged system calibrated to deliver 6 gpa at 47 psi through TSX-8 nozzles (2/row) in each test. Application timing for each treatment was determined by selecting and tagging 20 plants per plot from the 2 center rows before spraying. For the 50% panicle emergence treatments, 10 of 20 tagged plants had panicles completely emerged with no open flowers. For the 20% and 50% flowering treatments, 4 of 20 and 10 of 20 tagged plants per plot, respectively, had flowering panicles. A panicle was considered to be flowering when its upper 20% portion had open spikelets. Sorghum midge damage to seed heads was evaluated in the field on 1 Aug, 20 Aug, and 13 Sep for the 17 Apr, 6 May, and 3 Jun plantings, respectively, by visually estimating the percent blasted kernels on each tagged plant. Sorghum seed yields were obtained on 19 Aug, 28 Aug, and 29 Sep, respectively for each planting date, by mechanically harvesting the 2 center rows in each plot. Sorghum midge damage data were transformed [arcsin (x/100)] prior to ANOVA and LSD. Actual means are presented in tables.

Grain Yield of Sorghum Midge-Resistant Sorghum Hybrids, 1997

Abstract Resistance to sorghum midge (SM) was evaluated ir a 3-replication RCB design experiment composed of 52 sorghum hybrids at the Texas A&M University Agricultural Research and Extension Center a Corpus Christi and the Texas A&M University Agricultural Experiment Station Research Farm near College Station. Evaluated were 43 resistant X resistant experimental hybrids, 3 resistant X resistant checks, 1 resistant X susceptible check, and 5 susceptible checks. Three weeks before planting the experimental hybrids, eight rows of a susceptible hybrid were planted adjacent to the area to be used for the experiment. This sorghum provided a source of sorghum midge to infest the experimental sorghums. At Corpus Christi, seed of the experimental sorghum was planted 8 Apr in 6-m-long plots, with 97.1 cm between rows. Sorghum was planted at College Station 8 May, with 76.2 cm between rows. The sorghums were planted approximately 1 month late than normal for the region. SM were not controlled with insecticide. Performance of the experimental hybrids was compared with that of the checks (resistant X resistant, resistant X susceptible, and susceptible X susceptible) for grain yield (kg/ha) and damage caused by SM. Sorghum at physiological maturity was rated by plot for SM damage based on a scale of 1 to 9: 1 = 1-10%, 2=11-20%, to 9 = 81-100% of kernels failed to develop. Sorghun panicles in plots (0.0025 ha) were hand harvested and threshed using a stationary plot thresher. Threshed grain weight (g) was converted to kg/ha to ob tain grain yield. ANOVA and LSD0.05 were used for data analysis and mean separation, respectively.

Evaluation of Insecticides for Management of Sorghum Midge in Sorghum, 1997

Abstract Selected insecticides were evaluated for control of sorghum midge in grain sorghum at the Macon Ridge location of the Northeast Research Station near Winnsboro, LA. Grain sorghum was planted in a Gigger-Gilbert silt loam on 6 Jun in 4-row plots (40-inch centers) X 60 ft. Treatments were arranged in a RCB design with four replications. In-secticide treatments were applied with a high clearance sprayer equipped with a CO2 charged system calibrated to deliver 6 gpa at 47 psi through TX-8 hollow-cone nozzles (2/row). All insecticides were applied on 5 Aug when an average of 25% of sorghum plants in the field were at some stage of yellow flowering. Sorghum midge damage to seed heads was evaluated in the field on 5 Sep by visually estimating the percent blasted kernels on each of 20 randomly selected panicles from the 2 center rows per plot. Sorghum seed yields were obtained on 10 Sep by mechanically harvesting the 2 center rows in each plot. Sorghum midge damage data were transformed [arcsin (x/100)] prior to ANOVA and DNMRT. Actual means are presented in the table.

Sorghum Midge Suppression on Sorghum, 1997

Abstract The efficacies of 13 insecticides to control sorghum midge were evaluated at the Texas Agricultural Experiment Station Research Farm near College Station, TX. Seed of 6 sorghum hybrids of different maturity rates was mixed together and planted 29 May on rows spaced 30 inches apart. Mixtures of hybrids of different maturity were used to simulate sorghum with nonuniform flowering as often occurs in fields of sorghum. Insecticide treatments were compared in plots 6 rows X 40 ft long, arranged in a RCB design with 4 replications. Insecticides were applied to the middle 2 rows of plots. Insecticides were applied between 0630 and 0930 h CDT by using a hand-held backpack sprayer with 4, TX-3 hollow cone nozzles at 35 psi, producing a finished spray volume of 4.9 gpa. Insecticides were applied on 24 (when 10-50% of the panicles were flowering), 28, and 31 Jul (when 95-100% of panicles were flowering). Just before the 1st application, 10 panicles per replication of each treatment were enclosed individually in clear plastic bags, and the panicles were cut and taken to a laboratory where the number of adult sorghum midges were counted. Prior to the 2nd and 3rd applications, 5 panicles per plot were visually inspected and the number of adult sorghum midges recorded. Data on kernel loss and grain yield were collected from the 2 treated rows of each plot. Panicles in these rows were visually rated for kernel damage based on a scale of 1-9, where 1 = 0-10% to 9 = 81-90% kernel loss. Panicles were hand harvested from 2, 8.75-ft sections of treated row per plot and mechanically threshed. Data were statistically analyzed by using ANOVA and LSD.