J. E. Slosser

Cotton Aphid (Homoptera: Aphididae) Abundance in Relation to Cotton Leaf Sugars

Abstract A 4-yr study (1997–2000) was conducted in the northern Texas Rolling Plains to determine whether the pyrethroid insecticide λ-cyhalothrin was associated with changes in nutritional quality (nonstructural carbohydrates, percentage leaf nitrogen and moisture, and total amino acids) of cotton leaves. Another objective was to determine the relationship between nonstructural carbohydrates (glucose, fructose, sucrose, and starch) in cotton leaves and change in cotton aphid, Aphis gossypii Glover, numbers during late summer and on formation of dark-colored morphs. Carbohydrate concentrations, percentage leaf moisture and nitrogen, and total amino acids were not significantly altered in cotton leaves by λ-cyhalothrin. Glucose, fructose, sucrose, sugar ratio [(glucose + fructose)/sucrose concentrations], leaf nitrogen, and moisture were significantly influenced by year and irrigation treatment. Regression analysis indicated that change in aphid numbers was influenced by numbers of aphids per leaf, temperature, leaf moisture and nitrogen, and sugar ratio. A negative linear relationship was observed between change in aphid numbers and sugar ratio; population growth was limited by high levels of glucose and fructose in cotton leaves, especially when temperatures were high and leaf moisture low. Percentage of dark-colored aphids was negatively correlated with temperature and daylength and positively correlated with leaf moisture and nitrogen and the sucrose/glucose ratio. Some of the nutritional and abiotic environmental variables that interact to regulate the occurrence of dark morphs are also interacting with other variables to influence the extent to which the reproductive potential will be expressed.

Relationship between Aphis gossypii (Homoptera: Aphididae) and sticky lint in cotton.

Abstract The study was conducted in the northern Texas Rolling Plains in 1999 to define the relationship between number of cotton aphids, Aphis gossypii Glover, and resulting contamination of cotton lint by honeydew. Whole-plot treatments were three furrow irrigation management treatments: cotton grown without supplemental irrigation (dryland), irrigated cotton with last irrigation in mid August, and irrigated cotton with last irrigation in late August. Subplots within each irrigation treatment included an untreated check, a plot treated with lambda-cyhalothrin to stimulate aphid population increase, a plot treated with lambda-cyhalothrin followed by pymetrozine after aphids began to increase, and a plot treated with lambda-cyhalothrin followed by thiamethoxam after aphids began to increase. Cotton aphids were counted on leaves picked from the top and bottom half of the plant. Cotton lint was analyzed for contamination by glucose, fructose, sucrose, and melezitose secreted by cotton aphids, and percentage leaf moisture and nitrogen and leaf sucrose concentrations were determined. The manual sticky cotton thermodetector was used to determine degree of lint stickinesss. There was a significant relationship between thermodetector counts and melezitose contamination on lint, and a melezitose concentration of 90.9 μg/g of lint was associated with a thermodetector count of 10, the threshold for sticky lint problems in textile mills. An equation was developed to estimate melezitose concentration on lint as a function of average numbers of aphids per leaf and the interaction between percentage leaf moisture and nitrogen. The number of aphids per leaf associated with a melezitose concentration of 90.9 μg/g of lint ranged from 11.1 to 50.1, depending on percentage leaf moisture and nitrogen. The threshold for sticky lint problems occurred when aphid numbers ranged between 11.1 and 50.1 per leaf after bolls open.

A Model for Predicting Boll Weevil (Coleoptera: Curculionidae) Overwintering Survivorship

Abstract Boll weevil, Anthonomus grandis grandis Boheman, overwintering survivorship was quantified monthly throughout the overwintering period (October to May) in Texas High Plains and Rolling Plains for 12 yr. A negative exponential model was developed to dynamically predict survivorship throughout the overwintering months. Survivorship was modeled as a function of the number of days that weevils were in the habitat, negative degree-days (<0.0°C), positive degree-days (>6.1°C), rainfall, and mortality during the first month of overwintering. First month mortality was modeled as a function of overwintering survival potential of weevils determined by dissection examination of their body lipid content and gonad atrophy. A nonlinear iterative multiple regression analysis showed that the model explained 94% of the variability in parameterization-verification data; a goodness-of-fit test showed that 97% of the estimated survival values did not significantly depart from their corresponding observed values. With independent validation data, 94% of the variability was explained by the survival model; a goodness-of-fit test for validation data showed that 96% of the predicted survival values did not significantly depart from their corresponding observed values. This model offers a greater understanding of boll weevil overwintering biology as it demonstrates a link between biological and climatic parameters. The model can be used to forecast weevil survivorship throughout the overwintering period in the Texas Plains.

Economic Evaluation of Boll Weevil (Coleoptera: Curculionidae) Management Options for Early-Planted, Irrigated Cotton in the Texas Rolling Plains

A full-season, threshold-based insecticide control program, primarily for boll weevils, Anthonomus grandis grandis Boheman, was compared with no insecticide control in irrigated cotton in the northern Texas Rolling Plains from 1993 to 1995. These 2 insecticide-use options were compared in 3 cotton varieties (Paymaster HS-26, TAMCOT HQ95, All-Tex Quickie) and 2 row spacings (76 cm and 102 cm). Cotton was planted in late April–early May each year, which is an earlier planting time than the preferred period of late May–early June. Net returns per hectare were calculated for each treatment and used to determine the most cost-effective chemical and row spacing management option for each cultivar. Of the 29 insecticide applications for thrips, boll weevils, bollworms, Helicoverpa zea (Boddie), and cotton aphids, Aphis gossypii Glover, during the 3-yr study, 23 were for management of boll weevils. Boll weevil damage was lowest in HS-26 in the 76-cm spacing and in Quickie in the 102-cm spacing; these cultivars produced fewer squares and bolls in these 2-row spacings, respectively, than did the other 2 cultivars. Average yield and net income were higher in the 76-cm row spacing than in the 102-cm spacing, and yield and income were higher in HS-26 and HQ95 than in Quickie in both row spacings. Average yields were higher in treated plots, but average net income was higher in untreated cotton. In the 76-cm row spacing, highest net return was obtained from untreated Quickie (

Variations in Damage to Wheat Caused by Russian Wheat Aphid (Homoptera: Aphididae) in Texas

161.10/ha), whereas in the 102-cm row spacing, highest net returns were obtained in treated HS-26 (

Parasites of Heliothis spp. (Lepidoptera: Noctuidae): Parasitism and Seasonal Occurrence for Host Crops in the Texas Rolling Plains

174.47/ha) and untreated HQ95 (

Seasonal activity of Helicoverpa zea and Heliothis virescens (Lepidoptera: Noctuidae) detected by pheromone traps in the Rolling Plains of Texas

167.25/ha). In treated and untreated cotton, the break-even yields were 697 and 514 kg lint per hectare, respectively. In treated plots, yields for HS-26 in both 76- and 102-cm row spacings and for HQ95 in the 76-cm row spacing exceeded the break-even yield. Yields in all untreated plots, except Quickie in the 102-cm row spacing, exceeded the break-even yield. Based on comparisons of net returns between treated and untreated plots of each cultivar, only HS-26 in the 102-cm row spacing responded adequately to a full-season insect control program. In all other comparisons, net returns were numerically higher in untreated plots. Insecticidal control should be tailored to the cultivar and row spacing used.