Physiological consequences of laboratory rearing of Lygus hesperus (Hemiptera: Miridae).

Several aspects of the basic biology of the western tarnished plant bug, Lygus hesperus Knight, are poorly known despite the economic importance of this species. Among these are the factors regulating the adult diapause. Reports of recent studies questioned the validity of earlier reports of diapause in L. hesperus, in part because of the demonstrated loss of diapause response in insects obtained from long-standing laboratory colonies. However, use of laboratory reared insects would facilitate additional diapause research, so long as those insects exhibit a diapause response similar to that of the field population. L. hesperus, originating as eggs from field-collected insects, were reared in the laboratory for four generations to examine corresponding changes in selected biological characteristics. Over the course of the four generations, incidence of diapause in both L. hesperus genders decreased whereas the frequency of oviposition by virgin females increased. Measurable changes were not observed in frequency of occurrence of a specific fat body type (glass bead fat) or nymphal development time. These results suggest L. hesperus used in diapause research should be as close to the field population as possible, but no further removed than three generations. Results further demonstrate variability among different biological characteristics in their responses to selection from laboratory rearing. Collectively, these findings demonstrate the importance of understanding the influences of rearing on specific biological characteristics under study, and the need to verify the similarity of laboratory-reared insects to their native counterparts in studies used to draw inferences regarding the field population.

Among-sampler variation in sweep net samples of adult Lygus hesperus (Hemiptera: Miridae) in cotton.

The sweep net is a standard sampling method for adults of the western tarnished plant bug, Lygus hesperus Knight (Hemiptera: Miridae), in cotton (Gossypium spp.). However, factors that influence the relationship between true population levels and population estimates obtained using the sweep net are poorly documented. Improved understanding of these factors is needed for the development and application of refined treatment thresholds. Recent reports of significant among-sampler differences in sweep net-based population estimates of the adult tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), seem to preclude meaningful comparisons of population estimates collected by different samplers. We used a mark-release-recapture method and the standard sweep net to evaluate among-sampler differences in population estimates of L. hesperus adults. Adult lygus, marked with fingernail polish to facilitate identification and prevent flight, were released into 10-m sample rows on the evening before 10-sweep samples were collected the following morning. The experimental design was a randomized complete block with three replications of three treatments (sampler). Separate experiments were conducted in two plantings each of Pima (Gossypium barbadense L.) and Acala (Gossypium hirsutum L.) cotton. Collections of marked bugs from each study were evaluated for effects of sampler, sample date, and their interaction. Although differences in lygus collections were observed among sample dates in some tests, no differences were detected in the population estimates by different samplers. These results demonstrate that the sweep net technique can be sufficiently standardized to allow direct comparison of population estimates obtained by different samplers.

Efficacy of Four Soil Treatments Against Fusarium oxysporum f. sp. vasinfectum Race 4 on Cotton.

Fusarium wilt, caused by race 4 of Fusarium oxysporum f. sp. vasinfectum, is a critically important disease problem in California cotton (Pima, Gossypium barbadense; Upland, G. hirsutum). Because few cultivars with resistance to race 4 are available, alternative management strategies for this disease are needed. Four soil treatments (50:50 methyl-bromide + chloropicrin as a positive control; 60:40 chloropicrin + 1,3-dichloropropene; 6 weeks of solarization; and metam-sodium) were evaluated for efficacy against race 4 in a naturally infested, heavy clay soil. Treatments were evaluated based on plant mortality, height, number of mainstem nodes, vascular discoloration ratings, and soil counts of F. oxysporum. Two cultivars each of Pima and Upland cotton varying in resistance to race 4 were used. Plant mortality was lowest in methyl-bromide + chloropicrin, solarization, and chloropicrin + 1,3-dichloropropene treatments, and highest in the nontreated and metam-sodium treatments. Although most plant mortality occurred within 5 weeks after planting, substantial mortality of the susceptible Pima cultivar DP 744 accumulated for up to 10 weeks. Seven to eight weeks after planting, plants in methyl-bromide + chloropicrin and chloropicrin + 1,3-dichloropropene treatments were taller and had more mainstem nodes than in other treatments. Vascular discoloration was reduced in methyl-bromide + chloropicrin and solarization treatments compared with the nontreated control, metam-sodium, and chloropicrin + 1,3-dichloropropene treatments. Soil counts of F. oxysporum were significantly reduced only in the methylbromide + chloropicrin, chloropicrin + 1,3-dichloropropene, and solarization treatments. Six weeks of solarization and 60:40 chloropicrin + 1,3-dichloropropene (295 liters a.i./ha) proved effective for reducing Fusarium wilt of cotton in heavy clay soil.

Comparisons of boll weevil (Coleoptera: Curculionidae) pheromone traps with and without kill strips.

Boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), eradication programs typically equip pheromone traps with an insecticide-impregnated kill strip. These strips are intended to kill captured insects, thereby simplifying trap servicing and reducing the loss of weevils from predation and escape. However, the effectiveness of kill strips has not been extensively evaluated. We examined the influences of kill strips on weevil captures, trap servicing, and the incidences of weevil predation and trap obstruction (e.g., by spider webs). Evaluations were conducted weekly during three different production periods (pre- to early-, late-, and postseason) of cotton, Gossypium hirsutum L., to represent different environmental conditions and weevil population levels. Within each period, mean weekly captures of weevils in traps with and without kill strips were statistically similar. On average, traps with kill strips took 9 s longer to service than traps without kill strips, but statistical differences were only detected during the late-season period. Overall, the mean weekly proportion of traps with evidence of weevil predation or trap obstruction was significantly lower for traps with kill strips (0.25) than for traps without kill strips (0.37). However, this reduction in the frequency of weevil predation or trap obstruction was too small to produce a corresponding increase in the numbers of weevils captured. In light of these findings, the use of kill strips is likely unnecessary in eradication programs, but may be a consideration in situations when the numbers of deployed traps are reduced and chronic problems with weevil predation or trap obstruction exist.

New Genotypes of Fusarium oxysporum f. sp. vasinfectum from the Southeastern United States.

Sixty-one isolates of Fusarium oxysporum f. sp. vasinfectum were collected from cotton plants (Gossypium spp.) with symptoms of Fusarium wilt to determine the composition of races present in the southeastern United States. Analysis of partial sequences of the translation elongation factor gene revealed four novel genotypes, as well as the presence of races 3 and 8 for the first time in the United States outside of California. The majority of isolates (16 of 27) sampled from Arkansas were novel genotypes. A subset of isolates representing the novel genotypes was compared with previously described races using sequences from translation elongation factor, phosphate permase, and ß-tubulin genes and their pathogenicity on a total of six Upland (Gossypium hirsutum) and Pima (G. barbadense) cotton cultivars. Two of the novel genotypes belonged to a clade containing races 1, 2, 4, 6, and 8 and two shared ancestry with race 3. All new genotypes were pathogenic to at least some of the cotton cultivars tested. The Pima cv. Phytogen 800 was relatively resistant to all genotypes of the pathogen. These results indicate that the population of F. oxysporum f. sp. vasinfectum in the southeastern United States is more diverse than previously recognized.

Captures of boll weevils (Coleoptera: Curculionidae) in traps associated with different habitats.

Programs to eradicate the boll weevil, Anthonomus grandis grandis Boheman, from cotton, Gossypium hirsutum L., in the United States rely heavily on pheromone traps for monitoring weevil populations in both active and posteradication maintenance programs. Modifications to trapping protocols that increase trap effectiveness should contribute to this eradication effort. Between October 1996 and May 1997 and between September 1997 and April 1998, we compared trap effectiveness, indicated by the numbers of captured weevils, in relation to selected habitat types. Each study period was divided into fall, winter, and spring seasons. Traps were closely associated with seven habitat types, including four types with prominent erect vegetation (brush-lined irrigation canal, brush, sugarcane, and resaca or ox-bow lake) and three types with only low-growing or sparse erect vegetation (irrigation drainage canal, unimproved pasture, and fallow fields). Captures of male and female weevils were statistically similar regardless of season or trapping habitat. Although captures differed significantly among habitats, these differences varied among seasons. Trapping habitats with prominent vegetational features generally produced higher weekly captures of weevils than habitats lacking these features. Also, captures in traps associated with prominent vegetation indicated seasonal differences in weevil activity, with highest captures occurring during the fall. Traps associated with habitats lacking prominent vegetation did not statistically demonstrate seasonal differences. Our results indicate that immediate trap surroundings strongly influence the effectiveness of the boll weevil pheromone trap. These results also suggest that effectiveness of current trapping programs may be improved through purposeful association of traps with selected vegetational features.

Comparisons of standard and extended-life boll weevil (Coleoptera: Curculionidae) pheromone lures.

The Southeastern Boll Weevil Eradication Program has proposed reducing maintenance program costs in eradicated zones by using an extended-life "superlure" in traps to detect populations of the boll weevil, Anthonomus grandis grandis Boheman. However, superlure effectiveness has not been extensively evaluated. We compared the superlure (30 mg of eugenol plus 25 mg of grandlure) to a standard lure (10 mg of grandlure) based on captures of weevils and changes in lure pheromone content. Lure treatments (standard and superlure, replaced biweekly or not replaced) were compared in 4-mo-long trapping periods. Captures of weevils did not generally reflect differences among lure treatments indicated by assays of lure contents. During the first 2 wk of exposure, amounts of pheromone released by the superlure were generally comparable with those of the standard lure, but pheromone composition was more stable. During the second 2 wk of exposure, the superlure usually released more pheromone than similarly aged standard lures, but less than half as much as the standard lure replaced biweekly. Based on numbers of captured weevils during the last 2 wk of an extended trapping period, the superlure performed similarly to the standard lure replaced biweekly. However, corresponding pheromone releases by the superlure were less than those by the standard lure replaced biweekly. This inconsistency suggests that numbers of captured weevils alone may be inadequate for evaluation of pheromone formulations. Our results suggest that better understanding of the consequences of reduced pheromone release during an extended trapping period is needed before adoption of the superlure can be recommended.

Size-dependent feeding and reproduction by boll weevil (Coleoptera: Curculionidae).

The considerable variation in adult size of the boll weevil, Anthonomus grandis grandis Boheman, has been well documented, but the influences of adult size on reproductive rate are not known. We examined the relationship between the size of boll weevils and their feeding and oviposition. Weevils weighed to the nearest milligram were grouped into five categories based on pupal weight: < or =5, 6-10, 11-15, 16-20, and >20 mg. Numbers of lifetime punctures produced in flower buds (squares) of cotton, Gossypium hirsutum L., by both sexes of adults tended to increase with pupal weight. Boll weevil females with pupal weights >10 mg produced progeny with significantly higher survival to adulthood and also produced a higher percentage of female progeny than those with pupal weights < or =10 mg. The population growth indices for females having pupal weights >10 mg averaged 1.8-fold higher than those of females weighing < or =10 mg. Survivorship of adults of both sexes also tended to increase with pupal weight. The percentage of females laying eggs on any given day averaged 2.1 times higher when their pupal weights were >10 mg than when their pupal weights were < or =10 mg. Although small size negatively affected female reproductive potential, even extremely small females produced some viable offspring. However, the penalties of small adult size, in terms of longevity and reproductive potential, suggest that cultural practices that result in the production of small adults may be used to impact weevil populations.

Population patterns of Mexican corn rootworm (Coleoptera: Chrysomelidae) adults indicated by different sampling methods.

The Mexican corn rootworm, Diabrotica virgifera zeae Krysan & Smith, is a serious pest of corn, Zea mays L., in several areas of Texas. Recent demonstrations of areawide adult control suggest this tactic has promise for rootworm management, but additional information regarding treatment thresholds and sampling methodology is needed. In 2000 and 2001 we examined the influence of distance into the field on rootworm captures by CRW and Pherocon AM traps, the fidelity of trap captures to population estimates from visual counts of beetles on plants (whole plant samples), and the seasonal population patterns indicated by each sampling method. Only the CRW trap consistently indicated reduced trap captures at the field margin compared with other distances. However, trends for the AM trap and whole plant samples suggested sampling on the field margin should be avoided. Population estimates at other distances into the field (2-30 m) were usually statistically similar. Thus, monitoring does not require trap placement far into the field. Both trap types indicated population peaks after flowering in corn, whereas plant samples indicated peak populations during tasseling and flowering. Both the CRW trap and plant samples showed the proportion of female beetles increased as the season progressed, but the CRW trap underestimated the proportion of females until after flowering. Regressions relating captures by traps to counts from plant samples indicated efficiency of both traps increased with increasing plant development. Our findings should increase acceptance of the CRW trap by producers and consultants and provide a rationale for development of improved, plant growth stage-specific treatment thresholds.

Early-season colonization patterns of the boll weevil (Coleoptera: Curculionidae) in Central Texas cotton.

It is commonly believed that colonization of early-season cotton, Gossypium hirsutum L., by overwintered boll weevils, Anthonomus grandis grandis Boheman, is concentrated on field margins. However, supporting experimental evidence is not available. In 1999 and 2000, we examined colonization patterns of overwintered boll weevils in Central Texas cotton on the bases of adult collections by a pneumatic sampler and hand collections of abscised infested squares. Samples were taken from sites arranged in a grid that extended inward >70 m from the field margin. Adults were collected from shortly after seedling emergence until the flowering stage, and infested squares were collected during the one-third grown square stage. Despite numerical trends, the numbers of adult weevils collected were not significantly different between years or sexes, or among plant phenological stages. Field-to-field variation among collections was considerable and likely prevented detection of differences among these factors. Spatial patterns represented by adult weevil and infested square collections were examined by logistic regressions fitted to the respective probabilities of weevil detection at each designated sample site. Although we observed trends for slightly decreased probability of weevil detection with increased distance from the field margin, these trends were too weak to be demonstrated statistically. Our results indicate the boll weevil does not consistently exhibit a strong edge-oriented colonization pattern, and that management tactics that are predicated on these patterns, such as border sprays, should be used with caution.