Dale W. Spurgeon

Preferred technique for adult sex determination of the boll weevil (Coleoptera : Curculionidae)

Abstract Determining the sex of boll weevil adults is complicated by the lack of a discreet, gender-specific, externally visible character. However, the posterior edge of the male’s 8th tergite is distinctively notched and can be revealed by gentle probing. Although completely accurate as originally described 35 yr ago, this method has not gained wide acceptance because of the perception that it is too slow for processing large numbers of weevils, and because the original illustrations are not entirely adequate. Here we illustrate the tergal-notch method of sexing with photographs and interpretive drawings. This method is quickly learned and fast enough for most applications. Experienced users in our laboratories routinely sex 200–300 weevils per hour. The common alternative technique of sexing by relative snout characteristics is only 88–90% accurate.

Variation in boll weevil (Coleoptera: Curculionidae) captures in pheromone traps arising from wind speed moderation by brush lines

Abstract Paired trap lines of five boll weevil pheromone traps each were placed on opposite sides of a brush line at six different sites on a plantation in the Lower Rio Grande Valley of Texas. Temperature, wind speed, and wind direction were monitored with a nearby weather station. We observed a strong negative relationship between mean daily wind speed and total daily capture of boll weevils. About half of the day-to-day variation in weevil captures was explained by wind speed alone. In addition, our data indicate that much of the variation between traps within days may arise from differences in local wind speed as governed by local vegetation depending on wind direction. Brush lines in this study slowed the wind by 40–70% and mitigated its effects such that traps on the leeward side averaged 2.5–2.9 times higher captures than traps on the windward side. The magnitude of the effect of windward or leeward placement of traps on weevil captures depended on the relative strength of the wind. Under light winds (<10 km/h), there were no increases in leeward trap captures. However, on days of moderate (10–20 km/h) or strong (>20 km/h) winds, leeward trap captures averaged 3.9- or 2.4 times greater than windward captures, respectively. By accounting for the ability of vegetation to dampen the effects of wind on boll weevil trap captures, we should be able to dampen daily and positional variation in trap captures by more careful placement of traps. Furthermore, judicious placement of traps in locations protected from prevailing winds should improve detection efficiency in areas where early warning of weevil presence is critical, such as in eradication and posteradication zones.

Diapause Response of Laboratory Reared and Native Lygus hesperus Knight (Hemiptera: Miridae)

ABSTRACT Inconsistencies among results of previous studies on the induction of adult diapause in the western tarnished plant bug, Lygus hesperus Knight, could be either attributed to differences in the criteria being applied to assess diapause status or to differences in the sources of the bugs. To determine the cause of variation and to refine the criteria used to define diapause status, we obtained eggs from adults collected from the field and from long-term laboratory colonies. Concurrent experiments were conducted at two locations. L. hesperus were reared from egg to adulthood under photophases of either 10 or 14 h and at a constant temperature (26.6°C, Shafter, CA; 27.5°C, Maricopa, AZ). Adults were dissected at 10-d posteclosion to measure diapause status using both stringent and relaxed morphological criteria. Under both sets of criteria, offspring of field caught parents were more likely to enter diapause with short day exposure than those offspring from stock colonies. The relaxed criteria appeared appropriate for distinguishing diapause in males, whose reproductive organs can appear undeveloped after mating, although the stringent criteria appeared appropriate for females. Additional analyses were conducted to determine whether extended laboratory rearing influenced other behavioral or physiological traits. Bugs from stock colonies were more likely to mate, and to oviposit as virgins, than were bugs originating from the field. The results demonstrate a pronounced effect of colony rearing on L. hesperus physiology and behavior, and also have led to refinement of the criteria used to assess diapause status.

Laboratory Studies of Variations in Feeding Behaviors Among Lygus hesperus (Hemiptera: Miridae) of Different Gender and Reproductive States

ABSTRACT Lygus hesperus Knight (Hemiptera: Miridae) is a key pest of cotton (Gossypium spp.) in the western United States that injures floral buds (squares) and developing fruit (bolls). Levels of Lygus-induced damage to cotton can vary by Lygus ages or gender, and these variations complicate interpretation of studies designed to elucidate Lygus and cotton interactions. Variations in observed injury may reflect different behaviors among Lygus gender or ages. We compared times allocated to feeding and trivial movement between male and female adult L. hesperus of different reproductive states: prereproductive, reproductive and mated, and reproductive and unmated. Prereproductive adults exhibited less trivial movement and spent more time stylet-probing compared with reproductive unmated and mated adults. Mated females stylet-probed more times than other classes of adults, whereas mated and unmated reproductive females exhibited more test probes (≤10 s duration) than prereproductive females. Reproductive females probed the anther region of squares less than prereproductive females. Instead, reproductive females tended to stylet-probe squares below the bracts, which is also where they oviposited. Each oviposition event was preceded by a short duration stylet-probe at the oviposition location. Unmated reproductive males exhibited more test probes but fewer ingestion probes (>1 min) compared with prereproductive and mated males. These results indicate a pattern in which prereproductive adults are less active and feed more compared with reproductive adults, but behaviors vary among reproductive adults of different gender and mating states. We propose that differences in behaviors exhibited among adult L. hesperus are related to the different requirements imposed by mate seeking, mate attraction, and oviposition.

Age Dependence of Pheromone Production by the Boll Weevil (Coleoptera: Curculionidae)

Abstract Factors influencing pheromone production by the boll weevil, Anthonomus grandis Boheman, have been extensively studied, yet recent research using new methods suggests much remains unknown in this regard. The studies reported herein examined age-related changes in production and composition of pheromone at ages from 0 to 6 d, and at 3, 6, 9, and 12 d, respectively, and evaluated the association between accessory gland condition and pheromone production. Estimates of pheromone from extracted feces were near the upper values previously reported. However, ≈95% of the total pheromone was obtained from headspace collections. Based on feces extractions, pheromone production increased with age until the sixth day, while headspace collections indicated an increase in production until the ninth day of adulthood. The boll weevil pheromone is composed of two alcohols (components I and II) and two aldehydes (components III and IV), and ratios of these components changed with age. Component I was dominant for the first days of adult life, but the composition subsequently stabilized at ≈42.5:42.5:5:10 (I:II:III:IV). Also, high levels of pheromone production were associated with well-developed accessory glands, while weevils with small, transparent glands produced little or no pheromone. These results demonstrate production of pheromone in greater quantities, and at earlier ages, than was previously recognized. Additional study using the methods reported herein should provide improved understanding of the dynamics of boll weevil pheromone production that will facilitate ecological interpretation of field data and improvements in trapping systems.

Influence of Feeding Status and Physiological Condition on Supercooling Points of Adult Boll Weevils (Coleoptera: Curculionidae)

Abstract Cold bath studies were conducted to examine the impacts of midgut content and fat body condition on supercooling points of adult boll weevils, Anthonomus grandis Boheman. The presence of solid food in weevil midguts significantly raised the supercooling points of weevils. Supercooling points of recently fed weevils with solid food present in their midguts ranged from −6.2 to −16.0°C, with a mean ± SD of −10.9 ± 1.9°C, whereas supercooling points of unfed weevils with empty midguts ranged from −10.2 to −20.2°C, with a mean ± SD of −16.0 ± 2.1°C. The mean supercooling point of weevils whose midguts contained colored traces of food from previous feeding was between those of recently fed weevils containing solid food and those having empty midguts. These findings indicate that the influence of feeding on supercooling points of weevils is related to the quantity and/or condition of midgut contents in weevils. No relationship was detected between the supercooling capabilities of weevils and amounts of hypertrophied fat bodies present for either fed or unfed weevils. Additionally, there was no significant difference in mean supercooling points between male and female weevils, and no relationship was detected between the supercooling points of weevils and their age. These results show that presence of food residues in the boll weevil midgut can have a significant effect on supercooling points of weevils, and indicate that the recent feeding history and midgut condition of weevils should be documented or at least considered in future supercooling and overwintering survival studies.

Temperature-Dependent Reproductive Development of Lygus hesperus (Hemiptera: Miridae)

ABSTRACT Recent studies to elucidate relationships between the western tarnished plant bug, Lygus hesperus Knight, and injury to cotton (Gossypium spp.) have identified important differences in feeding behaviors among adults of different gender and reproductive states. These findings suggest a need for improved knowledge of L. hesperus temperature-dependent reproductive development. We used nonlinear biophysical development rate models to describe the reproductive development of adult L. hesperus held under constant temperatures from 12.8 to 35.0°C. In addition, abdominal coloration was measured as a nondestructive indicator of reproductive maturity. The nonlinear models indicated most stages of ovary, seminal vesicle, and medial accessory gland development were subject to mild high-temperature inhibition, except for the stage of filled seminal vesicles. Development of filled seminal vesicles was subject to minor low-temperature inhibition and severe high-temperature inhibition. Estimated development times reflected extensive interindividual variation, especially at low temperatures. This variation suggests the opportunity to select for more rapid or more consistent reproductive development under the conditions of laboratory culture. Although presence of a dark aqua coloration of the ventral abdominal cuticle was statistically associated with reproductive maturity, especially in female L. hesperus, color-based predictions of reproductive maturity were often inaccurate for bugs reared at temperatures above or below 26.7°C. Therefore, use of abdominal coloration as an indicator of reproductive development in ecological studies seems inappropriate. The results provide mathematical descriptions of L. hesperus temperature-dependent reproductive development which should facilitate improved planning and interpretation of studies involving manipulation of adult reproductive status.

Temperature dependent development of Lygus hesperus (Hemiptera: Miridae) nymphs.

ABSTRACT Lygus hesperus Knight (Hemiptera: Miridae) is a key pest of fruit and vegetable crops, forages, and cotton (Gossypium spp.) in the western United States. Accurate models describing relationships between temperature and L. hesperus development are critical to the study of seasonal L. hesperus population dynamics. Development of L. hesperus nymphs was assessed at nine constant temperatures from 10 to 37.8°C. The relationships between temperature and development for each L. hesperus instar, and for the entire nymphal stage, were best described by six-parameter biophysical models indicating both low- and high-temperature inhibition of development. Development rates asymptotically approached zero with decreasing temperature in the lower thermal range, and decreased with increasing temperatures above 32.2°C. Nymphs did not survive from egg hatch to adulthood at either 10 or 37.8°C, and nymph mortality was >90% at both 12.8 and 35.0°C. The fifth instar exhibited the longest stadium, whereas the shortest stadia were associated with the second and third instars. Development rates of males and females did not differ, and the ratio of males to females was not different from 1:1 at any temperature. Our temperature-dependent development rate models for L. hesperus nymphs will facilitate control of insect physiological age in controlled laboratory experiments, and should be useful in planning and interpreting field studies on L. hesperus population dynamics.

Boll Weevil (Coleoptera: Curculionidae) Adult Diapause Responses to Selected Environmental and Dietary Conditions

Abstract Diapause in the boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), remains a contentious issue despite intense investigation. In particular, the roles and interactions of photoperiod, temperature, and adult diet are poorly understood. We reexamined the influences of these factors by using a feeding regime typical of previous studies (group, one square per five weevils per day) as well as one known to promote reproductive development (single, one square per weevil per day). Three separate studies each incorporated all combinations of the following: photoperiod (short day, 11:13; long day 13:11 [L:D] h), temperature regime (constant 29.4°C; warm day, cool night, 29.4°C:10°C [day:night]), feeding regime, and age at dissection (3, 6, 9, and 12 d after adult eclosion). Studies differed in the source of weevils (Lower Rio Grande Valley, Brazos Valley of Texas), and the photoperiod used to rear weevils to adulthood (11:13 or 13:11 [L:D] h). Interpretable effects of photoperiod on diapause induction were not observed. Cool nights delayed development of fat bodies, oocytes with yolk, and the occurrence of diapause, especially in weevils fed in groups. However, these effects were not apparent by day 12. The most marked responses were to feeding regime. Hypertrophied fat bodies and diapause generally occurred more often, and oocytes with yolk less often, for weevils fed in groups compared with those fed singly. Fundamental differences in diapause response corresponding to rearing conditions or source of weevils were not observed. These results suggest improved understanding of the dynamics of adult diapause could be achieved through continued investigation of the effects of adult diets.

Injury to Cotton by Adult Lygus hesperus (Hemiptera: Miridae) of Different Gender and Reproductive States

ABSTRACT Lygus hesperus Knight (Hemiptera: Miridae) is a key pest of cotton (Gossypium spp.) in the western United States that injures floral buds (squares) and developing fruit (bolls). However, no clear relationship between Lygus population level and plant injury has been established. Agedependent feeding activity by L. hesperus is a possible source of variation that has not been examined for its influence in studies of the impact of Lygus on cotton. Recent video-based laboratory studies indicated that feeding behaviors and trivial movement varied among L. hesperus adults of different gender and reproductive states (prereproductive; reproductive and unmated; and reproductive and mated). We compared within-plant distributions and accumulations of feeding injury to intact cotton plants corresponding to adult L. hesperus of different gender and reproductive states. Adult females, regardless of reproductive state, were observed on squares and axillary buds more often than were males. Additionally, prereproductive adults were observed on squares and axillary buds more often than were mated or unmated reproductive adults, regardless of gender. Plants that were exposed to prereproductive adults exhibited more abscised squares and more squares with injured anthers compared with plants exposed to reproductive adults. However, feeding injury did not differ by insect mating status or gender. These results are consistent with results of our previous video-based assays, and indicate adult reproductive state represents a source of variation that should be controlled in studies to evaluate Lygus-induced injury to cotton and other crop plants.