Charles P.-C. Suh

Geographic pattern of host-associated differentiation in the cotton fleahopper, Pseudatomoscelis seriatus

Host‐associated differentiation (HAD) is the occurrence of genetically distinct, host‐associated lineages. Most of the cases of HAD in phytophagous insects have been documented in specialist insects inhabiting feral ecosystems or in generalist parthenogens in agroecosystems. Herein we report HAD in the cotton fleahopper, Pseudatomoscelis seriatus (Reuter) (Hemiptera: Miridae), a native, generalist, non‐parthenogenetic insect feeding on native wild hosts [horsemint, Monarda punctata L. (Lamiaceae) and woolly croton, Croton capitatus Michx. (Euphorbiaceae)] and on cotton [Gossypium hirsutum L. (Malvaceae)] in the USA. Examination of genome‐wide genetic variation with AFLP markers and Bayesian analyses of P. seriatus associated with three different host plant species at five locations in Texas revealed a geographic pattern of HAD. The geographic pattern of HAD corresponded with differences in precipitation among the locations studied. In three locations, two distinct lineages of P. seriatus were found in association with horsemint and cotton/woolly croton, whereas in two other locations, populations associated with the different host plants studied were panmictic. We suggest that precipitation differences among locations translate into heterogeneity in vegetation distribution, composition, and phenology, which altogether may contribute to the observed geographic pattern of HAD.

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.

Host-Free Survival of Boll Weevils (Coleoptera: Curculionidae) Previously Fed Vegetative-Stage Regrowth Cotton

The need to minimize populations of overwintering boll weevils (Anthonomus grandis Boheman) in eradication programs is widely recognized, but the potential contribution of nonfruiting regrowth cotton (Gossypium hirsutum L.) to weevil survival has not been directly examined. We conducted experiments in 2002 and 2003 to examine the host-free survival of weevils previously supplied vegetative-stage regrowth cotton. Weevils, 1-3 d after eclosion, were caged with vegetative regrowth cotton under ambient environmental conditions (2002), or in a controlled environment (23.9 ± 2°C, 13:11 [L:D] h photoperiod, 2003). Four cohorts of each weevil sex were examined each year. Of the 60 weevils dissected in 2002, 12% possessed the hypertrophied fat bodies associated with diapause and extended host-free survival. In both years, the majority of weevils (>75%) died during the initial 3 wks of the host-free period. No differences in host-free survival were detected between weevil sexes, but differences were observed among replicates of the experiment established on different dates. Survival patterns among replicates, however, did not strictly follow a seasonal pattern. Overall, approximately 5% of the weevils survived >=12 wks beyond the feeding period. Maximum observed host-free longevities were 21 wks in 2002 and 19 wks in 2003. Our findings suggest vegetative-stage regrowth cotton should not be disregarded in eradication zones, particularly in those experiencing unsatisfactory progress or resurgence of weevil populations.

Comparisons of Boll Weevil (Coleoptera: Curculionidae) Pheromone Traps With and Without Kill Strips

ABSTRACT Boll weevil, Anthonomm 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.

Supercooling in the Adult Boll Weevil (Coleoptera: Curculionidae) Relative to Physiological Condition and Diet

Severe winter temperatures are known to reduce overwintering survival of the boll weevil (Anthonomus grandis Boheman), but the mechanisms responsible for overwintering mortality are poorly understo...

Temperature Influences on Diapause Induction and Survival in the Boll Weevil (Coleoptera: Curculionidae)

Abstract The boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), has been the most important pest of cotton (Gossypium spp.) wherever it occurs. Although eradication programs in the United States have reduced the range of this pest, the weevil remains an intractable problem in subtropical Texas, Mexico, and much of South America. A key to managing the weevil in the subtropics and tropics might lie in better understanding its diapause and overwintering survival in regions characterized by relatively high late-season temperatures. We examined the temporal patterns of acquisition of diapause characters at 18.3, 23.9, and 29.4°C, and the effects of temperature during the diapause-induction period on subsequent host-free survival at 23.9°C. Occurrence of the diapause characters generally increased with weevil age at all temperatures but appeared more rapidly at higher temperatures. Acquisition of the diapause characters tended to occur slightly earlier in female weevils compared with the male weevils. Despite the slower development of diapause characters at lower temperatures, when adult weevils were fed under low temperatures, subsequent host-free survival was enhanced. These results are consistent with reports of increased weevil survival with delayed entry into overwintering. Our findings also suggest that the potential host-free survival facilitated by diapause occurring in subtropical or tropical production regions may be reduced compared with dormancy developing in southern temperate regions. This reduced survival potential emphasizes the importance of a maximized host-free season and suggests that the late-season diapause spray intervals should be short enough to ensure that the number of dormant weevils developing in late-season cotton is minimized.

Relative collection efficiency of the keep-it-simple-sampler for cotton fleahoppers (Hemiptera: Miridae) in cotton

Reassessment of action thresholds to initiate treatment of the cotton fleahopper, Pseudatomoscelis seriatus (Reuter), has recently been identified as a research priority by extension and research entomologists in Texas. Imperative to the success of this mission is the ability to accurately and efficiently estimate fleahopper population levels in cotton, Gossypium hirsutum L. The standard method for quantifying fleahopper abundance involves direct counts of adults and/or nymphs on plants (wholeplant examination method). This sampling method, however, typically requires a considerable level of experience and keen vision to obtain accurate counts. Consequently, there is a need among researchers to identify or develop alternative sampling methods for fleahoppers in cotton. Beerwinkle et al. (1997, pp. 1330-1333, In Proc., Beltwide Cotton Conf.) introduced a hand-held pneumatic device, the Keep-It-SimpleSampler (KISS), as a portable tool that could be used to efficiently sample arthropod populations in various row crops. However, the potential use of the KISS as a tool for sampling cotton fleahoppers in cotton has not been examined. Presented herein are estimates of the collection efficiency of the KISS for fleahopper adults and nymphs in cotton relative to the whole-plant examination method, and some observed advantages and disadvantages of using the KISS. Commercial cotton fields located in the Brazos River Bottom production area of Texas were sampled for cotton fleahoppers in 2005 (4 fields) and 2006 (5 fields) using both the KISS and whole-plant examination method. Fields were sampled 3 times weekly during the initial 3 wks of squaring except when rain or insecticide applications prevented sampling. All fields were sampled between 0830 and 1400 h (CDT) and in the same order on each sampling date to minimize potential time-of-day sampling effects. Additionally, node counts and plant height measurements were taken twice weekly on 50 plants in each field to provide supporting information. Within each field, sampling was confined to a 0.4-ha area partitioned into 25

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.

Morphology, Diet, and Temperature-Dependent Host-Free Survival of the Boll Weevil, Anthonomus grandis (Coleoptera: Curculionidae)

Abstract The boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), is an important pest of cotton (Gossypium spp. (Malvales: Malvaceae)) in South America, Mexico, and southernmost Texas in the United States. A key factor in the persistence of the boll weevil is its ability to survive the noncotton season. Mechanisms facilitating this survival in subtropical and tropical areas are incompletely known, and our understanding has been further complicated by recent reports of overwintering on noncotton hosts. In addition, the nature and even existence of the adult dormancy, and validity of the criteria used to distinguish it, have been questioned. We manipulated the boll weevil diet to produce a range of diapause responses and observed the corresponding host-free survival patterns. The estimated proportion of diapause was associated with subsequent survival. In addition, different diets producing similar incidences of diapause resulted in substantially different host-free longevity, and the longest survival was observed for weevils fed bolls as adults. Survival patterns exhibited a negative temperature dependence except for the lowest observed temperature (12.8°C), where survival was reduced compared with that at 18.3°C. This suggests chronic chilling injury in response to extended exposure to low, above freezing temperatures. All but the highest temperature (29.4°C) facilitated survival >6 mo, and survival of >1 yr was observed at 18.3°C. The temperatures permitting the greatest survival are typical for many subtropical and tropical regions during the noncotton season, indicating that diapausing boll weevils are capable of surviving the noncotton season in these regions without the benefit of presumptive noncotton hosts.

Diapause Response of the Boll Weevil (Coleoptera: Curculionidae) to Feeding Period Duration and Cotton Square Size

Abstract Distribution of the boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), in the United States has been greatly reduced by eradication efforts. Still, it remains a key pest of cotton (Gossypium spp., [Malvales: Malvaceae]) in the New World, and has proven difficult to eliminate from Mexico and from southern Texas. In those regions, improved knowledge of boll weevil overwintering ecology may benefit efforts by eradication and management programs. Adult diapause in the boll weevil is well documented, but influences of the feeding period duration between adult eclosion and assessment of diapause remain unstudied. We examined diapause incidence and associated survival for weevils fed for 7, 14, or 21 d after adult eclosion. Diapause incidence of females was less influenced by feeding duration compared with males. For males, highest diapause incidence occurred after 14 d of feeding compared with 7 or 21 d. Host-free survival tended to be higher after 14 d of feeding compared with 7 or 21 d, although many weevils were long-lived (≥80 d) after each feeding period duration. Males exhibited higher survival compared with females, and survival was higher for weevils fed large flower buds (squares) compared with smaller squares. Survival was most influenced by temperature; longevity increased with decreasing temperature except at the lowest temperature (12.8°C). These results suggest an optimal feeding period for induction of diapause and maximized host-free longevity. These findings may permit improved timing of late-season insecticide treatments aimed at reducing overwintering populations, and thereby improve effectiveness of eradication and management programs.