Shoil M. Greenberg

Interactions Among Two Species of Eretmocerus (Hymenoptera: Aphelinidae), Two Species of Whiteflies (Homoptera: Aleyrodidae), and Tomato

Abstract Laboratory experiments were conducted to determine the influence of two tomato (Lycopersicum esculentum Miller) varieties (‘Trust’ and ‘Floridade’) on the biology of two whitefly species, Bemisia argentifolii Bellows & Perring and Trialeurodes vaporariorum (Westwood), and the interactions of host plant and whiteflies on the biology and parasitization of two parasitoid species, Eretmocerus eremicus Rose & Zolnerowich (native) and Eretmocerus mundus Mercet (exotic). Natural mortality, developmental time, and fecundity of B. argentifolii were not significantly different from those of T. vaporariorum on either tomato variety. The two species of Eretmocerus responded differently to the whitefly hosts. Eretmocerus mundus developed significantly faster, produced more progeny, and had greater parasitism and rate of emergence in B. argentifolii than in T. vaporariorum. Eretmocerus eremicus performed similarly on both whitefly species except that its females deposited more eggs in B. argentifolii than in T. vaporariorum nymphs. Females of both parasitoid species emerging from T. vaporariorum were significantly larger than those emerged from B. argentifolii. Tomato variety had no significant effect on the two parasitoid species. Eretmocerus eremicus attack both whiteflies efficiently and it can be used as a single species for whitefly management.

Beet Armyworm (Lepidoptera: Noctuidae) Host Plant Preferences for Oviposition

Abstract Beet armyworm, Spodoptera exigua (Hübner), oviposition preferences were determined on five host plants: cabbage (Brassica oleracea capitata L.), cotton (Gossypium hirsutum L.), bell pepper (Capsicum annuum L.), pigweed (Amaranthus retroflexus L.), and sunflower (Helianthus annuus L.) in no-choice, two-choice, and five-choice tests. Tests were conducted in the laboratory, greenhouse, and field cages. Oviposition preferences were compared on the basis of two measurements, the proportion of eggs laid on the plants to total that were deposited, and the oviposition preference index defined as [(number of eggs laid on the plant) - (number of eggs laid on the cage)] × 100/total number of eggs laid. The proportion of eggs laid on the plants to total that were deposited was highest for pigweed and lowest for cabbage in all tests. Beet armyworm females were significantly deterred from laying eggs on cabbage and sunflower, while pigweed and cotton elicited a positive oviposition preference. Pepper tended to be neutral or slightly unattractive. Apparent interactions among plant species in choice tests produced measurable shifts in oviposition preference. Most notably, female response to pepper was enhanced in the presence of cotton or pigweed. Egg masses laid on the plants contained significantly higher numbers of eggs than those laid on the surface of the cage, except in the case of cabbage leaves. Knowledge of hierarchies of host plant oviposition preference by beet armyworm females will be useful in understanding the population dynamics of this important agricultural pest, and for developing effective monitoring and management strategies.

Temperature-Dependent Life History of Eretmocerus eremicus (Hymenoptera: Aphelinidae) on Two Whitefly Hosts (Homoptera: Aleyrodidae)

Abstract The effects of temperature on insect life history were studied for two whitefly hosts (Homoptera: Aleyrodidae), the silverleaf whitefly, Bemisia argentifolii Bellows & Perring, and the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), as well as the parasitoid, Eretmocerus eremicus Rose & Zolnerowich (Hymenoptera: Aphelinidae) attacking both hosts. Mean egg numbers as a function of time were fitted to models for age-specific oviposition for each whitefly. For B. argentifolii, numbers of eggs laid increased with time at 15, 21, and 24°C. At 28 and 32°C, the curve declined after 6 d, although the model fit was poor. The model did not fit the oviposition data at 32°C. Maximal oviposition rate occurred at 24°C (12 eggs per 48-h period), and the model was almost linear. For T. vaporariorum, the model closely fit mean eggs laid, with highest rates of ≈12 eggs per 48 h at 21 and 24°C. Numbers of whitefly eggs as a function of time and temperature were described by a three-dimensional surface model that was also used to estimate temperature thresholds for oviposition (12.5°C for B. argentifolii and 10.9°C for T. vaporariorum). Increasing temperatures produced decreased preoviposition periods in B. argentifolii, whereas temperature extremes resulted in longer periods for T. vaporariorum. Development times from egg to adult, percentage mortality, and estimated degree-days for development were measured at 15, 21, 24, 28, and 32°C for both whiteflies, and for E. eremicus reared on both hosts. Development rate was higher for B. argentifolii than T. vaporariorum at 24 and 28°C. Development of E. eremicus was faster using B. argentifolii as hosts than T. vaporariorum at 24, 28, and 32°C. By extrapolation of development rates, lower developmental thresholds (°C) were estimated as follows: T. vaporariorum, 2.92; B. argentifolii, 10.32; E. eremicus on T. vaporariorum, 5.44; and E. eremicus on B. argentifolii, 8.7. Mean degree-day requirements for egg to adult development were calculated for T. vaporariorum, 483.4; B. argentifolii, 319.7; E. eremicus on T. vaporariorum, 417.3; and, E. eremicus on B. argentifolii, 314.4. Percentage mortality also was significantly affected by temperature in both species of whitefly. For T. vaporariorum, higher temperatures caused higher levels of mortality, with almost 98% killed at 32°C. The reverse occurred in B. argentifolii, where highest levels of mortality were found at the lowest temperatures. Mortality patterns in E. eremicus reflected those of the host: increasing with temperature on T. vaporariorum, while decreasing on B. argentifolii. The life history of E. eremicus was profoundly affected by that of its host.

Comparison of functional response and mutual interference between two aphelinid parasitoids of Bemisia argentifolii (Homoptera: Aleyrodidae)

Functional responses and mutual interference were compared in an indigenous parasitoid, Encarsia pergandiella Howard (Hymenoptera: Aphelinidae), with that of an exotic parasitoid, Eretmocerus mundus Mercet (Aphelinidae) from Spain, attacking the silverleaf whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae). Type II functional response curves were fitted to the data and were used to calculate handling time. Eretmocerus mundus attacked more whitefly nymphs than E. pergandiella. Handling times estimated from the functional responses were 72 min for E. pergandiella and 12 min for E. mundus, suggesting that lower attack rates for the former parasitoid may be attributed to longer handling times. The statistically estimated handling time for E. mundus was compared with an estimate derived from empirical observations of parasitoid behavior. Actual observations of handling time, defined as oviposition, host feeding and associated preening, yielded a mean handling time of <2 min, suggesting...

Influence of Different Cotton Fruit Sizes on Boll Weevil (Coleoptera: Curculionidae) Oviposition and Survival to Adulthood

Abstract Understanding the critical host plant factors that determine oviposition behavior and survival of boll weevil, Anthonomus grandis grandis Boheman, on cotton, Gossypium hirsutum L., is important for developing successful pest management strategies. However, published information is both conflicting and limited regarding how different cotton fruit sizes affect boll weevil oviposition choices and subsequent larval survival to adulthood. Consequently, we used a standard based on fruit size diameter to evaluate boll weevil feeding and oviposition punctures, and survival to adulthood on 10 different cotton fruit sizes: squares of diameter 1.5–2.0 (pinhead), 3.0–3.5 (matchhead), 5–6, 7–8, or 9–10 mm; candle; and bolls of diameter 10–15, 15–20, 20–30, or >30 mm. Oviposition and feeding punctures were significantly affected by cotton fruit size. Females did not oviposit in pinhead squares. The fewest eggs were oviposited in boll sizes >30 mm. The highest number of eggs was recorded in square sizes of 5–6 and 7–8 mm. Boll weevil survival to adulthood was highest on square sizes of 7–8 or 9–10 mm (58.6–59.7%). No survival occurred in matchhead squares or bolls >30 mm. Duration of development was longest on boll sizes of 15–20 and 20–30 mm (18.2–18.8 d). The growth index (percentage immature survival divided by immature developmental time) of female boll weevils was 2.8-fold higher in 7–8- or 9–10-mm diameter squares than in 20–30-mm diameter bolls. This study will improve our capacity to develop methods to predict fruit losses and changes in boll weevil populations in the field, given a starting density of fruit suitable for oviposition, and a corresponding initial population density of weevils.

Effect of Age of Transgenic Cotton on Mortality of Lepidopteran Larvae

Abstract. Leaves from cotton, Gossypium hirsutum L., plants containing transgenic traits (Bollgard®, Bollgard II®, and WideStrike™) were assayed for bioactivity against bollworm, Helicoverpa zea (Boddie); beet armyworm, Spodoptera exigua (Hübner); fall armyworm, Spodoptera frugiperda (J. E. Smith); and cabbage looper, Trichoplusia ni (Hübner); in a laboratory. Fifty leaves from the middle of the plants were collected every 20 days, starting at 40 days and until 120 days after cottons were planted. The leaves were fed to 15 first-instar larvae of each insect pest until they died or pupated. Most larvae died when fed Bollgard II or WideStrike. At the end of the season (100–120 days after planting), more larvae died when fed Bollgard II than WideStrike. Conclusions from the results are that (1) mortality was not significantly different when the larvae were fed dual Bt cotton leaves of different ages; (2) survival duration depended on consumption of the amount of endotoxin that caused larval mortality; (3) biological characteristics of surviving lepidopterans (pupal weight, emergence, and developmental time) were significantly better on non-Bt cotton.

COTTON POLLEN RETENTION IN BOLL WEEVILS: A LABORATORY EXPERIMENT

Abstract Cotton pollen is thought to exist in the gut of boll weevils for at least 24 hours. Because finding whole or broken cotton pollen grains are valuable clues in determining when boll weevils were living in cotton, the retention of cotton pollen in the gut of the weevil was examined. Boll weevils were fed cotton buds, cotton flowers, and water for five days, then they were placed into cleaned cages without any food. However they were allowed to drink water freely. Weevils were examined at 0, 24, 48, 72, 96, and 120 hours after the buds and flowers were removed. One hundred weevils were dissected at each time interval, and each gut was placed onto a glass slide. Light microscopy was used to determine the presence or absence of cotton pollen. More weevils (63%) at 0 hours contained pollen than weevils at any other interval. Whole pollen grains were not found in weevils after 24 hours. The 120 hour interval contained a single weevil with pollen. In the laboratory test, whole cotton pollen grains indicated that feeding on cotton occurred within 24 hours. When only halves and fragments of pollen were found, cotton feeding occurred 48–120 hours earlier. Additionally, when six or more cotton pollen grains were found in a weevil, it had fed on cotton within 24 hours. This information narrows the boll weevil dispersal times and distances from cotton fields and overwintering areas.

ALMOND, MELON, AND PIGWEED POLLEN RETENTION IN THE BOLL WEEVIL (COLEOPTERA; CURCULIONIDAE)

Abstract The boll weevil (Anthonomus grandis Boheman) remains a devastating insect pest on cotton (Gossypium hirsutum C. Linnaeus), particularly where it has not been eradicated. Identifying and understanding the survival of overwintered boll weevils, when cotton is not available, is important in designing mitigation programs and controlling this insect pest, especially in areas where boll weevils are active all year. Many non-malvaceous taxa have been listed as possible overwintering adult food sources. The purpose of this research was to determine if pollen from three non-malvaceous taxa could be used as food sources for overwintering boll weevils in the Lower Rio Grande Valley of Texas, where weevils are active year-round and during cotton-free periods. The taxa are almond [Prunus dulcis (Mill.) D.A. Webb], melon (Cucumis melo C. Linnaeus subsp. melo), and pigweed (Amaranthus sp.), Furthermore, it was intended to determine the retention of these pollen types in the boll weevil gut. More weevils (84%) contained almond pollen when they had additional water than those (30%) that did not have additional water. Almond pollen was found in 100% of the examined weevils after 72 hours, and in 80% after 96 hours. Pigweed and melon pollen were never found after 48 hours. Overall, almond pollen was found in more weevils (96%) than pigweed (40%), or melon (12%). In addition, a greater number of almond pollen grains (539) were found than pigweed (41), or melon (21). This shows that these types of pollen were consumed, and could play a part in the survival of overwintering weevils. However, more research is needed to examine the nutritional value of these, and other, pollen grains for overwintering boll weevil survival, and to determine the length of time weevils can survive feeding on these pollen types.

Bemisia tabaci (Homoptera: Aleyrodidae) instar effects on rate of parasitism by Eretmocerus mundus and Encarsia pergandiella (Hymenoptera: Aphelinidae)

Studies were conducted to compare preference among Bemisia tabaci Gennadius, biotype B instars for parasitization by Eretmocerus mundus Mercet and Encarsia pergandiella Howard when provided one instar only, two different instars, and four different instars simultaneously. In the single‐instar no choice treatment, Er. mundus was more successful in parasitizing the younger host instars, while En. pergandiella parasitized a greater proportion of the older instars. Similar results were observed when parasitoids were provided a choice of two instars in six different pair combinations. When all four instars were provided simultaneously, the numbers of first, second, and third instars parasitized by Er. mundus were not significantly different from each other (range 10.3–16.4%), but all were significantly higher than parasitism of fourth instar nymphs (2.1%). The highest percentage parasitization by En. pergandiella was in third instar (17.2%), and the lowest in first instar (2.8%).

Effects of burial and soil condition on postharvest mortality of boll Weevils (Coleoptera: Curculionidae) in fallen cotton fruit.

Abstract Effects of soil condition and burial on boll weevil, Anthonomus grandis grandis Boheman, mortality in fallen cotton, Gossypium hirsutum L., fruit were assessed in this study. During hot weather immediately after summer harvest operations in the Lower Rio Grande Valley of Texas, burial of infested fruit in conventionally tilled field plots permitted significantly greater survival of weevils than in no-tillage plots. Burial of infested squares protected developing weevils from heat and desiccation that cause high mortality on the soil surface during and after harvest in midsummer and late summer. A laboratory assay showed that burial of infested squares resulted in significantly greater weevil mortality in wet than in dry sandy or clay soils. Significantly fewer weevils rose to the soil surface after burial of infested bolls during winter compared with bolls set on the soil surface, a likely result of wetting by winter rainfall. A combination of leaving infested fruit exposed to heat before the onset of cooler winter temperatures and burial by tillage when temperatures begin to cool might be an important tactic for reducing populations of boll weevils that overwinter in cotton fields.