A Comparison of Rearing Creontiades signatus Distant on Green Bean Pods or Pea Plants

For the last 15 yrs, a green mirid, Creontiades signatus Distant, has been reported to infest cotton, Gossypium hirsutum L., grown in the Lower Rio Grande Valley and the Coastal Bend regions of south Texas causing injury to developing lint and seed inside cotton bolls (Armstrong et al. 2007, Proc. Beltwide Cotton Conf., pp. 335 - 37). This mirid is taxonomically related and morphologically similar to an important plant bug pest of cotton in Australia, Creontiades dilutus (Stål), (Kahn 1997, Int. J. Pest Manage. 43: 197 - 202; Ward 2005, Australian J. Entomol. 44: 310 - 315). However, recent molecular and taxonomic work (Coleman 2008, Southwest Entomol. 33 - 111 - 117) identified C. signatus as being native up to several miles inland from the Gulf Coast of the U.S. and Mexico. The known host plants have not been published, but Armstrong et al. (2009, Ann. Entomol. Soc. Am. 102: 196 - 200) reported that C. signatus oviposited and survived on cotton and preferred the normal-leaf genotypes compared with the okra-leaf types. Although survey numbers of this pest are not provided here, C. signatus is increasing in densities and pest status in cotton in part because soybeans are a viable host (JSA unpublished data) and the soybean acreage has increased significantly in South Texas over the last 5 yrs.

Our laboratory has been rearing C. signatus since January 2005 to determine its susceptibility to insecticides, conduct seasonal reproductive studies, and characterize the injury/yield relationships to cotton. In an effort to increase the numbers of specimens available for experimentation, we routinely rear and maintain C. signatus on green bean, Phaseolus vulgaris L., pods as well as whole cowpea, Vigna unguiculata L., plants. In the absence of knowledge or use of an artificial diet, these 2 in vivo methods of rearing have been successful in supplying the number of bugs needed, but both require time, labor and resources. The study reported herein was designed to determine the most consistent and efficient method for rearing C. signatus based on the number of eggs, nymphs and adults produced on green bean pods versus pea plants. The green bean “pod” method of rearing C. signatus in this study followed that of Wilson (1973, J. Econ. Entomol. 156: 810 - 811) who reared Lygus hesperus Knight on green beans pods in ice cream cartoons. This method allowed for less disturbance of the females during routine changing of the food and oviposition substrate. The current study for pod rearing used 2-L ice cream cartons with 12 openings (1.5 cm²) cut in side walls of the carton (Fig. 1).The openings were covered with a 2-cm² piece of organdy cloth hot-glued to the inside of the container. Small slits were cut in the organdy cloth that allowed a green bean pod to be stuck through the opening to serve as both a food and an oviposition substrate. The second method of rearing used whole cowpea plants grown in potting soil (Sunshine Mix, Bellevue, WA) in 2-L plastic pots (Dellin Products, Middlefield, OH) in a greenhouse maintained at 29.0 C ± 5.0. Plants in the pots (Fig. 2) were thinned to 3 plants at 10 d of age (20 - 25 cm in height) and used as an oviposition and food source just as the green bean pod cages. The bugs were enclosed on the plant for the 48 h oviposition period by using a rolled sheet of cellulose acetate to fit inside the pots with ventilation provided by muslin cloth at the top (Fig. 1). Once a week, for 12 consecutive wks, 3 replications of the bean pod containers (12 pods in a container) and pea plants (3 plants per pot) were infested with 20, 7-d-old female C. signatus from the laboratory colony, and 15 7-d-old males. The males and females were reared together from the time of adulthood, and the females were assumed to be mated. After the first 24 h of the 48 oviposition period, any dead females or males were replaced. Following the 48 h oviposition period, the bugs were removed, and the green bean pods and pea plants were placed in ventilated, 6-LTupperware® (Tupperware Corp., Orlando, FL) containers in a walk-in environmental chamber maintained at 28.5°C, 65% RH and on a 14:10 (L:D) h photophase. Fresh green bean pods and whole corn, Zea maize (L.), were provided to the rearing containers every 3 d. The roots of the pea plants were gently washed and placed in 25-ml aqua-pick tubes with MaxiGro hydroponic solution (General Hydroponics, Sebastopol, CA) with parafilm covering the juncture of the stem and opening of the tube. Five days after the insects were removed, the number of viable (maturing) eggs were identified and counted on the pods and plants by placing a red mark near the operculum with a red fine-point pen (Sharpie Permanent Marker, Sanford Corp., Oak Brook, IL). Creantiades signatus eggs are inserted into the plant with the operculum nearly level with the oviposition surface. The operculum turns dark black in 4 - 5 d when a viable egg matures (Coleman et al. 2008). After 9 d the number of live nymphs was counted, and after 16-d the number of live adults was counted. The total number of eggs, nymphs and adults from the 3 replications of bean pod cages (12 pods per cage) and whole pea plants (3 plants per pot) were square-root transformed to stabilize the variance, then subjected to a one-way ANOVA using PROC GLM (SAS Institute 2002, Cary, NC).The overall model included the fixed effects of rearing substrate, life-stage (egg, nymph and adult) and week (1 - 12) of rearing. The means for the different life-stages reared over the 12 wks of study were tested (α = 0.05) using Student-New-man-Keules test (SAS Institute 2002). Untransformed data are presented. The models for the rearing substrate (F = 23.58; df = 1, 215; P < 0.001), life-stage (F= 85.92; df = 2, 115; P > 0.001) and week of rearing (F = 31.07; df = 11, 115; P > 0.0001) were all significant effects. The number of viable eggs oviposited on the green bean pods were significantly higher than for the pea plants, whereas the number of successful nymphs and adults was not significantly different (Fig. 2). From the 12 wks of study, 37% (647) of the total number of eggs was oviposited in the pea plants as opposed to 63% (1063) of the total number of eggs being laid on the bean pods. However, the number of nymphs and adults that successfully developed on the pea plants and green bean pods were comparable. The bean pods shrunk considerably as they were held for egg hatch. This reduced the quality of the pods as an oviposition substrate, resulting in a decrease in the number of eggs that matured, eclosed, and developed to first-instar nymphs. The humidity in the walk-in growth chamber was maintained at 65%. Past efforts to increase the humidity have increased fungal growth that rapidly degrades the host quality and decreases the survival of the insects. Other means of increasing the efficiency of rearing C. dilutus might be found by discovering a more suitable host or by decreasing the shrinkage and host quality of bean pods by exploring ways to keep them from desiccating without increasing the humidity. Other improvements, such as the development of artificial diet, could be explored for rearing C. dilutus. The current methods described herein are the first laboratory procedures reported for rearing this relatively new pest of cotton. Future increases in the efficiency and quality of insects produced by mass rearing efforts will help contribute to conducting studies on the biology and damage of this relatively new pest of cotton.

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