Coccinellids Associated with the Cotton Aphid (Homoptera: Aphididae) in Northeast Arkansas Cotton' Hugh E. Conway 2 and Timothy J. Kring3 USDA-APHIS-PPQ-CPHST-PDDML Pest Detection, Diagnostic & Management Laboratory, Edinburg, Texas 78541 USA J. Entomol. Sci. 45(2): 129-139 (April 2010) Abstract Adult and larval coccinellids are important predators of the cotton aphid, Aphis gos- sypii Glover, in cotton, Gossypium hirsutum L. Adult and larval Coccinellinae and Scymninae were sampled by beat-pan method and identified to species in a 3-year field study (1999-2001) conducted in northeast Arkansas. The coccinellids observed in descending order of abundance were Hippodarnia convergens Guerin, Scymninae (Scymnus spp. and Diomus spp.), Coleome- gil/a macu/ata (Degeer), Harmonia axyridis (Pallas), and Coccinella septempunctata L. Popula- tion dynamics and community structure by species for C. maculata, H. axyridis and C. septempunctata were unpredictable within and among years. Based on population densities, timing of colonization, and consistent delayed density-dependent relationship with aphid popula- tions, H. convergens and Scymninae genera (Scymnus spp. and Diomus spp.) were the most important predators of cotton aphids in northeast Arkansas. Key Words coccinellid, Coccinellinae, Scymninae, cotton aphid, Aphis gossypii The diverse beneficial insect fauna found in cotton, Gossypium hirsutum L., is well described (Whitcomb and Bell 1964, van den Bosch and Hagen 1966, Wells et al. 2001). The natural enemies commonly credited with controlling the cotton aphid, Aphis gossypll Glover, are the predaceous arthropods including the coccinellids, chrysopids, and syrphids, the parasitoids (braconids), and the entomogenous fungus, Neozygites fresenll (Nowakowski) Batko (Chambers 1986, Frazer 1988, Rosenheim and Cisneros 1994, Steinkraus et al. 1991). Coccinellid predators are commonly cred- ited in providing cotton aphid regulation in several locations (Hodek and Honek 1996, Kidd and Rummel 1997, Wells et al. 2001). Adult and larval Hippodamia con vergens Guerin, Coleomegiia maculata (Degeer), Coccinella septempunctata L., Harmonia axyridis (Pallas), Cycloneda munda (Say), Scymnus spp., and Diomus spp. are com- monly collected from southern cotton fields (Knutson and Ruberson 1996). Sampling techniques that provide accurate estimates of predator densities are critical for evaluation studies on natural enemies and must provide precise density estimates while minimizing costs (Frazer 1988, Obrycki and Kring 1998). The drop cloth is one of the most efficient methods for sampling coccinellids (Wade et al. 2006), although in warm weather the adults quickly fly away after falling on the cloth, thus making identification difficult (Michels and Behle 1992, Hagerty et al. 2002). The beat-pan 'Received 24 March 2009; accepted for publication 17 July 2009. 2Address inquiries (email: hugh.e.conway@ aphis.usda.gov). 3Department of Entomology, University of Arkansas, Fayetteville, AR 72701. 129 130 J. Entomol. Sd. Vol. 45, No. 2 (2010) method is another effective sampling method that allows additional time for species identification because adult natural enemies often land on the screen cover before flying away. This method is a modification to the "thrips-catcher" (Newsom et al. 1953) used by research entomologists to sample insects in cotton. Insects are dislodged by striking the plants onto a wire mesh covering a light-colored pan (Elkassabany 1994). The annual development of insect communities within agricultural crops is influ- enced by the species pool composition of potential colonists and by habitat develop- ment within the field (Liss et al. 1986). For coccinellids, the species pool of potential colonists is influenced by several factors, including availability of adequate overwinter- ing habitat, the population densities of coccinellids entering diapause, and the extent of overwintering mortality (Honek 1986). Predation of aphids by these coccinellids is potentially important in maintaining cotton aphid densities below the treatment levels during early, stages of cotton development (Kerns and Gaylor 1993, Knutson and Ruberson 1996, Wells et al. 2001, Conway et al. 2006). Our goal in this study was to characterize the development of coccinellid communi- ties in cotton both within and between the growing seasons. The objectives were to identify trends in the seasonal patterns of population density and occurrence of coc- cinellid adults and larvae inhabiting cotton fields, to establish the predictability of de- velopment of the coccinellid communities, and to quantify the relative abundance of individual species. Materials and Methods A 3-yr (1999 - 2001) field study was conducted at the University of Arkansas Delta Research Station at Clarkedale, AR. Sixteen plots, each —0.3 ha, were planted with Gossypium hirsutum L. (NuCott 3313, Delta and Pine Land Company, Scott, MS 38772) on 4 May 1999, 2 May 2000, and 28 April 2001 in —1.0-rn wide rows. Cotton was grown under standard cultural practices including preplant herbicides, in-furrow aldi- carb application (targeting thrips and nematodes), fertilization as required, cultivation 3 . 4 times yearly, insecticide applications after reaching plant bug or boll weevil thresholds, in-furrow irrigation, monthly manual removal of in-field weeds, and bi- weekly mowing of field edges. Data were obtained from a cotton aphid threshold experiment with 4 replicates ap- plied to cotton fields using 3 treatments: (1) untreated, (2) conventional treatment threshold, and "(3) experimental treatment threshold (Conway et al. 2006). Conven- tional plots reached aphid treatment thresholds and were treated by applications of 0.22 L / ha of imidacloprid on 18 and 28 June 1999, 28 June and 3 July 2000, and 7 and 12 July 001. Experimental threshold plots reached aphid treatment thresholds and were treated with 0.22 L / ha of imidacloprid on 28 June 1999, 3 July 2000, and 19 July 2001. In 2001, there were treatments of malathion for the boll weevil eradica - tion program applied to all test plots in Clarkedale, AR, on Sand 15 June, and on 3, 11, 18, and 24 July. Natural enemies were sampled 2x per wk using the beat-pan method whereby plants were struck onto a hardware mesh (4 cells / cm 2) covering a white, plastic wash basin (35.5 X 28.5 X 13.5 cm). Population density levels of beneficial insects were estimated by collecting 8, 1 row-rn samples per plot with 128 samples (64 from un- treated control, 64 from the treated plots) taken on each sample date. In 2000 and 2001, larval coccinellids were collected from the beat-pan samples for later identification CONWAY and KRING: Coccinellids in Arkansas Cotton 131 and plaäed into vials of ethanol marked with date, plot number and treatment. Sam- pling began each year in early June and continued until late July in 1999 and 2000, and early August in 2001. Total adult and larval coccinellids were enumerated each year, and Coccinellinae were identified to species in 2000 and 2001 (Gordon and Vandenberg 1991, Chapin and Brou 1991, Rees et al. 1994). Adult Scymninae were grouped together in counts, and larval Scymninae were identified as either Scymnus spp. or Diomus spp. Principal components analysis was conducted on the mean yearly values for cot- ton aphids, adult and larval Scymninae, and adult and larval Coccinellinae from each of the 16 plots each year (48 total per factor) over the 3-yr period of the study (SAS Institute 2006). Multivariate analysis was conducted for Hippodamia convergens Guerin, Scymninae (Scymnus spp. and Diomus spp.), Coleomegilla maculata (Deg- eer), Harmonia axyridis (Pallas), and Coccinellinae larvae and adults per plot (16 plots) over years (2 years) with Kendall's tau coefficients of association for pairwise combinations (SAS Institute 2006). Results and Discussion A total of 3634, 3562, and 1644 beneficial insects were collected from the cotton fields in 1999, 2000, and 2001, respectively. The dominant predators of cotton aphids from these samples were coccinellid adults and larvae, which together composed 79%, 68%, and53% of the captured beneficial insects in 1999, 2000, and 2001, respectively. Coccinellids also were the most abundant arthropod natural enemies identified from cotton in Georgia (Abney et al. 2008) and Mississippi (Weathersbee and Hardee 1994). The coccinellid species observed in Arkansas during the 3-yr study (in descending order of abundance) were: H. convergens, Scyrnninae (Scymnus spp. and Diomus spp.), C. maculata, H. axyridis, and C. septempunctata L. (Table 1). Wells et al. (2001) reported a similar composition of coccinellid species in Georgia cotton during a 2-yr (1997 - 1998) field study, with H. convergens and Scymninae (Scymnus spp. and Dio- mus spp.) being the most abundant coccinellids collected. Hippodamia convergens and C. maculata were the most abundant coccinellids found in a study in Oklahoma cotton (French et al. 2001). In our study, there were 3 - 4 times more adult and 9 times more larval H. convergens than any other coccinellid species in 2000 and 2001 (Table 1). An overall decrease in abundance of adult and larval Scymninae and of larval Coccinel- mae occurred over the 3-yr study period, likely due to the yearly decrease in aphid densities (Fig. 1). The number of Coccinellinae adults fluctuated less predictably among years with 2000 having a higher abundance of adults than either 1999 or 2001 (Table 1). Adult and larval densities of each species fluctuated widely within the- field and among the years. In 1999, beneficial insect sampling was conducted for 6 wks from 17 June to 19 July. During that time, Coccinellinae densities increased over time peak- ing the first full week of July at (mean ± SE, 1.01 ± 0.12) adults per row-m. Adult Scyrnninae densities increased through time, peaking at (1.30 ± 0.17) adults per row-rn during the second week of July (Fig. 2). Similarly, Coccinellinae and Scymninae larval densities increased over time peaking on the first full week of July at (3.45 ± 0.41) and (1.05 ± 0.19) larvae per row-m, respectively, before densities declined after an epizootic of N. fresen/iincotton aphids (Fig. 3). 132 J.