In the southeastern US, the population dynamics of stink bugs (Heteroptera: Pentatomidae) have changed over the past several years; a clear indication of this phenomenon is the emergence of stink bugs as major pests of cotton, Gossypium hirsutum L. (Greene and Turnipseed 1996). Grain sorghum [Sorghum bicolor (L.) Moench spp. bicolor] has been reported to be an important host plant for stink bugs, including the southern green stink bug, Nezara viridula (L.), the brown stink bug, Euschistus servus (Say), and the rice stink bug, Oebalus pugnax pugnax (F.), and the leaffooted bug, Leptoglossus phyllopus (L.) (Hemiptera: Coreidae), in Georgia (Wiseman and McMillian 1971, McMillian and Wiseman 1972). Because stink bug distribution and abundance are closely linked to crop phenology and seasonal succession of host plants (Velasco and Walter 1992), changes in stink bug populations likely have occurred in other crops, including grain sorghum, in southeastern farmscapes. Consequently, the objective of this 4-yr study was to determine species composition and abundance of stink bugs and a leaffooted bug and their predators in sorghum in Georgia.

Materials and Methods

Sorghum. A 4-row strip (3.7-m-wide) of sorghum was established along the length (ranging from 230 - 480 m) of an edge of commercial cotton fields in Irwin Co., GA. DeKalb E57, a semiopen panicle variety of sorghum, was planted at a rate of 25,500 seeds/ha. Sorghum was planted in 5 fields on 5 May 2004, 3 fields on 19 May 2005, 3 fields on 14 April 2006, and one field on 10 May 2007. Sorghum panicle development was based on descriptions in the University of Arkansas production manual for sorghum (University of Arkansas 2006); after heading and flowering, sorghum seed development begins and progresses through the developmental stages of milk, soft dough, hard dough, and physiological maturity.

Insect sampling. Sorghum was examined for stink bugs, the leaffooted bug, and their natural enemies on a weekly basis. For each sorghum sample, the aerial parts of all plants within a 1.83-m length of row were visually checked thoroughly for these insects. Stink bug species, developmental stage (N viridula, O. p. pugnax, and E. servus), and sex [N. viridula, E. servus, Euschistus quadrator (Rolston), and Chinavia hilaris (Say) adults] were identified and recorded. In 2004, 5 random samples were taken from rows 1 and 2, and 4 random samples were taken from rows 3 and 4. For the remaining years, 9 random samples were obtained from each of the 4 rows. Voucher specimens are stored in the USDA-ARS, Crop Protection & Management Research Laboratory in Tifton, GA.

Statistical Analysis. Stink bug, leaffooted bug, and predator density data were analyzed using PROC MIXED (SAS Institute 2008). The fixed effect was species, and random effects were field within year and residual error. Least squares means were separated by least significant difference (LSD) (SAS Institute 2008) where appropriate. Density of bugs and their natural enemies were graphed over sorghum panicle development because stink bugs prefer to feed on plant “fruits/seeds” (Patel et al. 2006, Mizell et al. 2008). Means were obtained for number of N. viridula, O. p. pugnax, and E. servus nymphs and adults and Orius insidiosus (Say), Geocoris spp., spiders, lady beetles, and Solenopsis invicta Buren per sample over sorghum panicle development using PROC MEANS (SAS Institute 2008).

Results and Discussion

Panicle-feeding bug species. Nine species of phytophagous stink bugs and one leaffooted bug species were present in sorghum over the study (Table 1). Generally, bug adults were found in sorghum panicles primarily feeding on developing seed and sometimes on stems, branches, and glumes of sorghum panicles, as observed by Hall and Teetes (1982), but they also could be detected resting or mating in foliage. Only adults of Euschistus tristigmus (Say) and Euschistus ictericus (L.) inhabited sorghum. For the remaining bug species, sorghum served as a reproductive host plant. Adult female stink bugs generally laid eggs on leaves, but sometimes in panicles. Very young nymphs fed on leaves, small stems, or very young seed whereas older nymphs exhibited feeding habits similar to the adults. These bug nymphs completed development to adults on this host plant.

Table 1. Least squares means for number of phytophagous stink bugs and leaffooted bug per 1.83-m length of row and percentage of females (where known) in sorghum in Georgia in 2004 - 2007 and state references.

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Factorial analyses revealed a significant species effect (F = 191.18; df = 9, 27,000; P= 0.0001) for number of these pests per 1.83-m length of row in sorghum. Nezara viridula was the predominant species in sorghum; density of this stink bug was twice as high as that of the second most abundant stink bug, O. p. pugnax. Euschistus servus was the third most abundant stink bug species. Density of both L. phyllopus and E. quadrator was about half that of E. servus. Chinavia hilaris, Thyanta custator custator (F.), E. tristigmus, Piezodorus guildinii (Westwood), and E. ictericus were minor pests in sorghum.

Each of the first 7 sorghum panicle-feeding bug species listed in Table 1 previously have been reported to occur on sorghum (Dahms 1942, Wiseman and McMillian 1971, McMillian and Wiseman 1972, Hall and Teetes 1981, Mizell et al. 2008, Smith et al. 2008). Species composition varied by locality, but N. viridula, O. p. pugnax, E. servus, and L. phyllopus commonly have been found on sorghum (Table 1). Euschistus tristigmus and E. ictericus have not been reported to inhabit sorghum, but Euschistus spp. (Euschistus impictiventris Stål [Russell 1952] in Arizona and Euschistus consperus Uhler [Toscano and Stern 1976] in California), other than E. servus have been reported to feed on sorghum panicles. As far as is known, this is the first record of P. guildinii feeding on sorghum.

The 3 most predominant stink bug species, i.e., N. viridula, E. servus, and O. p. pugnax, were present on sorghum from the heading stage until maturation. Initially, O. p. pugnax adults were the most abundant stink bugs in sorghum (Fig. 1). Density of these adults remained relatively constant until declining near the hard dough stage. Soon after the appearance of O. p. pugnax adults, nymphs began developing on sorghum, peaking at the milk stage. Nezara viridula adults peaked during the milk to soft dough stage and were the most predominant stink bug adults from the milk to the hard dough stage. Subsequent N. viridula nymphs peaked at the soft dough stage. After E. servus adults peaked on sorghum in the soft dough stage, density of adults remained constant until the sorghum matured. Density of E. servus nymphs was relatively low, peaking a little during the soft to hard dough stage.

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Similarly, in earlier studies sorghum panicle-feeding bug adults were observed on sorghum panicles at or soon after completion of flowering (Wiseman and McMillian 1971, Hall and Teetes 1981). Hall and Teetes (1981) collected O. p. pugnax adults on flowering and milk panicles, L. phyllopus and E. servus on milk and soft dough panicles, and N. viridula on soft and hard dough panicles. In one study, the milk stage of sorghum panicles was the most preferred stage for feeding by E. servus adults (Mizell et al. 2008). Likely, stink bug density at each sorghum panicle developmental stage varies depending on farmscape composition and seasonal succession of stink bug host plants in these farmscapes (Tillman 2011a) and sorghum variety (McMillian and Wiseman 1972) and planting date (Tillman 2010a).

Predator species. Several species of generalist insect predators were found in sorghum over the study (Table 2). Factorial analyses revealed a significant species effect (F = 509.11; df = 7, 18,000; P = 0.0001) for number of these predators per 1.83-m length of row in sorghum. Orius insidiosus was the predominant species, but density of Geocoris spp. was still relatively high; twice as high as density of the lady beetles and spiders. Density of red imported fire ants, S. invicta, was about half that of lady beetles and spiders. The spined soldier bug, Podisus maculiventris (Say), and the spined assassin bug, Sinea diadema (F.), and damsel bugs, Nabis spp., were the least abundant predators.

Table 2. Least squares means for number of predators per 1.83-m length of row in sorghum in 2004 - 2007.

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The most predominant species of stink bug predators were present in sorghum from the heading stage to maturation (Fig. 2). However, O. insidiosus was very abundant on flowering panicles as reported earlier (Tillman 2006). Thrips (undetermined species) are not considered a pest of sorghum (Buntin 2009), but they were present in flowering panicles. Density of the heliothine Helicoverpa zea (Boddie), a known pest of sorghum (Buntin 2009), is higher on flowering panicles compared with all other stages of panicle development (Tillman 2006). Thus, O. insidiosus likely preyed on thrips and H. zea eggs as well as stink bug eggs. Indeed, this predator was observed on several occasions feeding on each of these prey items in sorghum as well as stink bug eggs on peanut and corn (Tillman 2008, 2010b). Orius insidiosus is an effective predator of adults and larvae of all thrips species (Ramachandran et al. 2001, Reitz et al. 2003). The prey-specific gut content enzyme-linked immunosorbent assay (ELISA) has been used to detect predation of H. zea eggs by this predator in cotton (Sansone and Smith 2001). When Ragsdale et al. (1981) tested 27 species for predation of N. viridula using ELISA, O. insidiosus was the second-most efficient predator of N. viridula eggs in soybean.

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Lady beetles were more abundant on milk and soft dough stage sorghum. Aphids, i.e., the corn leaf aphid (Rhopalosiphum maidis (Fitch) and the greenbug (Schizaphis graminum (Rondani), H. zea eggs, and stink bug eggs and nymphs were present on sorghum at this time. Lady beetles were observed preying on these pests in sorghum. Lady beetles can have a significant impact on aphids attacking grain sorghum (Kring et al. 1985, Rice and Wilde 1988). In addition, lady beetles are known to attack H. zea eggs (Nuessly and Sterling 1994, Pfannenstiel and Yeargan 2002). Coccinellids have been observed feeding on stink bug eggs on soybean, peanut, and corn (Stam et al. 1987, Tillman 2008, 2010b), and adults and larvae of Cycloneda sanguinea L. and C. maculata were efficient egg predators of N. viridula in soybean (Ragsdale et al. 1981).

Geocoris spp. were more abundant on soft to hard dough sorghum panicles. These big-eyed bugs were observed feeding on larvae of H. zea and stink bug nymphs infesting these panicles. Geocoris spp. also have been observed attacking stink bug eggs on soybean, peanut, and corn (Stam et al. 1987; Tillman 2008, 2010b). Geocoris punctipes has been regarded as a successful predator of Heliothis spp. in the field and laboratory (Whitcomb and Bell 1964, Lingren et al. 1968). In the ELISA experiments conducted by Ragsdale et al. (1981), G. punctipes was the top predator of N. viridula eggs and the second-most efficient predator of N. viridula nymphs.

Density of spiders and S. invicta was relatively constant throughout sorghum panicle development. Ragsdale et al. (1981) determined that the brown striped lynx spider, Oxyopes salticus Hentz, was the top nymphal predator of N. viridula, and the bold jumping spider, Phidippus audax (Hentz), also preyed on nymphs of this stink bug in soybean. Solenopsis invicta commonly attacked egg masses of N. viridula and E. servus on peanuts and corn (Tillman 2008, 2010b). Ragsdale et al. (1981) determined that S. invicta was the seventh most efficient egg predator of N. viridula in soybean. In predator exclusion studies in this crop, S. invicta was implicated as an important egg predator of N. viridula (Krispyn and Todd 1982). In a later study, this ant species was observed to be the dominant egg predator of N. viridula in soybean during the vegetative stages (Stam et al. 1987).

Podisus maculiventris was the only predatory stink bug found in sorghum; it is a generalist predator, feeding on a variety of insect prey, including nymphs and adults of pest stink bugs in a diversity of crop and noncrop ecosystems (Ragsdale et al. 1981, McPherson et al. 1982, Tillman 2008). Stink bug eggs and nymphs are known to be preyed upon by generalist predators in the Nabidae family (Ragsdale et al. 1981, Tillman 2008).

In summary, sorghum harbors many species of panicle-feeding bugs, but also many species of natural enemies of these pests including egg parasitoids of N. viridula and E. servus (Tillman 2011 b). In an earlier study, placement of a strip of sorghum along a cotton field edge enhanced abundance of lady beetles in cotton (Tillman and Cottrell 2012). Possible explanations for this enhancement included providing newly abundant prey during senescence of corn and rye, providing new or preferred prey for adults developing in peanut, and concentrating abundant prey next to cotton fields. Regardless of the mechanisms involved, a habitat of sorghum could possibly be used in conservation biocontrol of stink bugs in southeastern farmscapes.