The life history of the shield-backed bug, Pachycoris stallii Uhler (Heteroptera: Scutelleridae), immatures was studied on its host plant, Croton californicus Muell.-Arg. (Euphorbiaceae), in Baja California Sur, Mexico. Immature stages are described and illustrated. Pachycoris stallii is bi- or multivoltine and occurs in xeric areas with sandy soil where it is rarely encountered away from C. californicus. Nymphs and adults feed on seeds within C. californicus fruit. Bugs oviposit on the underside of leaves, and females guard their eggs and first-instar nymphs from natural enemies. Embryonic orientation of prolarvae is nonrandom; each embryo is oriented with its venter directed toward the ground. This orientation may facilitate aggregation of first instars. The longitudinal axes of eggs are always oriented upward at about a 16° angle of deviation from a line perpendicular to the leaf surface. This is the first recorded observation of this phenomenon in Pentatomoidea. Experimental removal of females guarding first instars results in 100% loss of nymphs, and this is attributed to disruption of the aggregative behavior of nymphs. Maternal guarding appears to be a net benefit to P. stallii, despite possible costs to the brooding female.
The Family Scutelleridae, or shield-backed bugs, occurs worldwide and is represented by about 80 genera and 450 species in four subfamilies (Lattin, 1964; Schuh etal.,1995). Scutellerids are members of the Pentatomoidea, and all are phytophagous. Perhaps because they are in frequently observed or collected in large numbers, little is known about scutellerids (Yonke, 1991; Schuh et al., 1995), although some are pests of wheat, cotton, and other crops (Cardona et al., 1983; Wilson et al., 1983; Grimm, 1999; Parker et al., 2002).
The genus Pachycoris Burmeister 1835, placed in the subfamily Pachycorinae, is distributed mainly in the New World. Pachycoris stallii Uhler was described in 1863; although its distribution is not well-demarcated, it is known mostly from subtropical regions of Mexico. Like several of its congeners (Hussey,1934; Wolcott, 1951; Grimmet al., 1998; Peredo, 2002), P. stallii is host-specific on a Euphorbiaceae, and exhibits aposematic coloration and subsocial behavior. Females oviposit on leaves and guard egg masses and first-instar nymphs from natural enemies (Williams et al., 2001). Wilson (1979) postulated that exceptional environmental challenges (e.g., intense pressure from natural enemies) might select for evolution of subsocial behavior in insects. Brooding P. stallii females apparently do not feed and therefore may be subjected to costs associated with guarding offspring, including shortened lifespan due to starvation, and increased vulnerability to natural enemies. Williams et al. (2001) reported that the chemical defense system of P. stallii is based on short-chain carbonyl compounds, as appears to be the norm for Heteroptera (Aldrich, 1995).
The population of P. stallii that we studied was discovered by one of us (LW) in Baja California Sur, México. At this site P. stallii is apparently highly host specific to Croton californicus Muell.-Arg. (Euphorbiaceae) as it was rarely encountered on other plants. Croton californicus is a perennial shrub that occurs in the southwestern United States and northwestern México (Shreve et al., 1964; Wiggins, 1980). In Baja California this shrub occupies sandy areas near beaches and drainages where annual precipitation and temperatures average 260 mm and 26°C, respectively (Johnson, 1977). The plant is drought-deciduous and at the study site grows as an erect, sparse shrub up to about 1 m in height. Leaves measure several centimeters in length, are gray-green, lanceolateor elliptical, and hang vertically. Croton californicus flowers throughout the year at the study site, producing inconspicuous male and female flowers borne on terminal racemes. Fruit are dehiscent tri-lobed capsules (ca. 5 mm diam). Phytochemical investigations have shown that leaves, stems, and fruit produce numerous toxic compounds, including terpenoids and phorbol esters (Wilsonet al., 1976; Luzbetaketal., 1979; Chavezetal., 1982; Williams et al., 2001). Most herbivores, both vertebrate and invertebrate, generally avoid this chemically well-defended plant.
The purpose of this study is to describe the immature stages of P. stallii and present life history information on its field biology, including an experimental field study of maternal guarding of nymphs.
Materials and Methods
Field observations and collections of P. stallii eggs and nymphs were made on its host plant, C. californicus, in Baja California Sur, México (Fig. 1). The field site was visited at irregular intervals throughout the calendar year from 1988 to 1997. Some field-collected egg masses were placed in 9 dram ventilated plastic vials (Thornton Plastics, http://thorntonplastics.com) and maintained alive at about 27° C to allow the observation of color change and embryonic orientation during development.
A study was conducted to determine the effect of maternal guarding on survival of first-instar nymphs, and to provide insight on the tradeoffs inherent in subsocial behavior. Croton californicus plants harboring newly emerged (≤ 1 day old) first-instar nymphs were chosen in a 600 x 100 m study area. The duration of the study (6 days, 27 July-1 August 1994) approximated the developmental period of first-instar nymphs during the summer. Minimum and maximum temperatures during the study ranged from 26 to 44° C (shaded temperatures 2 m above the soil; Schultheis Quick Reading Thermometer, http://www.millerweber.com). In total, 15 groups of guarded nymphs were chosen for study; five control groups and ten treatment groups. In each treatment group the adult female bug was carefully removed from the leaf and placed on another plant several meters away. Previous observations confirmed that female bugs did not return to their offspring after being displaced in this manner. Care was taken to minimize disturbance to the nymphs during this procedure. In control groups adult females were not removed. The nymphs in each of the 15 groups were then counted (day=0); on each day thereafter (day = 1 to 5) the entire plant was searched and all unguarded first instar nymphs were counted. We assumed that these nymphs represented those included in the study. The effect of female brooding on survival of first instars was assessed by comparison of the appropriate confidence intervals (binomial distribution was used for control, normal distribution was used for treatment) on each day of the study.
While conducting fieldwork we noticed that when nymphs were disturbed they sometimes dropped from plants and hid in the leaf litter where they were probably exposed to greater risk from natural enemies as well as high surface temperatures. Therefore, we tested the ability of nymphs to survive if they fell to the sand or attempted to disperse to different plants during the summer (July). At mid-day several leaves with second or third instar nymphs were placed on the sand in direct sunlight and their behavior was recorded. A Schultheis Quick Reading Thermometer was used to measure substrate surface temperatures (direct sunlight = 54° C, in shade = 42° C).
Field-collected specimens to be used for descriptions were killed and preserved in 70% ethanol. Descriptions and terminology follow those of Decoursey and Esselbaugh (1962), Miller (1971), and Javahery (1994). Atleast ten specimens of each nymphal instar were measured using an ocular micrometer on a Wild M-stereomicroscope. Length of nymphs was measured from tip of clypeus to tip of abdomen, and width was measured at the widest point of the body. The fifth instar is described in detail and only major differences that are present in previous instars are described. All measurements of dimensions are given in millimeters (mean ± SD and range). Drawings of nymphs were made with a drawing tube on a Wild M-stereomicroscope.
Sample sizes varied for eggs and are given in the description. However, measurements of individual eggs were made from at least 66 individuals randomly chosen from 39 egg masses. The degree of deviation of eggs from being perpendicular to leaf surface was estimated by observing the lateral aspect of each egg mass under a dissecting scope. A protractor was positioned under the egg mass and the degree of deviation from perpendicular was recorded from the grid on the protractor. Specimens to be examined by scanning electron microscopy were dehydrated in a graded ethanol series, mounted on aluminum stubs with silver paint, and sputter-coated with gold-palladium alloy prior to examination with a JEOL T-100 scanning electron microscope.
Voucher specimens of P. stallii are deposited in the Museo de Arthrópodos de Baja California (CODEN: CICESE), Centro de Investigación Cientifica y de Educación Superior de Ensenada, Ensenada, Baja California, Mexíco, and in the Museo Departmento Científico de Entomología (CODEN: MLP), Facultad de Ciencias Naturales, Universidad Nacional de La Plata, La Plata, Argentina.
Results and Discussion
Eggs cylindrical with convex ends. Length, 1.46 ± 0.13 (1.20-1.69, n = 101); width, 0.87±0.06 (0.71-1.02, n=101). Chorion with hexagonal reticulations surrounding smaller irregular sculpturing (Fig. 2). Cephalic pole with a ring of unevenly spaced micropylar processes, 19± 2.08 (14-23, n = 66). Pseudoperculum circular, sometimes askew, ovate; egg burster dark brown, subtriangular, T-shaped. Chorion relatively thin, weak, and pliable, similar to that of the pentatomid Euschistus servus (Say). Egg masses consisted of 53.8 ± 8.86 (33-74, n =40) eggs laid in roughly hexagonal groups. The average number of rows per egg mass was 8.47 ± 0.97 (6-11, n = 30). Egg masses averaged 6.80 ± 0.63 (5.43 - 8.23, n=39) in length and 5.95 ± 0.55 (4.75-7.20, n = 39) in width.
Leaves with egg masses averaged 35.0 ± 7.2 in length, and 13.0 ± 4.2 (n = 35) in width. The position of egg masses on leaves corresponded to the approximate center of the leaf (92% of maximum width, 44% of maximum length measured from leaf base). Egg masses usually overlapped the midvein, but occasionally were positioned entirely between the leaf margin and midvein. The longitudinal axes of eggs were not perpendicular to the leaf surface, but instead were always oriented toward the leaf base, (i.e., upward) at an angle. This angle of deviation from a line perpendicular to the leaf surface was 15.9° ± 12.4 (5-45, n=125). To our knowledge, this is the first recorded observation of this phenomenon in Pentatomoidea. Its function, if any, is unknown. Dimensions of egg masses were considerably less than those of adult females (egg masses: 6.80 x 5.95mm; adult females: 11.73 x 7.83 mm), thus egg masses were completely covered by the brooding females. The longitudinal axis of an egg mass was usually parallel to that of the leaf.Eberhard (1975) also reported this egg mass orientation for the pentatomid Antiteuchus tripterus limbativentris Ruckes and suggested that it might be due to the consistent position of the defending bug in relation to the orientation of the egg mass, as influenced by selection pressure of parasitic wasps.
Immediately after oviposition, eggs were cream-colored. As embryonic development proceeded, the embryos acquired a concolorous pigmentation of pale pink except for a cream-colored band extending from the caudal to cephalicregion. With further development, the eggs became pinkish orange, noticeable first in the eyes, clypeus, pronotum, and appendages. Finally, about 4 days after oviposition the embryos transformed to salmon, then darkened to red prior to eclosion. Hatching was observed on two occasions, both in the morning (28 July 1994, 0741-0751h, 34°C, two separate egg masses). When embryonic cuticles were recovered (n=83), they were matted together either on the portion of the egg mass nearest the leaf tip, or occasionally on the leaf just distal to this portion of the egg mass. Some of the eggs failed to develop (≤10%), and these undeveloped eggs remained creamy yellow. Color changes observed during development of P. stallii eggs were similar to those reported for other Pachycoris species (Hussey, 1934; Grimmetal., 1998; Peredo, 2002). Most P. stallii egg masses were parasitized by a wasp, Telenomus pachycoris (Costa Lima) (Hymenoptera: Scelionidae); parasitism rates averaged about 47% (Fig. 3). Parasitized eggs turned from gray to black as the parasitoids developed. Antagonism by ants, Dorymyrmex bicolor Wheeler (Hymenoptera: Formicidae), sometimes caused brooding P. stallii females to abandon their egg masses, after which the ants devoured the eggs (Fig. 4).
Lockwood and Story (1986a) postulated that nonrandom embryonic orientation, i.e., dorsoventral polarity, of prolarvae is an adaptive function that reduces mortality of first instars. We assessed the embryonic orientation for P. stallii prolarvae within egg masses that were not parasitized by T. pachycoris. These unparasitized egg masses were similar to those described previously, consisting of 55.4 ± 11.06 (34-73, n = 14) eggs. Embryonic orientation in these egg masses was not related to position of egg with in the mass. Instead, each embryo was oriented with its venter directed down (i.e., toward the ground) regardless of the location of the egg within the mass. Other studies also reported uniform embryonic orientation in scutellerids. Javahery (1994) reported that eggs of Eurygaster alternata (Say), E. integriceps Puton, and E. maura (L.)were oriented in one direction, as did Reid and Barton (1989) for Chelysomidea guttata (Herrich-Schaeffer). However, neither study indicated the direction of the embryos. Eberhard (1975) found that embryos of the pentatomid A. t. limbativentris were all oriented in the same direction in each egg mass, although neither the direction of embryos in an egg mass nor differences in embryonic orientation between egg masses were reported. However, embryonic orientation of other pentatomid species was dependent on egg location with in the mass, as influenced by form of egg mass, number of eggs, and number of rows in an egg mass, and could be described as a ‘center orientation’ for emerging nymphs(Lockwoodetal., 1986a; Javahery, 1994). Lockwood and Story (1986a) demonstrated that embryonic orientation in Nezara viridula (L.) varied with egg position within the mass, was extrinsically mediated, and facilitated formation of first-instar aggregations. This ‘center orientation’ of embryos enhanced survival and development of N. viridula (Lockwood etal., 1986b), and may serve a similar function in other pentatomoids. Although the embryonic orientation of P. stallii differed from that described for most pentatomoids, it may nevertheless provide an adaptive function. The uniform downward embryonic orientation of P. stallii may facilitate formation of first-instar aggregations at a relatively safe location on the leaf (i.e., between the egg mass and the distal tip of the leaf). This would position the brooding female bug between the nymphs and the only route by which crawling natural enemies could gain access to the leaf. Thus, the mother would be in a position to defend her offspring.
(Fig. 5a). Color as for fifth instar, without metallic green. General color red. Legs pale brown, tinged with red. Eyes red. Total length,2.57 ± 0.05 (2.50-2.60). Length of head, 0.62 ± 0.02 (0.60-0.63); width of head, 0.88 ± 0.01 (0.87-0.90). Width of eye, 0.12 ± 0.01 (0.12 - 0.13); interocular space, 0.64 ± 0.01 (0.63 - 0.65). Rostral length, 1.17 ± 0.07 (1.08 - 1.25). Antennal length, 1.11 ± 0.05 (1.03-1.15); ratio of segment lengths about 1:1.78:1.64:3.43. Length of pronotum, 0.25 ± 0.01 (0.24-0.25); width of pronotum, 1.09 ± 0.04 (1.03-1.13). Abdominal length, 1.41 ± 0.04 (1.37-1.47); abdominal width 1.66 ± 0.06 (1.60-1.73).
(Fig. 5b). Color as for fifth instar, without metallic green. Legs dark brown. Abdomen red. Eyes red. Total length, 3.27 ± 0.09 (3.20-3.40). Length of head, 0.78 ± 0.02 (0.75-0.80); width of head, 1.19 ± 0.03 (1.15-1.22). Width of eye, 0.19 ± 0.01 (0.18-0.20); interocular space, 0.81 ± 0.02 (0.78-0.83). Rostral length, 2.14 ± 0.1 (2.00-2.22); ratio of segment lengths about 1:2:1.34:1.29. Antennal length, 1.75 ± 0.05 (1.70-1.82); ratio of segment lengths about 1:1.65:1.5:2.58. Length of pronotum, 0.38 ± 0.01 (0.37-0.40); width of pronotum, 1.50 ± 0.06 (1.42-1.57). Abdominal length, 1.65 ± 0.08 (1.57-1.75); abdominal width, 2.10 ± 0.12 (2.00-2.27).
(Fig. 5c). Color as for fifth instar, without metallic green. Eyes dark red. Abdomen red. Total length, 4.29 ± 0.1 (4.18-4.43). Length of head, 0.94 ± 0.13 (0.75-1.17); width of head, 1.69 ± 0.04 (1.63-1.75). Width of eye, 0.29 ± 0.02 (0.27-0.32); interocular space, 1.11 ± 0.04 (1.05-1.17). Rostral length, 2.93 ± 0.07 (2.89-3.05); ratio of segment lengths about 1:2.11:1.21:1.31. Antennal length, 2.65 ± 0.04 (2.64-2.75); ratio of segment lengths about 1:2.15:2.03:2.85. Length of pronotum, 0.62 ± 0.03 (0.58-0.65); width of pronotum, 2.28 ± 0.16 (2.08-2.51). Abdominal length, 2.34±0.37 (1.98-2.81); abdominal width, 2.87±0.36(2.48-3.35). Stridulitrum on venter of abdomen consisting of a series of coarse fingerprint-like elongate ridges on sternites 5, 6, and anterior portion of 7, less pronounced than in fourth instar.
(Fig. 5d). Eyes dark red. Total length, 5.23 ± 0.21 (4.94-5.38). Length of head, 0.98 ±0.17 (0.83-1.24); width of head, 2.27 ± 0.03 (2.23-2.30). Width of eye, 0.39 ± 0.01 (0.30-0.40); interocular space, 1.48 ± 0.02 (1.45-1.50). Rostral length, 3.32 ± 0.02 (3.30-3.35); ratio of segment lengths about 1:2:1.19:1.25. Antennal length, 3.64 ± 0.04 (3.60-3.69); ratio of segment lengths about 1:1.92:1.86:2.37. Length of pronotum, 0.97 ±0.05 (0.90-1.00); width of pronotum, 3.82 ± 0.08 (3.74-3.94). Wing pad length, 1.22 ± 0.06 (1.15-1.30). Abdominal length, 2.76 ± 0.21 (2.53-3.04); abdominal width, 4.28 ± 0.17 (4.05-4.43). Stridulitrum on venter of abdomen consisting of a series of coarse fingerprint-like elongate ridges on sternites 5, 6, and anterior portion of 7, less pronounced than in fifth instar.
(Fig. 5e). Total length, 6.77 ± 0.46 (6.33-7.41). Length of head, 1.01 ± 0.30 (0.63-1.37); width of head, 2.91 ± 0.09 (2.79-3.00). Body broadly oval; shiny, coarsely punctate. Head dark brown, almost black, with a metallic green sheen; laterally dark brown except under the eyes, brown. Bucculae pale brown. Clypeus not surpassing the jugae, with short, sparse setae. Eyes prominent, rounded from lateral view, nearly contiguous with pronotum. Eyes dark red. Width of eye, 0.56 ± 0.02 (0.53-0.58); interocular space, 1.90 ± 0.05 (1.83-1.93). Rostrum dark brown. Rostral length, 4.46 ± 0.02 (4.57-4.60); ratio of segment lengths about 1:1.79:0.96:1.2. Antennae dark brown, almost black, union between segments pale brown; short abundant setae. Antennal length, 5.31 ± 0.09 (5.20-5.40); ratio of segment lengths about 1:2.41:2.18:2.22.
Abdominal length, 3.58 ± 0.08 (3.48-3.67); abdominal width, 5.57 ± 0.23 (5.38-5.89). Abdomen pale brown medially, laterally tinged with orange, rugose; dark brown, almost black with metallic green plates. Ventrally brown, medially pale brown, rugose; laterally tinged with orange, dark brown plates (Fig. 5f); sparsely setose. Paired ostia of dorsal abdominal scent glands located between terga 3 and 4, 4 and 5, and 5 and 6, with reticulate microsculpturing near ostiole and occlusion arm (see Williams et al. 2001). Using the criteria of Remold (1962), ostioles of the anterior dorsal abdominal glands of type 1 (undivided ostiole), while median and posterior ostioles intermediate between types 1 and 2 (divided ostiole). Stridulitrum on venter of abdomen consisting of a series of coarse fingerprint-like elongate ridges on sternites 5, 6, and anterior portion of 7 (Fig. 6a, b). Ventral surface of metathoracic tibia with ridges with ca. 20 small tubercules (Fig. 6c, d).
Thorax with three distinct segments. Pronotum dark brown, almost black, with metallic green sheen, except edges brown tinged with orange, flat laterally; punctate. Length of pronotum, 1.60 ± 0.14 (1.43-1.77); width of pronotum, 5.55±0.12 (5.38-5.63). Meso-and meta sternum brown. Wing pad length, 2.98±0.04 (2.93-3.03); dark brown, almost black, with metallic green sheen. Ventrally pale brown, tinged with orange. Legs dark brown, almost black; sparsely setose. Tarsi with pulvilli and parempodia as seen in Figure 6e.
Dimensions of P. stallii nymphs were similar to thoseof P. klugii studied in México (Peredo, 2002), although P. stallii had a slightly shorter antenna, rostrum, and head. Fourth- and fifth-instar P. stallii had smaller bodies overall (i.e., length and width) than P. klugii (Peredo, 2002). Dimensions of P. klugii nymphs studied in Nicaragua (Grimmet al., 1998) were considerably larger than P. stallii. As with eggs, these differences may be proportional to adult body size.
Life History and Development of Nymphs. All nymphal instars, as well as adults and eggs, were found throughout the year; thus, P. stallii appears to be bi-or multivoltine. After eclosion, first instars remained aggregated on or near the egg mass from which they hatched, and were guarded by the mother (Fig. 7). These nymphs were relatively inactive and were usually in contact with one another and sometimes the mother. Aggregation of first instars with the attending mother is consistent with other species of Pachycoris (Hussey, 1934; Grimm et al., 1998; Peredo, 2002). Female bugs brooding eggs were never observed feeding. First-instar P. stallii also appeared to abstain from feeding, as do first instars of many other Pentatomoidea (Walt et al., 1972; Eberhard, 1975; Oetting et al., 1975; Peredo, 2002).
The effect of maternal brooding behavior on survival of first-instar nymphs was evaluated by experimental removal of female bugs guarding nymphs that had just emerged. Our results indicated that brooding of first instars significantly increased their survival (P<0.05) (Fig. 8). Removal of guarding female bugs resulted in a rapid increase in nymphal mortality, reaching 100% after 4 days. Mortality of untreated controls was less than 4% at the end of the 6-day study. Behavior of first instars in the absence of the guarding adult was noticeably different from nymphs that were guarded. Unguarded nymphs often separated into two or three groups within a day after removal of the guarding female, and dispersed to other parts of the plant, often the apex of stems. Although predation of nymphs was not observed during the study, it is possible that disruption of nymphal aggregations predisposes nymphs to greater predation. However, Grimmand Somarriba (Grimm et al., 1998) found that first-instar P. klugii reared in outdoor cages that excluded most if not all predators also suffered 100% mortality when abandoned by the mother.Antiteuchus t. limbativentris first instars suffered substantial losses (≥40%) when guarding mothers were experimentally removed (Eberhard, 1975). Results of these studies indicate that the presence of the brooding mother plays a critical role in the aggregative behavior of nymphs and their subsequent survival. The mechanism is unknown, but it is possible that brooding Pachycoris females provide a cue, chemical and/or physical, that is necessary for the stability of nymphal aggregations. Our results suggest that maternal guarding behavior is a net benefit to P. stallii. Possible costs to the brooding female, such as starvation and increased risk from natural enemies, await further study. Eberhard (1975) also showed that the net effect of maternal guarding of first-instar A. t. limbativentris was positive.
Second-instar nymphs remained aggregated, but did not remain near the brooding female. We did not observe separation of these nymphs and the adult, but P. klugii nymphs moved away from the mother after molting to second instar (Peredo, 2002). However, Eberhard (1975) reported the reverse situation for A. t. limbativentris; mothers abandoned their second-instar offspring. Aggregations of second-instar P. stallii were most commonly observed feeding on C. californicus fruit (Fig. 9a) or resting on leaves (Fig. 9b). They were much more active than first instars and readily crawled away when disturbed. Third through fifth instars were also very active, and exhibited similar feeding and resting behavior (Fig. 10) as second instars. However, aggregations of third through fifth instars consisted of progressively fewer individuals with age, so that fifth instars were usually solitary. This may be a resul tof cumulative nymphal mortality and/or normal decay of aggregative behavior. In contrast to what we observed, Grimm and Somarriba (Grimm et al., 1998) found that aggregations of P. klugii nymphs remained intact until after adult metamorphosis. Moltingin P. stallii was observed on two occasions (28 July 1994, 0701 hours, 31° C, emergence of a fourth instar; 19 January 1997, 1353 hours, 27°C, a callow fourth instar on leaf in shade 7.5 cm above soil). On 28 December 1990, 1200-1415 hours the behavior of nymphs and adults was observed during a light/moderate rain. These condition shad no apparent effect on the behavior of P. stallii ; bugs were active feeding and resting as usual. During the summer months nymphs were generally observed either resting in shaded portions of the plant canopy, or if in direct sunlight, were nearly always feeding on fruit or occasionally on the stem.
Pachycoris stallii nymphs are subject to biotic and abiotic mortality factors. Predation on P. stallii nymphs was rarely observed. Adult female Zelus sulcicollis Champion (Heteroptera: Reduviidae) were observed feeding on third-instar nymphs on two occasions (15 April 1990, 1500-1530 hours; 1 November 1995, 1515 hours in the plant canopy about 3 cm above soil surface). Ants, Dorymyrmex bicolor, were sometimes observed attacking nymphs on the soil beneath C. californicus. When leaves harboring nymphs were placed on the sand indirect sunlight the nymphs crawled to and froon the leaf and within 5 minall had fallen or crawled off the leaf on to the sand; within 2 min they became immobilized by the heat and after an additional 2 min they died. Within 30 min they were found by D. bicolor. These observations indicate that typical surface temperatures during summer days are lethalto P. stallii nymphs that fall from plants, and thus interplant dispersal by nymphs is unlikely. However, it is possible that nymphs could survive surface temperatures during summer nights or during other seasons when temperatures are more moderate, although interplant dispersal via the ground might still expose them to greater risk of predation. These limits to nymphal dispersal suggest that nymphs usually mature to adulthood on their natal host plant. Clearly, muchre mains to be learned about the biology of P. stallii. Future studies will address P. stallii -natural enemy associations, and the effect of environmental factors on the life history of this bug.
This study is dedicated to the memory of Vivienne E. Harris, an avid fan of the Pentatomoidea, a gifted teacher, and a vibrant spirit. We express our gratitude to W. H. Clark, T. J. Henry, D. B. Thomas, and anonymous reviewers for valuable comments on the manuscript. We are also grateful to the following individuals for determination of insects: J. E. Eger, Jr. (P. stallii), N. F. Johnson (T. pachycoris), R. R. Snelling (D. bicolor), and R. C. Froeschner (Z. sulcicollis). This work was supported in part by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIPn°0545), and the Universidad Nacional de La Plata, Argentina.
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