Influence Of Host Pheromone On Egg Parasitism By

Journal of Insect Behavior, Vol. 13, No. 2, 2000

Influence of Host Pheromone on Egg Parasitism by Scelionid Wasps: Comparison of Phoretic and Nonphoretic Parasitoids R. Bruni,1 J. Sant’Ana,2 J. R. Aldrich,3,4 and F. Bin1 Accepted September 23, 1999; revised October 27, 1999

The kairomonal activity of the attractant pheromone for the ‘‘spined soldier bug,’’ Podisus maculiventris (Hemiptera: Pentatomidae), was investigated by exposing fresh pentatomid egg masses in field traps with or without synthetic pheromone. Predominantly two parasitoids were recovered from exposed eggs of P. maculiventris and Euschistus obscurus: Telenomus podisi Ashmead (a generalist pentatomid egg parasitoid) and Telenomus calvus Johnson (a phoretic specialist on Podisus eggs) (Hymenoptera: Scelionidae). The incidences of T. podisi from P. maculiventris and E. obscurus eggs placed in pheromone-baited and nonbaited traps were not significantly different, suggesting that this oophagous wasp does not use the spined soldier bug attractant pheromone as a kairomone. However, T. calvus was reared almost exclusively from egg masses of P. maculiventris placed inside pheromonebaited traps. These results suggest that T. calvus females orient to volatile chemicals emitted by spined soldier bug males as a searching strategy to find areas likely to contain host eggs, in addition to the previously discovered strategy of using the pheromone to guide their phoretic behavior. The data also indicate that T. calvus can distinguish between the egg masses of these pentatomid hosts at close range.

1

Istituto di Entomologia Agraria, Universita` di Perugia, Perugia, Italy. Departamento de Fitossanidade, Faculdade de Agronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil. 3 Insect Chemical Ecology Laboratory USDA—ARS, Beltsville, Maryland, USA 20705. 4 Correspondence should be sent to Jeffrey R. Aldrich, USDA—ARS, Insect Chemical Ecology Laboratory, Bldg. 007, Rm. 326, BARC-West, Beltsville, Maryland USA 20705; e-mail: [email protected] 2

165 0892-7553/00/0300-0165$18.00/0  2000 Plenum Publishing Corporation

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KEY WORDS: Telenomus; Podisus maculiventris; Euschistus obscurus; Scelionidae; kairomone; biological control; phoresy.

INTRODUCTION Egg parasitic Scelionidae (Hymenoptera) are efficient biological control agents of phytophagous true bugs (Hemiptera) and other insects (Masner, 1976; Orr et al., 1986a; Jones, 1988; Orr, 1988; Orr and Boethel, 1990; Foerster and Queiro´z, 1990; Correˆa-Ferreira and Moscardi, 1995; Weber et al., 1996), but also adversely affect beneficial predators such as predatory stink bugs (Pentatomidae: Asopinae) (Aldrich et al., 1984a; Orr et al., 1986b) and praying mantids (Orthoptera: Mantidae) (Clausen, 1976). One such beneficial is the spined soldier bug, Podisus maculiventris (Say) (Hemiptera: Pentatomidae), an important predator of several agricultural and forest pests in North America (McPherson, 1980, 1982). This predator is a potentially useful biocontrol agent for a wide variety of pests, including the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) (Couturier, 1938; Biever and Chauvin, 1992; HoughGoldstein and Whalen, 1993; Hough-Goldstein and McPherson, 1996), and the Mexican bean beetle, Epilachna varivestis Mulsant (Coleoptera: Chrysomelidae) (Sant’Ana et al., 1997). The aggregation pheromone of P. maculiventris has been identified, synthesized, and is commercially available (Aldrich, 1988; Aldrich et al., 1984a). Techniques are being developed to utilize the synthetic pheromone to promote augmentation and conservation of this predator (Sant’Ana et al., 1997; Aldrich, 1998), and one aspect of the effort is to blunt the kairomonal exploitation of the spined soldier bug’s pheromone by its parasitoids. The most important egg parasitoids of P. maculiventris are scelionids: Telenomus podisi Ashmead, Trissolcus euschisti (Ashmead) (Yeargan, 1979; Okuda and Yeargan, 1988a), and Telenomus calvus Johnson (Aldrich et al., 1984a; Johnson, 1984; Orr et al., 1986b). Telenomus podisi parasitizes the eggs of various pentatomids in agroecosystems, including those of Euchistus spp. (Yeargan, 1979; Orr et al., 1986a; Correˆa-Ferreira and Moscardi, 1995), Nezara viridula (L.) (Buschman and Whitcomb, 1980), Piezodorus guildinii (Westwood) (Correˆa-Ferreira and Moscardi, 1995), and P. maculiventris (Yeargan, 1979; Aldrich et al., 1984a; Orr et al., 1986b). Similarly, Tr. euschisti is a generalist, but its females reportedly prefer stink bug eggs deposited on woody plants (Okuda and Yeargan, 1988b). Telenomus calvus, however, is a specialist egg parasitoid whose females are known to be phoretic on females of P. maculiventris and females of the closely related species, P. neglectus Westwood (Johnson, 1984; Aldrich, 1995).

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Phoresy, the attachment of one animal to another for transportation, was reviewed by Howard (1927), who suggested that the phenomenon ‘‘may be found to be not at all rare, especially in the Scelionidae,’’ a prediction borne out by subsequent observations (Clausen, 1976). The phoretic search strategy of T. calvus females for spined soldier bug eggs is relatively circuitous. Initially the female wasp is attracted by the male-produced Podisus pheromone, although the wasps are not phoretic on male bugs (Aldrich et al., 1984a). A female wasp becomes phoretic only on a Podisus female after the female bug finishes mating (Buschman and Whitcomb, 1980), possibly recognizing female-specific volatiles from small glands underneath the wings (Aldrich et al., 1984b; Aldrich, 1995). At the time of oviposition, the T. calvus female dismounts the bug to parasitize her freshly laid egg mass. Thus, in addition to usurping two host pheromones, this remarkable behavior probably also involves recognition of and stimulation by chemicals emanating from the eggs themselves—the ultimate mechanism used by parasitoids to recognize eggs at very close range (Vinson and Piper, 1986; Bin et al., 1993). Hemipteran pheromones (Yasuda and Tsurumachi, 1995) and allomones (Mattiacci et al., 1993; Bin et al., 1993) are also exploited by some nonphoretic scelionid wasps as indirect cues to locate egg masses, and one phoretic egg parasitoid, Telenomus euproctidis Wilcox, has been unequivocally shown to locate females of its tussock moth hosts (Lepidoptera: Lymantriidae) by flying to their sex pheromones (Arakaki et al., 1997). The objective of the present study was to evaluate the kairomonal activity of the synthetic pheromone of P. maculiventris to egg parasitoids, as well as to observe the efficiency and selectivity of these organisms toward eggs of two pentatomid species, thus clarifying the host-finding behaviors of these scelionid wasps.

MATERIAL AND METHODS Euschistus obscurus was obtained from a laboratory colony of Dr. Walker Jones (USDA-ARS, Weslaco, TX), and reared on fresh green beans, raw sunflower seeds, and water at 26 ⫾ 1⬚C with a 16:8 (L:D) photoperiod. The sunflower seeds were glued onto sheets of brown wrapping paper with wallpaper paste, and cut into 10-cm squares that were discarded after depletion by the insects. Brown paper towels were provided as an oviposition substrate. Podisus maculiventris for the study were caught in pheromone-baited traps on the grounds of the Beltsville Research Center. From March 13 to May 4, 1995, 832 male and 556 female wild spined soldier bugs were caught alive in 20 traps as previously described (Aldrich, 1998). These insects were

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stored at ca. 15⬚C, then reared as needed in order to provide a constant production of egg masses for testing. Rearing conditions were similar to those for E. obscurus except that pupae of Tenebrio molitor L. (Coleoptera: Tenebrionidae) were substituted for sunflower seeds, and cheesecloth was used as the oviposition substrate. The experiment included 10 control and 10 pheromone-baited traps, alternately hung ca. 1.5 m from the ground from branches of deciduous trees bordering an open field at the Research Center. The synthetic pheromone used was a blend of (E)-2-hexenal, 움-terpineol, and benzyl alcohol (Aldrich, 1998) mimicking the male-produced long-range attractant pheromone. Beginning May 4, 1995, every 3–4 days (depending on availability) P. maculiventris egg masses ⬍36-hr-old were placed inside the same type of traps used to capture adults (Aldrich et al., 1984a), but with the entrance holes of the traps plugged with cotton so as to only allow the passage of insects small enough to fit through the screen mesh around the normal trap entrance hole. Beginning May 25, 1995, E. obscurus egg masses from the laboratory culture ⬍36-hr-old were also added to the traps, so that in each trap there was one P. maculiventris egg mass and one E. obscurus egg mass of roughly equal size (ca. 15 eggs/mass). At 3–4 day intervals, 10 traps were rebaited with spined soldier bug pheromone (Aldrich et al., 1984a), while 10 traps received no pheromone. The experiment was terminated on July 25, 1995. The exposed egg masses were brought to the laboratory every 3–4 days for parasitoid emergence, and new egg masses were placed the traps. Parasitoids were preserved in alcohol and classified according to Johnson (1984). After emergences of parasitoids, the egg masses were observed under a stereoscopic microscope in order to assess the discovery efficiency of each parasitoid. Discovery efficiency was defined as the number of discovered egg masses (i.e., at least one parasitized egg) divided by the total number of egg masses for each treatment (Bin and Vinson, 1990). The data were analyzed by chi-square (Sokal and Rohlf, 1981).

RESULTS AND DISCUSSION A total of 695 masses were used in the experiment: 404 of P. maculiventris and 291 of E. obscurus (Table I). The discovery efficiency of P. maculiventris egg masses by parasitoids was significantly greater in pheromone-baited traps than in non-baited traps (␹2 ⫽ 14.8; 1 df; p ⬍ 0.001). However, the discovery efficiencies did not differ significantly for E. obscurus egg treatments (␹2 ⫽ 0.39; 1 df; p ⬎ 0.05). The results show that the synthetic spined soldier bug pheromone

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Table I. Egg Mass Discovery Efficiency (%) and Parasitism by Telenomus podisi, Telenomus calvus, and Other Scelionids on Eggs of Podisus maculiventris and Euschistus obscurus in Pheromone-Baited and Nonbaited Traps Trap treatment Host spp.

No. egg masses

P. maculiventris

404

E. obscurus

291

Parasitoid spp.

Pheromone no. (%)

Control no. (%)

T. podisi T. calvus Other spp. T. podisi T. calvus Other spp.

12 (5.9) 39 (19.3)a 1 12 (8.3) 1 (0.7) 2

15 (7.4) 0a 2 11 (7.5) 0 1

Significant difference: ␹ 2 ⫽ 43.2; 1 df: p ⬍ 0.001.

a

influences the parasitism of P. maculiventris eggs, but has no influence on the parasitism of E. obscurus eggs. The predominant parasitoid species observed were T. podisi and T. calvus, with much lower incidences of Trissolcus and Gryon spp. (Hymenoptera: Scelionidae) despite the fact that in Kentucky Tr. euschisti was more abundant in the arboreal habitat than was T. podisi (Okuda and Yeargan, 1988b). The first T. podisi specimens were obtained from eggs retrieved May 26, and the first specimens of T. calvus were from eggs retrieved June 5, in agreement with earlier data for T. calvus (Aldrich et al., 1984a). In-depth analysis of the data reveals behavioral differences between the two Telenomus spp. Telenomus calvus females located 39 egg masses (19.3%) in traps with pheromone and none in traps lacking the pheromone (a highly significant difference: ␹2 ⫽ 43.2; 1 df; p ⬍ 0.001), while T. podisi females located 12 egg masses (5.9%) in traps with pheromone and 15 (7.4%) in the control traps (an insignificant difference: ␹2 ⫽ 0.36; df ⫽ 1; p ⬎ 0.05) (Table I). Therefore, it seems that T. calvus females not only use the attractant pheromone of P. maculiventris as a kairomonal beacon to facilitate their phoretic behavior (Aldrich et al., 1984a), but also may forage directly for eggs in areas in which they perceive Podisus pheromones. It is important to point out that T. calvus needs fresh eggs to develop, preferably ⱕ 12 hr old (Orr et al., 1986a). The specialized phoretic behavior of T. calvus females obviously increases the probability that will encounter recently laid eggs. Searching for eggs in areas permeated with Podisus pheromone may also be an evolved foraging strategy of T. calvus to find fresh host eggs because the production of pheromone by spined soldier bug males is apparently associated with the discovery of prey patches making it more likely that P. maculiventris females will remain in the area to oviposit

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(Aldrich, 1988, 1995). Telenomus podisi and Tr. euschisti females are generally larger than are T. calvus females (Johnson, 1984), but the ability of T. calvus females to exploit host pheromones as kairomones enables them to successfully compete with larger, nonphoretic species having broader time windows of egg acceptability. Even though T. podisi is considered the main parasitoid of P. maculiventris eggs in soybeans (Orr, 1988) and Tr. euschisti is a significant parasitoid in the forest environment (Okuda and Yeargan, 1988b), neither species seems to use this predator’s attractant pheromone as a kairomone. Evidently these generalist parasitoids rely on other cues to locate host eggs, among which may be volatile and/or nonvolatile substances from the eggs (Vinson and Piper, 1986; Vinson, 1984, 1994; Bin et al., 1993; Vet et al., 1995) and semiochemicals from the flora associated with hosts (e.g., Turlings et al., 1995). The discovery efficiency of E. obscurus eggs by T. podisi was similar in the pheromone-baited (8.3%) and control (7.5%) traps (Table I), suggesting that females of this wasp are indifferent to the P. maculiventris pheromone. The reduced number of E. obscurus eggs parasitized by T. calvus, in spite of the presence of this parasitoid inside the traps, leads to the conclusion that females of this species are able to discriminate between the two egg masses. This discrimination is, in part at least, likely due to egg chemistry, as well as to physical and morphological factors (Sales et al., 1980; Vinson, 1984; Bin et al., 1993; Vet et al., 1995; Gazit et al., 1996). That synthetic pheromone for P. maculiventris does not affect the searching behavior of T. podisi or Tr. euschisti females (also important parasitoids of phytophagous stink bugs) suggests that pheromonal manipulation of P. maculiventris in the field will not interfere with biocontrol of pentatomid pests (e.g., see Orr and Boethel, 1986). In addition, the observation in the present study that egg parasitism does not begin until late May, plus the earlier discovery that most spined soldier bug adults have emerged from overwintering by the time their tachinid fly parasitoids appear in late April (Aldrich, 1995), means that early spring is a parasitoidfree time to mass-trap adults of this predator for biocontrol purposes (Aldrich, 1998).

ACKNOWLEDGMENTS Fellowships from CNPq of the Brazilian government, and the National Research Council of Italy, Special Project RAISA, helped support JS and RB, respectively. We also gratefully acknowledge Drs. David Orr (North

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Carolina State University, Raleigh) and Lester Ehler (University of California, Davis) for reviewing the manuscript.

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