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SUPERPARASITISM AND INTRASPECIFIC COMPETITION BY THE SOLITARY LARVAL-PUPAL PARASITOID ARCHYTAS MARMORATUS (DIPTERA: TACHINIDAE)
Stuart Reitz
Department of Entomology USA
Abstract
Superparasitism and intrinsic larval competition by the solitary larval-pupal parasitoid Archytas marmoratus (Townsend) were studied in vivo. In superparasitized hosts, when two parasitoids entered a host pupa, only one parasitoid completed development. The surviving A. marmoratus maggot eliminated the conspecific competitor through physiological suppression during the second stadium of the supernumerary maggot. Supernumerary parasitoids never survived beyond the second instar, regardless of the time interval between initial parasitism and subsequent superparasitism. Physical combat was not evident because the parasitoid eliminated did not show signs of physical injuries. Scramble competition for host resources was not a probable mechanism of elimination because puparial weights and adult eclosion rates from superparasitized host pupae, and those from singly parasitized pupae, were not signif-icantly different.
Key Words: Parasitoid competition, intraspecific interactions.
Resumen
El parasistismo y la competencia larval intrínseca del parasitoide larvo-pupal Archytas marmoratus (Townsend) fueron estudiados en vivo. En hospedantes superparasitados, cuando dos parasitoides entraron en una pupa hospedante, solamente uno completó el desarrolo. La larva sobreviviente de A. marmoratus eliminó el competidor conespecífico mediante supresión fisiológica durante el segundo estadio de la larva supernumeraria. Los parasitoides supernumerarios nunca sobrevivieron más allá del segundo instar, independientemente del intervalo entre el parasistismo inicial y el superparasitismo subsecuente. El combate físico no fue evidente debido a que el parasitoide eliminado no mostró señales de daño físico. La competencia por recursos de hospedante no fue un mecanismo probable de eliminación debido a que los pesos pupales y las tasas de eclosión de adultos de las pupas superparasitadas y de las parasitadas por una sola larva no fueron significativamente diferentes.
Archytas marmoratus (Townsend) (Diptera: Tachinidae) is a solitary larval-pupal parasitoid of numerous species of Noctuidae (Lepidoptera). Included in its host range are many important pest species in the genera Helicoverpa, Heliothis, Pseudaletia, and Spodoptera (Arnaud 1978, Ravlin & Stehr 1984).
A. marmoratus has a complex life history that allows it to parasitize a wide range of host instars. Females do not oviposit directly on hosts; instead they deposit numerous eggs in the vicinity of potential host larvae. The eggs soon hatch into planidia-type larvae (Wood 1987). Parasitism occurs when a host contacts a planidium which then burrows between the host cuticle and epidermis where it resides (Bratti et al. 1993). The first instar of A. marmoratus begins feeding on the host larva, but it does not molt until after the host pupates. The first instar must reenter the host following each larval-larval molt of the host. After the host undergoes its larval-pupal molt, the first-instar parasitoid penetrates the hemocoel under the host wing pad, where it induces the formation of a respiratory tunnel. A. marmoratus development within the host pupa is rapid. The maggot molts to the second instar 1 - 2 days following host pupation; the second and third stadia last 2 - 4 days each, with pupariation occurring within the host remains (Vickery 1929).
Because female A. marmoratus deposit multiple eggs at one time (Vickery 1929), and more than one female may oviposit in the same location, considerable potential for superparasitism exists. Despite this potential, only one A. marmoratus maggot completes development in a host (Vickery 1929, Hughes 1975). Among the possible mechanisms for the elimination of supernumerary parasitoids are physical combat, scramble competition for host resources, or physiological suppression (Salt 1961, Fisher 1971, Vinson & Iwantsch 1980). In this study, I examine aspects of superparasitism and intrinsic larval competition, including parasitism rates, elimination of supernumerary parasitoids, and effects on parasitoid development and emergence.
Materials And Methods
All tests were conducted at the Istituto di Entomologia “Guido Grande”, Universita degli Studi di Bologna, Bologna, Italy. A. marmoratus adults were reared in plexiglass cages (40 × 40 × 40 cm) in an environmental chamber maintained at 27 ± 2° C, 60 ± 10% R.H. and 14:10 (L:D) photoperiod (fluorescent light). To obtain A. marmoratus planidia for parasitization, pieces of pleated filter paper were placed on the bottom of cages as oviposition substrate the day prior to parasitism. Thus, all planidia were less than 24 h old at the time of parasitization. Larvae of the factitious host, Galleria mellonella L. (Lepidoptera: Galleriidae), were reared on artificial medium (Campadelli 1973) in plastic containers (23 × 11 × 8 cm) held at 30 ± 2° C, 60 ± 10% R.H., and 0:24 (L:D) photoperiod.
Penultimate-instar larvae of G. mellonella in apolysis were isolated in containers with fresh diet. The following morning, these groups were reexamined for newly-molted ultimate-instars. Each G. mellonella larva was infested by gripping it behind the head capsule with a soft forceps and transferring A. marmoratus planidia to the larval thorax with a fine camel hair brush. Each larva was held until the planidia burrowed into the cuticle. Then it was placed in a new plastic container with fresh diet.
Six parasitism treatments were used, with three groups of G. mellonella larvae being superparasitized and three groups being singly parasitized. The three superparasitized groups were: (A) Newly molted (day 1) ultimate-instar G. mellonella larvae parasitized with two A. marmoratus planidia (Superparasitized - Day 1), (B) Day 1 larvae parasitized once and superparasitized two days later (Superparasitized - Day 1, 3), and (C) Day 1 larvae parasitized and superparasitized four days later (Superparasitized - Day 1, 5). Only larvae with visible planidia were superparasitized. The three corresponding singly parasitized control groups were parasitized on day 1 (Day 1 Control), day 3 (Day 3 Control), or day 5 (Day 5 Control) of the ultimate stadium, respectively.
G. mellonella were weighed upon pupation and individually isolated until A. marmoratus pupariation, or G. mellonella eclosion or death. A. marmoratus puparia were weighed one day after pupariation and held individually until eclosion. Host remains were dissected to determine the number and status of A. marmoratus maggots. To verify that all parasitoid remains had been recovered during the initial inspection, the host remains were macerated in 10% KOH and reexamined. No additional A. marmoratus were recovered by this procedure. All parasitoid remains (bodies or exuviae) were identified to larval instar (Ravlin & Stehr 1984), and bodies were carefully inspected for signs of physical injury. The size and degree of sclerotization of the cephalopharyngeal skeletons of maggots from superparasitized hosts were compared at the stage at which the first maggot died. The A. marmoratus in each superparasitized host were classified according the developmental stage to which they survived. If neither parasitoid survived longer than the other, the competitive interaction was considered a “tie”. True winners of competitive interactions were those that actually survived to adult eclosion.
Chi square tests were used to examine differences in parasitism rates and parasitoid survival among the different treatments. To determine if parasitoid size and development were affected by parasitism treatment, data were analyzed by analysis of covariance (ANCOVA) with host pupal weight serving as a covariate. Pairwise comparisons between superparasitized treatments and their corresponding controls were made by least squares means t-tests.
Results
Because first-instar parasitoids had to reenter the host successfully after it pupated, not all G. mellonella pupae contained A. marmoratus (Table 1). However, parasitoid entries into host pupae were independent events because the proportion of superparasitized hosts across superparasitism treatments (19.8%) was approximately equal to the square of the proportion of the parasitized pupae in all control treatments (44.5%, X2 = 0.001, df = 1, P > 0.9). The first planidium that entered the host pupa did not exclude the entry of the other planidium because superparasitism rates were not significantly less than the values expected had entries been independent events. Even though approximately 20% of host pupae were superparasitized (Table 1), a maximum of one A. marmoratus survived per host.
Based on the condition of the parasitoid remains in host pupae, supernumerary parasitoids were almost always eliminated during their second stadium (test for differences among A. marmoratus instars, X2 = 133, df = 2, P < 0.001, Table 2); this difference was consistent across superparasitism treatments (test for differences among superparasitism treatments, X2 = 3.4, df = 2, P = 0.18, Table 2). Three of the A. marmoratus maggots that died as first instars (17%, n = 18) had failed to penetrate the host cuticle following host pupation. The remainder that did enter their host pupa grew (when compared with newly hatched planidia), but did not become successfully established in the host and molt. Only four (22%) of these first-instars showed signs of physical injury, such as melanized wounds. Few of those maggots that died as second instars had signs of physical injury (5%, n = 94), but they typically had smaller and/or less sclerotized cephalopharyngeal skeletons than those that survived to a later stage (75%, n = 94, X2 = 17.7, df = 2, P < 0.001). In two superparasitized pupae (2%, n = 112), the supernumerary A. marmoratus died before completing their final larval ecdysis.
Table 1. Parasitism and Superparasitism of G. mellonella Pupae by A. marmoratus When Parasitized at Different Intervals During the Ultimate Larval Stadium.
Number of A. marmoratus Maggots per G. mellonella Pupa 1See text for details of each treatment.
Adult eclosion of A. marmoratus did not differ across treatments (X2 = 10.1, df = 5, P > 0.07, Table 2), or when considering superparasitism versus single parasitism treatments (X2 = 3.6, df = 1, P = 0.06, Table 2). Overall, development times of A. marmoratus (from host pupation to A. marmoratus adult eclosion) from singly and superparasitized host pupae were not significantly different (overall x ± SE = 13.4 ± 0.2 days, F = 0.1, df = 1, 128, P = 0.80). In pairwise comparisons between superparasitized groups and their respective control groups, the only significant difference was between the Day 1, 3 superparasitized and Day 3 control groups; the controls emerged one day earlier (12.7 ± 0.3 days) than the parasitoids from the superparasitized group (13.9 ± 0.4 days) (t = 3.1, P = 0.0024).
The size of A. marmoratus puparia increased significantly with host weight (puparia from superparasitized hosts: y = 0.3 + 0.46x, r2 = 0.82; puparia from singly parasitized hosts: y = 1.3 + 0.42x, r2 = 0.73). However, puparia from singly parasitized hosts were not significantly heavier (71.1 ± 1.9 mg) than those from superparasitized hosts (68.2 ± 2.5 mg, F = 1.7, df = 1, 136, P = 0.19, test for homogeneity of intercepts). Host weight alone accounted for over 74% of the variation in A. marmoratus weights.
Discussion
A variety of mechanisms for the elimination of supernumerary larvae exist among the Tachinidae. Physical combat has been observed among first instars of Marquartia chalconota Meigen (Mellini & Baronio 1971). Anoxia is responsible for elimination of older supernumerary maggots of Lixophaga diatraeae (Townsend) (King et al. 1976). Superparasitism results in reduced body size for several potentially gregarious tachinids (Pschorn-Walcher 1971, Ziser et al. 1977, Reitz 1994). This variation in parasitoid size is attributed to resource depletion in superparasitized hosts. However, these species have significantly different life histories from A. marmoratus. Because of the high potential of superparasitism occurring in the field, and the relationship between parasitoid and host size (A. marmoratus puparia are > 50% of the weight of host pupae), A. marmoratus would be expected to have evolved an effective mechanism for eliminating conspecific competitors.
Given the consistent stage at which supernumerary maggots of A. marmoratus are eliminated and the lack of demonstrable physical injuries to these “losing” maggots, physiological suppression of conspecific competitors cannot be excluded as a mechanism of intraspecific competition. If competition were based solely on physical attacks, all “losing” maggots should show signs of injury (Mellini & Baronio 1971, Mellini 1990). Also, some encounters, especially in hosts superparasitized on the same day, could be expected to be resolved when both parasitoids were third instars.
In fact, if direct physical combat was responsible, it should occur most often among third instars. Unlike many solitary hymenopteran parasitoids that have free-roaming larvae adapted for fighting (Vinson 1985, Kfir & van Hamburg 1988, McBrien & Mackauer 1990), first and second instars of A. marmoratus reside in respiratory tunnels (Mellini 1990) that form along the wing pad margins of host pupae. Only third instars become mobile (Hughes 1975, Bratti et al. 1992). However, no host pupae contained two third instars. While second and third instars of A. marmoratus possess sickle-shaped mandibles that could inflict serious damage, any observed damage could have occurred after a competitor had already died from other causes. In addition, second instars are considerably smaller than G. mellonella pupae; therefore, unless parasitoid respiratory attachments are in close proximity, second instars would not encounter one another, further increasing the occurrence of encounters among third instars.
The possibility that supernumerary maggots are eliminated through scramble competition for host resources is also not supported by the present data. If scramble competition was operating, greater variation in the stage at which competitors are eliminated would be expected. In particular, larger hosts should more frequently support multiple third instars. Additionally, if scramble competition was responsible for elimination of supernumerary maggots, the size of A. marmoratus should vary with the number of maggots present in a host pupa. However, the relationship between A. marmoratus weights and host pupal weights did not vary with respect to whether a host was singly or superparasitized. Also, adult eclosion rates did not differ significantly as might be expected as a result of scramble competition.
A possible scenario for physiological suppression of supernumerary A. marmoratus is that older maggots (i.e., those parasitizing the host larva first) are more developmentally advanced, and initially have faster development rates in the host pupa, thus molting to the second and third instar sooner than subsequent maggots (Bratti et al. 1992, 1993). Because the host pupa dies by the time an A. marmoratus maggot molts to the third instar (Allen 1926), the maggot that molts to its final instar first could make the host environment unsuitable for younger maggots to continue their development. Therefore, maggots reaching their final instar first could suppress competitors by production of proteolytic enzymes (Mellini 1990) or degradation of the host. Further in vitro studies of intrinsic competition should elucidate the specific means of physiological suppression used by A. marmoratus.
Acknowledgments
A. Bratti, M. Mariani and the Staff of the Istituto di Entomologia “Guido Grande”, Universita degli Studi di Bologna provided valuable technical assistance. I appreciate the assistance of P. H. Adler, L. Correlli-Grappadelli, K. A. Luhring, and J. T. Trumble in making this study possible, and the comments of two anonymous reviewers which have improved this manuscript. This study was made possible with financial support from a W. C. Nettles Memorial Grant, a Sigma Xi Grant-in-Aid of Research, and a Florida Entomological Society Scholarship.
References Cited
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Arnaud, P. H., Jr. 1978. A host-parasite catalog of North American Tachinidae (Diptera). United States Dept. Agric. SEA Misc. Publ. No. 1319, 860 pp.
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Bratti, A., W. C. Nettles, Jr., and P. Fanti. 1992. Influence of Helicoverpa zea (Lepidoptera: Noctuidae) age during the last instar on rates of parasitization by the larval-pupal parasitoid, Archytas marmoratus (Diptera: Tachinidae). Environ. Entomol. 21: 1196-1201.
Campadelli, G. 1973. Allevemento di Galleria mellonella L. con dieta semiartificiale. Boll. Ist. Entomol. Univ. Bologna 32: 2-25.
Fisher, R. C. 1971. Aspects of the physiology of endoparasitic Hymenoptera. Biol. Rev. Cambridge Philos. Soc. 46: 243-278.
Hughes, P. S. 1975. The biology of Archytas marmoratus (Townsend). Ann. Entomol. Soc. America 68: 759-767.
Kfir, R., and H. Van Hamburg. 1988. Interspecific competition between Telenomus ullyetti (Hymenoptera: Scelionidae) and Trichogrammatoidea lutea (Hymenoptera: Trichogrammatidae) parasitizing eggs of the cotton bollworm Heliothis armiger in the laboratory. Environ. Entomol. 17: 664-670.
King, E. G., L. R. Miles, and D. F. Martin. 1976. Some effects of superparasitism by Lixophaga diatraeae of sugarcane borer larvae in the laboratory. Entomol. Exp. Appl. 20: 261-269.
McBrien, H., and M. Mackauer. 1990. Heterospecific larval competition and host discrimination in two species of aphid parasitoids: Aphidius ervi and Aphidius smithi. Entomol. Exp. Appl. 56: 145-153.
Mellini, E. 1990. Sinossi di biologia dei Ditteri Larvevoridae. Boll. Ist. Entomol. Univ. Bologna. 45: 1-38.
Mellini, E., and P. Baronio. 1971. Superparasitismo sperimentale e competizioni larvali del parassitoide solitario Marquartia chalconota Meig. Boll. Ist. Entomol. Univ. Bologna 30: 133-152.
Pschorn-Walcher, H. 1971. Experiments on inter-specific competition between three species of tachinids attacking the sugar cane moth borer, Diatraea saccharalis (F.). Entomophaga 16: 125-131.
Ravlin, F. W., and F. W. Stehr. 1984. Revision of the genus Archytas (Diptera: Tachinidae) for America north of Mexico. Entomol. Soc. America Misc. Pub. 58, 58 pp.
Reitz, S. R. 1994. Reproductive biology of Eucelatoria bryani and Eucelatoria rubentis (Diptera: Tachinidae), larval parasitoids of Helicoverpa zea (Lepidoptera: Noctuidae). Ph. D. Thesis, Clemson University.
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Table 2. Developmental Stage of A. marmoratus in G. mellonella Pupae Superparasitized at Different Time Intervals During the Ultimate Larval Stadium. Table Values Reflect the Final Developmental Stage Reached by Competing Parasitoids. No Hosts Produced Parasitoids that Survived to the Same Stage.
A. marmoratus Stage1 1(A) Stage reached by parasitoid surviving the longest, (B) Stage reached by first parasitoid that died. Figures 1-4. Scanning electron micrographs of dorsal and lateral views of Parvitermes collinsae n. sp. minor (1, 2) and major (3, 4) soldiers.
Figures 13-14. Scanning electron micrographs of P. collinsae (13) and P. wolcotti (14) right molar plates of worker mandibles.
Figure 15. Dorsal (D), right (R), ventral (V), and left (L) configurations of the digestive tube in situ of P. pallidiceps worker. CP, crop; M, mesenteron (stippled) including mesenteric part of MS, mixed segment; O, oesophagus; P1, first proctodeal segment; P2, enteric valve; P3, paunch; P4, colon; R, rectum; T, Malpighian tubules (darkly stippled). Scale bar = 0.5 mm.
University of California
Riverside, CA 92521
Treatment1
1 Maggot
2 Maggots
Total Number of Pupae Tested
Superparasitized
Day 1, 1
93 (36%)
47 (18%)
261
Day 1 Control
48 (41%)
—
118
Superparasitized
Day 1, 3
87 (44%)
40 (20%)
196
Day 3 Control
56 (62%)
—
91
Superparasitized
Day 1, 5
47 (44%)
25 (23%)
108
Day 5 Control
44 (36%)
—
123
Total
375
112
897
Number and Percent of Outcomes for Each Treatment2
(A)
(B)
Superparasitized
Day 1, 1
Superparasitized
Day 1, 3
Superparasitized
Day 1, 5
Total
Adult
II Instar
18 (40%)
19 (42%)
8 (18%)
45
Adult
I Instar
8 (57%)
3 (21%)
3 (21%)
14
Pupae
II Instar
10 (36%)
12 (43%)
6 (21%)
28
Pupae
I Instar
1 (50%)
0 (0%)
1 (50%)
2
III Instar
II Instar
9 (41%)
6 (27%)
7 (32%)
22
III Instar
I Instar
1 (100%)
0 (0%)
0 (0%)
1
Total
47 (42%)
40 (36%)
25 (22%)
112
2See text for details of each treatment.