The results of this study provide the first detailed information about the infection behaviors and life history of any species of Emblemasoma besides E. auditrix. Both laboratory and field observations reveal that E. erro find their hosts by eavesdropping on the sexual communication signals of male cicadas. When a female E. erro locates a calling cicada, she waits to attack until the host is in motion, and larviposition on flying cicadas is not uncommon. The results also show that male T. dorsatus are commonly parasitized by E. erro and that there can be substantial variation in population parasitism rates and parasitoid loads. I next discuss the behavior and life history of E. erro, especially in comparison to other acoustic parasitoids and other sarcophagid parasitoids; assess possible host defenses; and discuss possible causes of variation in host parasitoid loads and parasitism rates.
Host locating and infection behaviors of E. erro
E. erro’s use of phonotaxis to locate potential hosts is similar to that reported for other acoustically hunting parasitoids (Soper et al. 1976; Lehmann 2003; Lakes-Harlan and Lehmann 2015), but E. erro’s preference for attacking moving targets is apparently unique among known acoustic parasitoids. For example, E. auditrix, the only other Emblemasoma for which larviposition behaviors are known, will aggressively attack stationary or restrained cicadas. Upon finding a male cicada, female E. auditrix exhibit a stereotyped behavioral sequence in which the female fly immediately attempts to squeeze underneath the perched cicada’s wings to gain access to the cicada’s timbal region. She then uses specialized terminal abdominal sternites to cut through the cicada’s timbal membrane and injects larvae directly into the host’s body (Schniederkötter and Lakes-Harlan 2004). Female E. erro lack any comparable abdominal modifications, but larvipositing through the host’s timbal would likely be impossible for E. erro anyway, because male Tibicen dorsatus have timbals that are fully protected by well-developed timbal covers. In contrast, E. auditrix’s host cicada, O. rimosa, lacks timbal covers entirely.
Tachinid acoustic parasitoids of the tribe Ormiini will also attack stationary hosts, and they will even larviposit without visual or tactile confirmation of a host’s location. For example, Homotrixa alleni Barraclough, Ormia depleta (Wiedemann), and O. ochracea (Bigot) will all deposit larvae at a sound source regardless of whether or not a potential host insect is actually present (Cade 1979; Fowler 1987; Allen et al. 1999). For E. erro, the host’s calling song was never sufficient by itself to trigger larviposition, even when a potential host was present. In contrast to E. erro, ormiine tachinids are all nocturnal parasitoids of Orthoptera, and their willingness to larviposit in the absence of a host probably reflects an almost total reliance on acoustic cues at night. For acoustic parasitoids such as E. erro that are active during the day, requiring visual confirmation of a suitable host prior to larviposition allows for more precise placement of larvae and undoubtedly decreases the number of larvae that are wasted by the female fly.
In comparison to the larviposition behaviors of other acoustic parasitoids, E. erro’s tendency to attack flying cicadas is especially striking. One third of the successful attacks observed in the experiment cages took place while the cicada was in flight, but this is almost certainly an underestimate of the true frequency of flight-based attacks in nature. Due to the size of the cages used in the trials, most attempts by flies to follow cicadas in the air resulted in failure because the cicada crashed into a side of the cage before the fly could approach and orient itself to the flying cicada. It was hoped that the large’flight cage’ would alleviate this problem, but even it appeared to be too small for most aerial attacks to succeed. Nevertheless, flies seemed much more reluctant to attack potential hosts that were not in flight.
This conclusion is further supported by observations in the field, where nearly all apparent larviposition attacks occurred while cicadas were in flight. Flies sometimes even followed a single cicada from perch to perch, waiting patiently next to the cicada each time it landed, but never attempting to attack while the cicada was not flying. As an example, in 2013, I observed a male T. dorsatus calling from a grass flowering culm with a female E. erro perched on the opposite side of the stalk near the cicada’s abdomen. The fly was nearly motionless until the cicada backed a short distance down the stalk, causing the fly to move with him nearly in unison, but the fly made no move to attack the cicada. When the cicada flew a short distance (approximately 1 to 2 m) to a new perch, the fly closely followed him in the air, landed next to the cicada, and again remained nearly motionless while the cicada began calling. The cicada flew twice more, with the fly following both times, and after the final flight of at least 30 m, I captured the cicada and later reared two E. erro larvae from it.
While E. erro’s behavior of larvipositing on hosts while they are in flight or otherwise in motion might be different from E. auditrix and tachinid acoustic parasitoids, it is remarkably similar to the larviposition behaviors reported for some sarcophagid parasitoids of the genus Blaesoxipha that parasitize acridid grasshoppers. B. aculeata (Aldrich), B. caridei (Brethes), B. kellyi (Aldrich), B. redempta (Pandellé), and B. reversa (Aldrich), among others, have all been reported to attack grasshoppers while in flight (Coquillett 1892; Kelly 1914; Aldrich 1916; Lloyd 1951; Rees 1973; Povolný and Verves 1997). Kelly (1914) provided a detailed description of the larviposition behaviors of B. kellyi, reporting that grasshoppers were attacked either on the wing or on the ground, and that grasshoppers were only attacked when they were in motion (but see Smith 1915). Furthermore, both B. kellyi and B. reversa typically place larvae near the base of a host’s wings, much like E. erro (Kelly 1914; Rees 1973).
It is worth noting that early last century, Beamer (1928) and Kelly (1914), both working in Kansas, reported seeing cicadas pursued by flies while in flight. Beamer noted that ‘the flies follow but a few inches away, and sometimes seem almost to alight on the body of the cicada.’ Although their observations were largely adventitious and incidental, and neither author identified the flies involved, it seems plausible in retrospect that their papers might have been the first published records of E. erro’s host infection behavior.
Infection of female hosts
Given E. erro’s primary host-finding mechanism, male cicadas are clearly the primary targets of infection by this parasitoid. However, the observation of a fly larvipositing on a female cicada in the laboratory, along with the 2014 survey of female T. dorsatus in the field, confirms that female cicadas are also sometimes attacked.
Since female cicadas are silent, how are they discovered by E. erro in the field? One possibility is that, simply by chance, they happen to fly within the visual range of a perched female E. erro. Perhaps more likely, though, female T. dorsatus and female E. erro might sometimes encounter one another while seeking male cicadas. Like E. erro, female cicadas perform phonotaxis in response to males’ calls, so female cicadas could become parasitized if they were attracted to the same calling male as a female E. erro. In any case, despite many hours spent observing cicadas in the field, I never witnessed any interactions between female E. erro and female T. dorsatus, so such encounters must be rare in comparison to encounters between male cicadas and female E. erro. However, E. erro’s occasional use of female hosts is not unique. Several other species of acoustic parasitoids that primarily attack male hosts are also known to sometimes parasitize females (Soper et al. 1976; Lehmann 2003).
Phenology and fecundity of E. erro
Little is known of the seasonal phenology of E. erro. In this study, adult flies were observed in the field as early as June 13 (in 2012) and as late as September 4 (in 2014), and these were also the earliest and latest dates that I attempted to find them. The rearing data strongly suggest that E. erro is multivoltine in the geographic area covered by this study. With a total development time from larviposition to adult eclosion of about 22 days, it seems possible that there could be at least three generations per year. E. auditrix, in contrast, is apparently univoltine (Soper et al. 1976; de Vries and Lakes-Harlan 2005).
Female E. erro were observed with as many as 174 first-instar larvae, nearly 3.5 times the maximum of 50 observed for E. auditrix (De Vries and Lakes-Harlan 2005). The apparently large difference in fecundity between these two species might be at least partially explained by their larviposition behaviors and life histories. E. auditrix deposits larvae directly inside a host’s body, one larva per host, and all available evidence suggests that E. auditrix is a solitary parasitoid (Soper et al. 1976). By injecting larvae into its hosts, E. auditrix likely ‘wastes’ relatively few larvae during larviposition, and as a solitary parasitoid, it is plausible that multiple larvae inside a single host would physically attack one another (Godfray 1994). Under these conditions, females might benefit by producing fewer, larger larvae to increase their chances of survival. In contrast, because E. erro deposits its larvae on the exterior of a host, it is likely that some percentage of these larvae never manage to make it inside the host’s body. Moreover, E. erro is a gregarious parasitoid, and as such, larvae probably face little direct physical aggression from conspecifics (Godfray 1994). For E. erro, then, investing fewer resources in more larvae might increase a female’s lifetime reproductive success. Some tachinid acoustic parasitoid species, which deposit their larvae even more haphazardly, also have large larval complements (Wineriter and Walker 1990; Allen et al. 1999; Kolluru and Zuk 2001), and although behavioral data for other sarcophagid parasitoids is extremely limited, at least some parasitoid species in the genus Blaesoxipha also appear to follow this pattern (Middlekauff 1959).
Host defenses and mortality
Once discovered by a female E. erro, male T. dorsatus appeared to have relatively few viable options to defend themselves. When approached by a parasitoid fly, calling male T. dorsatus cicadas responded either by flying, immediately terminating their call and remaining motionless on their perch (hereafter referred to as ‘hiding’), or simply continuing their calling behavior. The latter seemed to be the most common. Cicadas often called repeatedly and walked freely about the walls of the experiment cages despite being followed by a fly only a few centimeters away. Cicadas sometimes even called with a fly perched right on top of them. However, stationary cicadas that were directly contacted by a fly would often vigorously flick their wings to try to repel the parasitoid. Unfortunately, given the relatively small space inside the cages, evaluating the effectiveness of any of these behaviors was nearly impossible because a cicada could never truly escape from the fly.
Nevertheless, observations in the field suggested that both the flight and hiding strategies do sometimes work. In at least one case, a fly lost interest in a hiding cicada and left before the cicada resumed calling, and in another, a cicada that was contacted by an approaching fly managed to escape by flying away. Most of the time, though, flies simply waited until a hiding cicada became active again, and they usually had little difficulty in following a flying cicada from one perch to another. As a defensive strategy, flying seems especially risky given E. erro’s aptitude for aerial larviposition.
After being larviposited upon, cicadas had yet another option for defending themselves. I repeatedly observed cicadas perform ‘wing flipping’ behavior immediately after being attacked, characterized by rapidly flapping their wings several times while perched. In this way, one cicada managed to completely dislodge the single larva that had been deposited on the cicada’s right fore wing, thus avoiding infection completely. This was the only case for which I confirmed that a cicada was able to remove all larvae from its body, but it is possible that some larviposition events were not detected during the behavioral experiments. Wing flipping by T. dorsatus appears to be functionally similar to the grooming behaviors used by the cricket Gryllus texensis Cade and Otte to prevent infection by the larvae of Ormia ochracea (Vincent and Bertram 2010).
Although the hosts of some other sarcophagid parasitoids have been reported to occasionally survive parasitism (Spencer and Buckell 1957; Danyk et al. 2000), infection by E. erro appears to be invariably fatal for T. dorsatus. In most cases, hosts died several hours before the parasitoid larvae emerged. Host death was usually preceded first by loss of wing function, then loss of leg function beginning with the hind legs and ending with the fore legs. Prior to death, a cicada’s antennae were typically the last appendages to display a visible response to external touch. After a cicada died, small, rhythmic movements of the legs or head capsule were often visible as the parasitoid larvae used their oral hooks to scrape muscle and other soft tissue from the integument.
Sometimes, though, when a cicada was infected with only a single larva, the larva emerged before the cicada died, leaving the host in a severely weakened, moribund state. Cicadas in this condition usually succumbed after a few hours. In one exceptional case, a large male T. dorsatus from the Prowers Co., CO site that was infected with a single E. erro larva survived for more than 24 h following parasitoid emergence. Although sluggish, it was still able to cling to and crawl on a perch, weakly flutter its wings (but not fly), and was even observed attempting to feed before its movements became uncoordinated and it, too, died. Overall, E. erro must be a major cause of mortality for adult male T. dorsatus, especially considering the very high parasitism rates observed in some cicada populations.
Variation in host parasitism rates among study sites
Host populations at the two westernmost field sites appeared to have consistently higher parasitism rates than sites further east (Table 1, Figure 3). The biogeography of potential host cicadas might offer one explanation for this pattern. The western sites were located on the semi-arid High Plains, where there are fewer species of large cicada present than on the more mesic midgrass prairies of the study sites further east. E. erro parasitizes other cicada species besides T. dorsatus (B. Stucky, in prep.), so higher parasitism rates of T. dorsatus on the High Plains could be a consequence of local differences in the communities of potential host species.
However, as noted in the ‘Results’ section, because these western sites were also sampled later in the season than the eastern sites, higher parasitism rates could have also been caused by seasonal effects rather than intrinsic differences among the sites. One might expect parasitism rates to increase throughout the season as E. erro populations reach their peak and host populations decline, as has been observed for several other species of dipteran parasitoids, including some acoustic parasitoids (e.g., Tamaki et al. 1983; Allen 1995; Lehmann 2008). It seems likely that this accounts for at least some of the among-site differences in parasitism rates found in this study. Furthermore, both host and parasitoid population sizes undoubtedly also play a role in determining parasitism rates. As evidenced by some of the small population sample sizes, host cicadas were uncommon and difficult to collect for some years at some field sites, which suggests that there was variation in host population sizes from year to year. Future studies that estimate host and parasitoid population sizes and sample both High Plains and central Plains sites multiple times throughout the season will be needed to fully disentangle the effects of these variables on host parasitism rates.
Superparasitism by E. erro
The strong, positive relationship between parasitism rate and parasitoid load (Figure 8), as well as the significant difference between the mean parasitoid load of field-collected hosts and the mean clutch size of larvipositing females (4.97 and 2.53 larvae/host, respectively), can both be explained as a consequence of superparasitism in the field. If at least some host cicadas are superparasitized in the field, then we should expect the mean parasitoid load of host cicadas to be larger than the mean clutch size of individual female flies. Furthermore, for gregarious parasitoids such as E. erro, superparasitism is expected to be more common when unparasitized hosts are rare, simply because female parasitoids have a harder time finding hosts that have not already been infected (Godfray 1994). Unparasitized hosts are rare when parasitism rates are high, so higher parasitism rates should correspond with increasing rates of superparasitism. Increased superparasitism would, in turn, likely result in larger parasitoid loads per host, which means that higher population parasitism rates should correspond with higher parasitoid loads. This prediction matches the pattern of the data quite well (Figure 8).
Additionally, anecdotal evidence of superparasitism was found in the relative sizes of larvae emerging from some of the most heavily parasitized hosts. In some cases, two distinct larval size classes were evident, presumably due to the smaller larvae having been deposited on the host later than the larger larvae. In other cases, though, all larvae emerging from heavily parasitized hosts were approximately the same size, suggesting that either a single female deposited all of the larvae at once, or more likely, that two (or more) female flies discovered an uninfected host at nearly the same time.