Encounters with camel crickets are common. In North America and Europe, camel crickets have long been noted in basements and cellars. Among the earliest examples of cave art is a carving of what appears to be a camel cricket on a bison bone. Clearly the relationship between humans and camel crickets is ancient. However, But these animals are far more likely to be encountered by citizens than they are, it seems, to be studied by scientists. As a result, while camel crickets are often observed as household visitors noted in houses, their biology is not well documented.
Camel crickets (Rhaphidophoridae) are a moderately diverse family of Orthoptera (the order of katydids, crickets and grasshoppers), represented by ca. 150 species (23 genera) across North America (Arnett 2000) and xxx species in the world. In North America, several species in the large genus Ceuthophilus (C. brevipes, C. agassizii, C. latens, and C. maculatus) have been reported as common or occasional inhabitants of human dwellings, particularly in cellars and basements (Vickery & Kevan 1983).
In nature, most species of Ceuthophilus (including those occurring in our houses) are found under rocks, logs, or surface debris in forested areas, although a few grassland species are also known (Vickery & Kevan 1983). The majority appear to be nocturnal (Vickery & Kevan 1983), hiding in protected spaces during the day to emerge and feed at night. Other Ceuthophilus are common residents of caves (e.g., C. carlsbadensis, C. longipes, C. secretus, and others), where the droppings and carcasses of these crickets are a major source of energy for troglobites (those organisms that live in caves). For this reason camel crickets are considered keystone species in many cave ecosystems (Lavoie et al. 2007, Taylor et al. 2005). Although our cellars and basements might in some ways be analogous to cave ecosystems, most native camel crickets found in our houses appear to be distinct from those typically collected in caves (Vickery & Kevan 1983). However, in at least one case (a camel cricket endemic to Tuscany, Italy, Dolichopoda schiavazzii), populations of an otherwise cave-inhabiting species may be found in subterranean environments of human origin, including cellars, tombs, and catacombs (Allegrucci et al. 1997).
In addition to a rich diversity of native camel crickets, at least one non-native camel cricket species has become established in North America. The ‘greenhouse camel cricket,’ Diestrammena asynamora (Adelung) (syn. D. marmorata Scudder, Tachycines asynamorus Adelung, Locusta marmorata Haan) is a species native either to Japan or the Sichuan region of China, and was first recorded in North America in 1898 from a greenhouse in Minnesota (Rehn 1944). D. asynamora was subsequently noted in many locations across the eastern and central United States and Canada, as well as throughout much of Europe. Many authors have considered this species to be associated largely with greenhouses (e.g. Bue & Munro 1939, Rehn 1944, Vickery & Kevan 1983), although a few early reports also document its presence in cellars (Rehn 1944). Contemporary reports of D. asynamora show that it is present in some basements, though it is uncertain whether the reports of this introduced cricket represent (a) isolated cases of localized abundance, or (b) a more extensive invasion. The Asian camel cricket is easily distinguished from native North American species such that photos provide a reliable measure of its presence/absence.
One challenge with studying species of any kind in homes is that privacy concerns make these areas homes relatively difficult to sample; people are, for good reason, reluctant to let strangers (particularly those with nets) into their basements. Surveys have the potential to take advantage of individuals’ abilities to study their own homes, while simultaneously engaging the public in science. However, obtaining accurate identifications of organisms from public survey data can be challenging. Although as a group camel crickets are distinctive in both morphology and behavior, untrained citizens can make mistakes in distinguishing camel crickets from other household arthropods (e.g., field crickets or even roaches or spiders). For this reason, photographic documentation is an invaluable addition to public survey data, and provides an easy way to confirm the presence of camel crickets in many of our homes. In addition, many of the characters that distinguish camel crickets at the species level may be visible in such photographs (e.g., coloration and tibial structure/armature). Because camel crickets include both introduced species and geographically and locally rare species, it is possible that basements and cellars might be important habitats for the spread of an introduced species and/or the persistence of native species.
In this study, we use citizen-contributed data to offer new insight into the distribution and composition of camel crickets taking shelter in our homes. Initially, in order to understand how common camel crickets are in houses, we surveyed citizens across the United States about the presence of camel crickets living in and around their homes. We then solicited photos of camel crickets from households with these insects present to evaluate the occurrence and geographical distribution of native versus nonnative species in homes. These survey results were augmented with trapping efforts to compare the composition of camel crickets living in houses to those present in urban yards.
We employed two types of surveys to understand the geographic distribution and composition of camel crickets in houses across the United States. First, as part of a broader study about the biology of organisms living in human homes (Missing Scenes of Nature project) we solicited volunteers across the United States to answer questions about organisms living in their houses. The survey included a question about whether participants had seen camel crickets in or around their homes; data were collected from December 2011 through July 2013. These survey data might have been biased in as much as only those individuals with camel crickets may have responded. As a result, we conducted a second survey in which each of the individuals participating in the Wild Life of Our Homes project (in which they sampled microbes in their homes) was required to fill out a questionnaire that included a question about the presence of camel crickets in their home. This second survey was run from October 2012 through April 2013 and included participants from all 50 states and the District of Columbia. Both the results of the original and subsequent surveys were used to map the presence of camel crickets in North American homes. Maps were created using the software Geocommons (geocommons.com) and Cartodb (cartodb.com). Only the second data set was used to estimate the prevalence of the crickets.
In order to understand the relative distribution of native versus nonnative camel crickets we next solicited photographs (or physical specimens) of camel crickets from citizen volunteers who had these insects in their homes. These volunteers included a subset of participants in the survey described above, as well as additional volunteers responding to an appeal for participation on a public science website for the project. We examined each photograph and/or specimen and identified it as D. asynamora or Ceuthophilus sp. based on relevant characters such as tibial armature described in Vickery and Kevan (1983).
Finally, to understand whether the Asian camel cricket, D. asynamora, is (a) living only in houses, (b) living in houses but foraging outside, or (c) living outside but foraging inside, we sampled camel crickets in a subset of urban yards at increasing distances from homes known to contain camel crickets. In July of 2013, ten participating households were recruited from central Raleigh, NC, and pitfall traps were placed in the yard of each. We constructed pitfall traps using plastic cups (7 cm across by 10 cm deep) and placed three traps per yard at distances of 1m, 4m, and 8m from each house along a haphazardly placed transect line. Traps were baited with a 1:1 dilution of molasses and water as per the methods of Hubbell (1936). We placed inverted plastic bowls elevated approximately 3 cm over the mouth of each trap to protect traps from rain and to encourage camel cricket visitation by offering cover. Contents of traps were collected daily and traps were left in place for two days. In some yards small mammals would disturb the traps, in which case we replaced the molasses bait with soapy water in all three traps so as to be less attractive to mammalian pests. We sorted the contents of each trap in the laboratory and identified all camel crickets to species with the aid of a dissecting microscope. We then used analysis of covariance (ANCOVA) linear regression to test for a relationship between the number of D. asynomora individuals in a trap and its distance from a house. Yards in which no camel crickets were recovered at any of the three traps were excluded from analysis. Statistical Regression analysis was performed in JMP v. 10.0 (SAS Institute, Cary, NC).
Individuals from 575 homes responded to our initial survey question about the presence or absence of camel crickets in or around their houses, offering positive reports of camel crickets for 244 homes across the country. An additional 1,719 households responded to the Wild Life of Our Homes questionnaire. Of these, 24.4% affirmed the presence of camel crickets (Table 1). Together, these surveys allowed us to evaluate the distribution of camel crickets associated with human houses across the United States. Participants from 39 states and the District of Columbia reported observing camel crickets in or around their homes (Fig. 1). Based on the proportion of photographs showing insects incorrectly identified as camel crickets by citizen scientists who responded to our call for photographs (see below), we estimated a 4.9% error rate associated with affirmative reports of camel crickets from all survey data.
Figure 1. Map of responses to the Wild Life of Our Homes survey question asking citizens about whether or not they have observed camel crickets in their houses. Red dots represent positive reports of camel crickets found in homes (N = 420), whereas blue dots indicate households where camel crickets have not been knowingly observed (N = 1,299).
Table 1. Responses to the Wild Homes survey by state, showing the percentage of households answering ‘yes’ to the question ‘Have you seen camel crickets in or around your home?’ Results are presented in order of decreasing prevalence.
Citizen scientists from 150 90 64 households submitted identifiable photographs and/or specimens of camel crickets from their houses. Submissions spanned 20 18 states and the District of Columbia, as well as and one Canadian province [numbers will be updated as new individuals participate], although the greatest proportion of submissions a majority (37% 39%) were from North Carolina. Out of all identifiable camel cricket submissions, 89% Of these, 86 85% of houses submitted evidence of the Asian D. asynamora whereas only 11% 17 19% of houses reported records were members of the native genus Ceuthophilus (Table 2, Fig. 2). In three two cases, evidence of both native and non-native Rhaphidophoridae were contributed from the same home. Out of all photographic submissions from citizen scientists, over 95% were correctly identified by citizen scientists as members of the Rhaphidophoridae.
Figure 2, Version 2. Showing the distribution of native Ceuthophilus spp. (red points; N = 17 15 12) versus the Asian D. asynamora (blue points; N = 134 77 54) found in homes, based on photographic and specimen submissions contributed by citizen scientists. (Figure 2, Version 2 (16 Oct 13); Original Figure 2)
Table 2, Version 3 (Revised 28 Jan 14). Results of citizen-contributed photographic or specimen submissions showing the relative number of households with the Asian Diestrammena versus native Ceuthophilus samples by state or Canadian province. Two houses in North Carolina each contributed specimens of both genera. (Table 2, Version 2 (16 Oct 13); Original Table 2)
We recovered a total of 158 camel cricket individuals in pitfall traps from urban yards in Raleigh, NC. Camel crickets were recovered from seven of the ten yards sampled, with as many as 52 individuals recovered from a single yard in the two day sampling period. All recovered specimens of Rhaphidophoridae were identified as the Asian species D. asynamora. In the case of three yards, no camel crickets were found in any traps, and these yards were excluded from analysis. The number of D. asynamora individuals recovered in traps was negatively correlated with a trap’s distance from a house (R2 = 0.66, P = 0.004 from ANCOVA after ln-transformation of the number of cricket individuals and after accounting for individual yard, as would reflect variation in local abundances of camel crickets = 0.18, P = 0.019 from linear regression after ln-transformation of the number of individuals; Fig. 3).
Figure 3. The number of D. asynamora individuals recovered from a pitfall trap was negatively correlated with the distance of a trap from a house (R2 = 0.66 0.18, P = 0.004 0.019 from ANCOVA, after accounting for individual yard effect linear regression). Each dot represents a single pitfall trap (N = 21 30, with three traps at each of seven ten yards). (Figure 3, 16 Oct 13)
Although camel crickets are common and include several species that may take up residence in our homes, little is known about the biology of these insects in association with human development. Using data contributed by citizen scientists, we found that camel crickets have a widespread distribution in houses across much of the continental United States (Fig. 1) and have been observed on average in nearly one of four American homes. Survey reports confirmed the widespread presence of camel crickets in homes particularly across the eastern half of the United States, the southwest, and west coast. Camel crickets were not reported in homes throughout much of the mountain west, although responses from this region were somewhat sparse relative to more populated parts of the country (Fig. 1, Table 1). Camel crickets were most prevalent in houses in the southeastern United States, with nearly 50% of households surveyed in Virginia, North Carolina, Missouri, Mississippi, Maryland, and Tennessee reporting the presence of camel crickets in their homes (Table 1).
Citizen scientists’ submissions of identifiable photographs and specimens of camel crickets found in their homes revealed that the Asian camel cricket D. asynamora has become a successful and widespread invader throughout the eastern United States (Fig. 2). Furthermore, across much of this region this species appears to be a much more common occupant of human homes compared to the native Ceuthophilus spp. (Table 2). For example, in North Carolina, the state for which we have the richest data, D. asynamora was present in 93% 95% comprised 92% of houses with camel cricket samples submitted (Table 2). The results of pitfall trapping in urban yards indicate that this species also can be extremely abundant (at least locally), with more than 50 individuals found over two days of sampling in a single yard in Raleigh, North Carolina. Photographic submissions showing large masses of Diestrammena individuals (dozens within a few cm) offer additional evidence as to the abundance of this invader, and indicate the possibility of a semi-gregarious habit in these crickets. How far this species ranges outside of the eastern United States is unclear, though this may be revealed as we obtain more identifiable submissions of camel crickets from homes in more western regions. Although more data from other regions are needed to test this hypothesis, the large number of D. asynamora reports from houses in the eastern United States may partly explain the greater prevalence of camel crickets in eastern homes (see Table 1).
Pitfall trapping of camel crickets in urban yards revealed that D. asynamora is not restricted to house environments, but is also a common forager in adjacent yards. Whether these same individuals trapped in yards are also moving in and out of houses is unclear. However, the fact that these crickets were significantly more abundant in traps placed within a meter of the house (Fig. 3) suggests that D. asynamora may be closely associated with human dwellings even when found in outdoor habitats. Surprisingly, no native camel crickets were recovered from any of these traps, despite the fact that molasses has been shown in other work to be highly profitable bait for sampling Ceuthophilus spp. (Hubbell 1936). This indicates that in some localities D. asynamora may be the dominant camel cricket not only in houses but also in urban yards. However, it is yet unclear whether D. asynamora has also invaded wilder habitats with less human disturbance, or if in North America the species persists exclusively in habitats in and around anthropogenic structures. The extent to which D. asynamora has actually displaced or is actively competing with native populations of Ceuthophilus (a genus that includes a number of rare or sensitive species) is also unknown, and further study is needed to determine whether this new invader poses an ecological threat, or is a merely a harmless visitor in our houses and yards.
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