Life history descriptions of two aquatic Florida moth species (Lepidoptera: Crambidae)

1 Introduction

Butterflies and moths (Lepidoptera) include approximately 157,500 described species (Mitter et al. 2017), most of which have terrestrial larvae. They are well adapted to the terrestrial environment, with adaptations such as adult flight, pollination, and breathing air. However, several lepidopteran lineages have successfully evolved ecological strategies to live an aquatic or semi-aquatic life in their immature stages. It is estimated that approximately 5 % of the currently described Lepidoptera species are truly aquatic (Pabis 2018), having at least one life stage that occurs under water. An even greater number of species have a looser association with aquatic environments; for instance, they may feed on aquatic plants without being submerged themselves or live in flood zones. Lepidoptera families with underwater stages include, but are not limited to, Cosmopterigidae, Crambidae, Erebidae, Noctuidae and Pyralidae (Adis 1983; Solis 2019). Aquatic and semi-aquatic moths display a large variation in life history, including stem boring, case making, and free living on plants (Lange 1956; Solis 2019).

The majority of truly aquatic species are found in the Crambidae. The crambid subfamily Acentropinae includes 800 described species (Mitter et al. 2017), nearly all of which are aquatic as larvae (Pabis 2018; Solis 2019). Nevertheless, details on how they have adapted to the aquatic environment remain largely unknown. Some aquatic and semi-aquatic species also are found in other, largely terrestrial crambid subfamilies such as the Pyraustinae (Solis 2019). Two crambid species, the aquatic waterlily leafcutter (Elophila obliteralis Walker, Acentropinae) and the semi-aquatic salvinia stem borer (Samea multiplicalis (Guenée), Spilomelinae) co-occur commonly in ponds and other slow-moving natural water environments in the southeastern United States (e.g., Center et al. 2002a; Tewari and Johnson 2011). Even though both species have been studied as potential biological control agents for invasive aquatic plants, our understanding of their life history, particularly their silk use, in the context of their aquatic lifestyle is limited.

Elophila Hübner is a highly polyphagous genus and is one of the only genera of aquatic Pyraloidea found on all continents except Antarctica (Mey and Speidel 2008). E. obliteralis is native to North America with a broad distribution in the eastern and midwestern United States and Canada (Habeck et al. 2021). Over the last century, this species also has been introduced to Hawaii, the UK (Habeck et al. 2021) and South Africa (Agassiz 2012), while potential records from India need confirmation (Sarkar 2022; Sen et al. 2019). It exhibits a fully aquatic lifestyle as a caterpillar and has been reported to feed on more than 50 aquatic plant species (Table 1 and references therein). Eggs are deposited underwater on the abaxial side of the host plant leaf in small ribbons (Dyar 1906), and caterpillars are generally submerged within proximity of the water surface. Soon after hatching, caterpillars generate an enclosure by chewing off a leaf fragment from their host plant and attaching it to the abaxial side of a leaf. This enclosure is used as shelter during the early stages of their lives (Dyar 1906; Gill et al. 2008). As they grow, caterpillars eventually generate a free-floating case, often consisting of two equally sized oval host plant pieces (Dyar 1906), which has led some to nickname this species “the sandwich man” (Habeck et al. 2021). E. obliteralis has been used as a biological control agent for invasive aquatic plants (e.g., Habeck et al. 2021; Tomasko et al. 2022) and can occasionally cause significant damage to ornamental plants, such as water lilies (Habeck et al. 2021).

Samea multiplicalis is distributed from Brazil and Argentina to the southeastern United States (Center et al. 2002a; DeLoach et al. 1979) and was introduced in Australia as part of a biocontrol effort to control Salvinia molesta D.S. Mitchel (Salviniaceae) (Sands and Kassulke 1984). It belongs to a small clade of species that evolved a semi-aquatic lifestyle, while its sister clades within the subfamily Spilomelinae remain terrestrial (Mally et al. 2019; James E. Hayden, personal communication). Its natural history is better understood than that of E. obliteralis, at least in part due to its suggested use as a biological control agent for several introduced ornamental aquatic plants in the United States and abroad (Bennett 1975; DeLoach et al. 1979; Sands and Kassulke 1984; Tewari and Johnson 2011). Its full lifecycle takes 25–40 days (Bennett 1966; Knopf and Habeck 1976; Sands and Kassulke 1984; Taylor 1988). Females lay eggs singly on either side of the leaf but prefer ovipositing on the adaxial side (Bennett 1966; Knopf and Habeck 1976; Sands and Kassulke 1984). The primary larval host plant of S. multiplicalis is water lettuce, Pistia stratiotes L. (Araceae), which is widely distributed across the world’s tropics and subtropics, but it also feeds on other aquatic plants (Table 1 and references therein). Larvae are reported to construct a gallery or tunnel composed of silk between the hairs of the host plant at early life stages and a silk cocoon for pupation (Knopf and Habeck 1976; Sands and Kassulke 1984), but their silk use has not been described further.

In the present study, we provide an updated and detailed account of the life history of these two aquatic moths distributed throughout Florida. Specifically, we report their development times, describe their behavioral adaptations to the aquatic environment, and characterize their different life stages and silk uses through high quality images.

2 Materials and methods

2.1 Sampling

All sampling for this study took place in 2024. Water lettuce (P. stratiotes) was sourced from multiple natural sites within and around Gainesville, Florida: Boulware Springs Park (Figure 1A, 29.6208056 °N, 82.3070000 °W), Chapman’s Pond and Nature Trails (Figure 1B, 29.6206111 °N, 82.4170556 °W), and Lake Alice (Figure 1C, 29.6428333 °N, 82.3633611 °W); on Florida Department of Agriculture and Consumer Services 08208 permit number 2024-024 and City of Gainesville Parks, Recreation & Cultural Affairs Special Use Authorization granted to B. Foquet. As per permit requirements, P. stratiotes were double-bagged and transported in a cooler to Florida Museum of Natural History’s McGuire Center for Lepidoptera and Biodiversity (MGCL), and all rearing took place in a United States Department of Agriculture (USDA) approved quarantine facility.

Figure 1:

Collection sites in Gainesville, Florida. (A) Boulware Springs Park. (B) Chapman’s Pond and Nature Trails. (C) Lake Alice.

Elophila obliteralis was collected at Chapman’s Pond and Nature Trails, either by light sheeting for adults using a LepiLED Maxi light (Brehm 2017) in front of a vertical white sheet or with an aquatic net for caterpillars. Samea multiplicalis was collected with an aquatic net from Boulware Springs Park and from Chapman’s Pond and Nature Trails.

3 Results

We recorded each life stage of S. multiplicalis and E. obliteralis, from egg deposition into adulthood, and describe their life histories in detail below. Both species were found at all three field sites. However, S. multiplicalis was very abundant at Boulware Springs Park (Figure 1A), while E. obliteralis was more common at Chapman’s Pond and Nature Trails (Figure 1B). At Lake Alice, both species were only occasionally encountered. Even though not statistically tested, we observed that each species seemed to prefer water lettuce with a different structure. S. multiplicalis seemed to prefer large plants with upright leaves, avoiding smaller plants or plants with leaves flat on the water surface. In contrast, E. obliteralis seemed to preferentially deposit eggs on leaves flat on the water surface, and similarly this is the type of plants where most caterpillars were found. These findings were based on our observations, and further study would be necessary to measure preference for different structures. The total life span of S. multiplicalis was 32–38 days from egg to adult. E. obliteralis had a longer life cycle than S. multiplicalis, lasting 47–54 days, with each life stage taking more time than for S. multiplicalis.

3.1 Samea multiplicalis

Eggs of S. multiplicalis (Figure 2A) were white and laid individually above the water surface, scattered between hairs on water lettuce. Eggs were generally laid on the adaxial surface of leaves, but were occasionally found on the abaxial side. Each egg was circular and protruded from the hairs with the bottom surface adhered to the leaf. After hatching, first instar caterpillars were 1 mm in size and translucent to cream in color. The first instar lasted 2–3 days and displayed a distinct silk spinning behavior of constructing a silk gallery that is only slightly wider than its own body width (Figure 2B). They fed under the silk by eating the epidermal layer of the plant and extended their gallery further as they grew and fed (Figure 2B). As a result, the gallery started out very narrow and gradually broadened as it expanded further on the leaf. The second instar measured approximately 3.8 mm in length and lasted 3–4 days (Figure 2C). The third instar lasted approximately 3–5 days, and larvae were approximately 6.25 mm in length (Figure 2D). During the second and third instar, setae and the sclerotization of spiracles became visible on the surface of the body. During the second and third instar, caterpillars extended their galleries to the abaxial side of the leaf. Second and third instar larvae typically did not leave their gallery; they fed, defecated, and molted within the confines of their silk cover.

Figure 2:

Life history of Samea multiplicalis. (A) Eggs. (B–F) First-through fifth-instar larvae, in respective order. (G) Fourth-instar larval feeding pattern. (H) Pupa. (I) Adult moth.

Third instar larvae began spinning a less dense web of silk, often near the base of the plant (Figure 3A). Larger third instar caterpillars generally fed on the upper surface of the plant, avoiding direct contact with the water surface. Fourth instars (Figure 2E) had a darker greenish-yellow body with a brown head capsule, providing effective camouflage on the leaf background. When at rest, caterpillars hid in their web among the base of leaves. Webs were often covered in frass. This instar lasted 4–5 days and measured approximately 9 mm. The fifth instar lasted 5–6 days, and was characterized by having a dark brown head capsule and spiracles, pronounced setae, and a long abdomen, with its total body length measuring up to 10 mm (Figure 2F). While larvae in the fourth and fifth instars primarily fed on the upper surface of P. stratiotes leaves, they occasionally swam between host plants to reach a new food source, especially when their previous host plant was fully consumed or when it was shared with conspecifics. Before pupation, larvae ate less and spent more time hidden at the base of the host plant leaves. Three to four days after turning into their final instar, feeding ceased and the larva entered the wandering stage, leaving their silk webs to find a suitable location for pupation.

Figure 3:

Silk spinning behavior of Samea multiplicalis and Elophila obliteralis. (A) Third-instar S. multiplicalis before molting under its silk cover. (B) Cocoon of S. multiplicalis. (C) Inner silk layer of S. multiplicalis cocoon. (D–E) Silk spun to maintain a ‘sandwich’ of leaves in final instar E. obliteralis. (F) Silk cocoon cover inside leaf ‘sandwich’ of E. obliteralis.

Pupation took place in a cocoon between two leaves or on a leaf with a strong indentation, and caterpillars occasionally swam when searching for an optimal pupation site. The pupation site was typically close to the base of the plant but above the water surface. Occasionally, pupation took place near leaf apices or on the abaxial side of a leaf in close proximity to the water. Caterpillars produced a dense, narrow, cylindrical silk cocoon with loose silk strands on the exterior that adhered it to the leaf surface (Figure 3B). Within the cocoon, there is a thin lining of silk that surrounded the pupa (Figures 2I and 3C). Pupae measured approximately 8.5 mm in length. Pupal duration of S. multiplicalis was approximately 4–5 days. In captivity, adults fed and mated within a day after eclosion. Adult S. multiplicalis had a tan to light-brown color with dark-brown and white markings, and a wingspan of approximately 17 mm (Guenée 1854; Figure 2I). Females seemed to prefer P. stratiotes plants with upright leaves, at a 45° angle with the water surface, possibly because its structure provided ample surface for oviposition. Egg laying continued for several days.

3.2 Elophila obliteralis

Eggs of E. obliteralis were laid underwater, near the P. stratiotes leaf margin in small clusters of four to seven. They were oval, translucent, and flattened, and the larvae hatched after 8–9 days (Figure 4A). After hatching, first instar caterpillars were up to 1 mm in length and translucent. Caterpillars wandered before starting to feed, often leaving the plant from which they originated. They chewed off small pieces of leaves from their host plant or obtained loose plant material from nearby plants such as Lemna sp. (Araceae). They created their case on the underside of their host plant leaf by cutting leaves around the margin of plants, avoiding the inner leaves of P. stratiotes (Figure 4B). At our primary field site for E. obliteralis, (Chapman’s Pond and Nature Trails, Figure 1B), cases were made of a large variety of plant materials, including other aquatic plants. The first instar, in our indoor captive rearing experiment, lasted 8–9 days. Second instar larvae are 1.5 mm in length and whitish green (Figure 4C). The second instar lasted 8–9 days. Before molting, the second instar wandered on the plant searching for a location to create a larger case. In these early instars, cases were not always made of a single piece of leaf but sometimes consisted of many smaller pieces that were woven together with silk.

Figure 4:

Life history of Elophila obliteralis. (A) Eggs. (B–E) First-through fourth-instar larvae, in respective order. (F) Pupal casing. (G) Various casings of E. obliteralis. (H) Pupa. (I) Adult moth.

The third instar larva was approximately 3 mm in length and was whitish green with a tan head capsule (Figure 4D). Caterpillars in this instar often cut out two oval pieces of host plant leaf and created a larger case. This instar lasted 6–8 days. The fourth instar was yellowish white (Figure 4E), and characterized by having a fully formed case, consisting of two oval leaf pieces of P. stratiotes measuring about 1 cm in length and that was tightly bound, sandwiching the caterpillar inside (Figure 3D and E). These cases had a small opening on one end, which allowed the larva to stick its head out of the case to feed. When threatened, larvae retreated into their case. This instar lasted 6–7 days. The fifth instar was white and covered with papillae, but as the caterpillar grew, the anterior portion of the larva (from the fourth abdominal segment forward) turned increasingly yellowish brown. This instar typically lasted 8–9 days and grew to about 6 mm in length. The fifth instar larva did not always generate a new case and often remained in its fourth instar case until pupation (Figure 4F). Pupation took place inside of the case, underwater on the stem or upper root of their host plant. The pupa (Figure 4H) was bound in a dense oval silk casing adhered to the insides of both P. stratiotes leaves, and the entry was sealed with silk (Figure 3F). In the field, cases were found to occasionally be made of other plant materials, including small sticks and dead plant matter (Figure 4G). Elophila obliteralis remained a pupa for 5–6 days before emerging as an adult (Figure 4I). In captivity, adults fed and mated within a day after eclosion. Adult E. obliteralis had a dark brown to black coloration with light markings (Figure 4I). Females seemed to prefer plants with leaves lying on the water surface, allowing easy deposition of eggs under the water surface.

4 Discussion

We have provided a detailed description of the natural history of S. multiplicalis and E. obliteralis. The two species overlap largely in distribution and share many host plants (Table 1). We found them to coexist in all three field sites in Florida. At Boulware Springs Park (Figure 1A), where P. stratiotes was the dominant aquatic plant, S. multiplicalis was more abundant. In contrast, at Chapman's Ponds and Nature Trails (Figure 1B), which hosts a diverse aquatic plant community, E. obliteralis was the more abundant species. Although these two species were found at the same sites, their specialization on different plant parts appeared to facilitate coexistence with minimal competition. S. multiplicalis primarily fed on and inhabited plant parts above the water surface, while E. obliteralis was largely restricted to the submerged portions of the same plants. Interestingly, when the two species were reared together in captivity, S. multiplicalis consistently outcompeted E. obliteralis due to its higher growth rate and tendency to destroy or heavily damage its host plant. S. multiplicalis did more damage to P. stratiotes, both in our laboratory and in the field where large numbers of S. multiplicalis resulted in large numbers of P. stratiotes plants being consumed significantly, only to regenerate when caterpillar abundance was reduced.

Data generated here on the development of S. multiplicalis is largely congruent with prior studies. Earlier work reported five to seven instars for S. multiplicalis and we observed five instars in our laboratory colony. The fewer number of instars that we observed may be caused by high nitrogen levels of our host plants (Taylor 1988). In our colony, the total lifespan of S. multiplicalis was 32–38 days, which is within the range of what was reported earlier (Bennett 1966; Knopf and Habeck 1976; Sands and Kassulke 1984; Taylor 1988). The total duration of the larval stage was 18–25 days, which was slightly faster than the 21–35 days reported by Sands and Kassulke (1984) at 26 °C on S. molesta but slower than the 15 days reported at 28 °C on P. stratiotes (Knopf and Habeck 1976). Pupal development in our study seemed a bit faster than reported before: 4–5 days in our study, 5–8 days in earlier studies (Knopf and Habeck 1976; Sands and Kassulke 1984). As we kept our rearing room at a constant temperature of 24 °C, there is likely an interaction between temperature and host plant for this species. While some earlier studies suggested that S. multiplicalis has up to three generations and cannot be found in the adult stage during extreme cold and hot temperature periods (DeLoach et al. 1979; de Sousa Oliveira and de Souza 2023), we confirm other reports (e.g., Queensland, Australia; Taylor 1988) that caterpillars can be found year round when temperatures are within an acceptable range. We also frequently observed adults, pupae, and caterpillars (at various developmental stages) for both S. multiplicalis and E. obliteralis at all localities, indicating that both species likely have overlapping generations in central Florida.

Our study sets the stage for many future studies. For example, evolutionary pressures that led to multiple repeated origins of aquatic or semi-aquatic behavior in caterpillars are currently unclear (Pabis 2018). A transition to an aquatic or semi-aquatic lifestyle may benefit the insect by allowing species access to additional resources, such as aquatic plants, where competition may be less intense than in terrestrial environments. Alternatively, a transition to living underwater may allow these insects to escape predators or parasitoids. We only observed one case of parasitism in our study; a caterpillar of E. obliteralis collected at Lake Alice was parasitized by Apsilops hirtifrons (Ashmead) (Hymenoptera: Ichneumonidae) (Judd 1953), which was deposited in the Florida State Collection of Arthropods (FSCA) under the accession number FSCA 00118657. Nonetheless, there are many observations of predation and parasitism for the two species studied here, including social wasps (Vespidae; de Sousa Oliveira and de Souza 2023), parasitic flies (Tachinidae; Knopf and Habeck 1976) and parasitic wasps (Ichneumonidae and Braconidae; Fernandez-Triana et al. 2020; Judd 1953; Knopf and Habeck 1976; Newton and Sharkey 2000; Semple and Forno 1987; Sharkey et al. 2011). A future study comparing predation and parasitism levels between closely related aquatic and terrestrial groups would be beneficial, as no such study has been conducted.

Studies have shown that both terrestrial and aquatic insects use silk for a variety of purposes, including constructing shelters, building retreats, creating feeding structures, and facilitating mobility in their environment (Mackay and Wiggins 1979; Sutherland et al. 2010). Our study revealed that both species used silk intensively throughout their life, making them well adapted to the aquatic environment. Additionally, we observed caterpillars of different instars altering their silk use based on their behavior during each life stage. For instance, early instar S. multiplicalis caterpillars exclusively fed and defecated under their tunnel of dense silk threads. We hypothesize that this silk cover provides protection from predation, parasitism, desiccation, and protection from water during early life stages, when caterpillars are very vulnerable. During the second and third instar, caterpillars started spinning a loose web of silk fibers, offering less protection but covering larger areas. They started to utilize the natural structure of their host plant for shelter, reducing their dependence on silk for protection. The prepupal caterpillar span a robust silk cocoon, which featured anchor lines that secured the plant material and a tight, durable silk coating in which pupation took place. Similarly, E. obliteralis caterpillars created a small dome-like case that consisted of one or multiple small leaf fragments during the first instar that was attached to the host plant. Although this restricted them to a very limited feeding area, it provided enough food at that life stage. As caterpillars grew and consumed more food, they changed their case to a sandwich-like case that was carried wherever they go. Even though they pupated within this case, they generated an additional tight silk layer, similar to the inner layer of cocoon silk in S. multiplicalis. As such, E. obliteralis seems to be much more dependent on silk throughout its life, as its case does not only provide a barrier against predation, but can also serve as camouflage.