Ottoe skipper (Hesperia ottoe) COSEWIC assessment and status report: chapter 6

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General

Like other butterflies, H. ottoe undergoes complete metamorphosis with adult, egg, caterpillar, and pupal stages. Each stage often has different resource and microhabitat requirements.

Adult activity period

Hesperia ottoe has only one generation per year. Adults are present for six to seven weeks, usually from mid-June to mid August, depending on the season (McCabe and Post 1977, Layberry et al. 1998, Nielsen 1999, Swengel and Swengel 1999). In Canada, adults have been collected in late July and mid-August (CNC collection database, Manitoba Conservation, Biological and Conservation Data System data; Klassen et al. 1989).

In a study by Dana (1991), H. ottoe males began to emerge in field plots five to six days earlier than females. The delay was expected as the duration of post-diapause development is longer in female larvae than in males (Dana 1991). Although protandry (males emerge before females) occurs in this species, emergence was protracted (21 days) in both sexes, resulting in considerable overlap in the emergence period of the two sexes (Dana 1991). In field plots, adults emerged over a 28-day period starting in early July. Dana (1991) estimated the potential adult life span of H. ottoe in nature to be around three weeks.

Adult food resources

Access to nectar is important to H. ottoe and other species of butterflies. Nectar provides adults with an energy source as well as water and allows females to attain maximal fecundity (Murphy et al. 1983).

No observations of nectaring are available for Canadian populations of H. ottoe. In a dry-mesic bluestem prairie in Minnesota (Hole-in-the-Mountain Prairie), Dana (1991) observed H. ottoe nectaring on ten species of flowers. The most frequently used species have relatively concealed nectar. In order of usage, these were Purple coneflower (E. angustifolia), Hoary Vervain (Verbena stricta), a thistle (Cirsium flodmanii), and a milkweed (Asclepias viridiflora) (Dana 1991). However, over 90% of all flower visitations were to E. angustifolia. In North Dakota, Fleabane (Erigeron strigosus) and Long-headed Coneflower (Ratibida columnifera) were the most common nectar sources of H. ottoe (McCabe 1981). Swengel (1994) observed H. ottoe on 16 species of flowers in Wisconsin. Most visits were to Wild Bergamot (M. fistulosa) and Blazingstar (Liatris sp.). Hesperia ottoe is probably opportunistic, foraging on the species of flowers that are most profitable at a given site.

Few of the above plant species were present at any of the historical sites for H. ottoe, although M. fistulosa was common in Spruce Woods Provincial Park. Echinacea was not present in the park, was rare at Aweme, and was frequent in the small, remnant, mixed-grass prairies west of Shilo. Other potential nectar sources, such as Snowberry (S. albus) were common on the edges of mixed-grass prairie openings, especially in Spruce Woods Provincial Park. Symphoicarpos albus was frequented by many butterfly and skipper species.

In Minnesota, freshly emerged males of H. ottoe were frequently observed puddling on muddy margins of stock ponds and on gravel roads after rain (Dana 1991). The adults were likely obtaining nutrients (mineral salts, amino acids) and water from these sites (Arms et al. 1974, Adler and Pearson 1982).

Courtship behaviour

No information is available on mating behaviour of populations of H. ottoe in Canada. However, detailed descriptions of the courtship behaviour of this species are provided by Dana (1991) from populations in Minnesota. Mate-seeking behaviour dominates the daily flight period of male H. ottoe and the mating system appears to be a form of scramble competition polygyny (Dana 1991).

Most commonly, courtship is of the waiting-perching-pursuit type. Males often perch on high vantage points above the grass canopy, such as the flower heads (capitula) of composites (E. angustifolia), and pursue any insects that fly nearby (Dana 1991). In hilly terrain, males often perch on hill and ridge tops. When a male encounters another male during the initial pursuit, the pursuit often develops into an aerial engagement with the two individuals whirling about each other, often to considerable heights, before the engagement is broken off (Dana 1991). Hesperia ottoe males often did not return to the same perch sites, as would be typical of territorial behaviour (Dana 1991).

If a male encounters a female H. ottoe, a different set of behaviours ensues. Perching males attempt to mate with any females that move within their visual range. Typically, the female flies a short distance and lands. The male pursues her, lands and quickly walks alongside her while curving his abdomen with claspers spread toward the abdomen of the female and attempts to copulate with her (Dana 1991). If receptive, the female extends her abdomen and they mate. If the male is rejected, the female holds her abdomen between closed wings and periodically jerks her wings forward. Unsuccessful males may attempt to mate a few additional times before flying to a nectar source (Dana 1991). Pheromones contained in androconial particles in the stigmata of males probably play a role in courtship and as a species isolating mechanism (Dana 1991).

A form of mate seeking or searching type behaviour is also often observed in H. ottoe. In this behaviour, males fly rapidly from one E. angustifolia capitulum to another, exhibiting a brief “bobbing” flight pattern near each capitulum without landing. Only one encounter of a male exhibiting this behaviour with a female was observed and a standard courtship sequence ensued.

Most mating attempts take place from late morning (10:00 h) to as late as 17:00 h. The peak number of pursuits, however, was observed between 14:00 and 16:00 h (Dana 1991). No data are available on the duration of copulation. If a mating pair is disturbed, it may take flight and travel several metres in a direct flight path. The female is the carrier in H. ottoe (Dana 1991).

All observed copulations were with freshly emerged females (unworn), suggesting females mate within a day or two of adult emergence (Dana 1991). It is not known if males and females mate more than once during their life span.

Oviposition behaviour and fecundity

Females do not start laying eggs until at least 36 hours after emerging (mature oocytes were not present in the ovaries until 36 hours after adult emergence) (Dana 1991). Once females commence laying eggs, oviposition continues throughout much of the female’s 3-4 week lifespan (Dana 1991). However, few data are available on daily egg production in this species. Based on the number of distinguishable oocytes in a young female, the potential lifetime fecundity was estimated to be around 225 eggs per female (Dana 1991).

In the USA, female H. ottoe lay eggs on a wide variety of grasses and forbs (Dana 1991). In a study at the Hole-in-the-Mountain Prairie in Minnesota, females oviposited on five species of grasses and several species of forbs (Dana 1991). The most common species of grasses used for oviposition, in decreasing order of usage, were A. scoparius, A. gerardii, Bouteloua curtipendula, and Dichanthelium wilcoxianum. Most of these grasses are also larval host plants. However, 50% of all observed ovipositions were on the capitula of purple coneflower (E. angustifolia).

Females ovipositing on grasses place eggs singly to the underside of leaves or the upper surface of erect grass blades, usually within the grass canopy (Dana 1991). Females fly slowly above the grass canopy and land on bare spots before crawling into the grasses. After laying an egg, the female flies to a new site.

Females laying eggs on the capitula of E. angustifolia fly rapidly and directly to a capitulum (the catpitula are usually much higher than the grass canopy), feed on the flower, then curl their abdomen down and affix a single egg on the outward-facing convex underside of a receptacular bract, and then fly to a new capitulum (Dana 1991). This behaviour is often repeated several times in succession, and oviposition occurs throughout the day (Dana 1991).

Larval resources

H. ottoe larvae use a variety of grass species. In Minnesota, larvae were found feeding on A. scoparius, B. curtipendula, A. gerardii and D. wilcoxianum (Dana 1991). Other common grasses, like Koeleria cristata and Stipa spartea, were not eaten, although they were consumed in no-choice experimental conditions (Dana 1991). Larvae generally feed on all grass species close to their shelters, excluding the avoided species (Dana 1991). Nielsen (1958) reported the grass Leptoloma cognatum as a host in Michigan, and McGuire (1982) reported ovipositions on A. scoparius from several locations within the skipper’s range.

The preferred host of H. ottoe in Minnesota was Little Bluestem (A. scoparius) (Dana 1991). Bunch grasses, like A. scoparius, have a dense cluster of erect blades and a mass of persistent basal material that remains edible throughout the summer and into the fall. MacNeill (1964) suggests that the architecture of these grasses makes them ideal for shelter construction by the larvae and provides a readily available food source close to the shelter. Although other species of grasses can be eaten by the larvae, some may not be suitable because of different architecture (too tall) or summer senescence (Dana 1991). The non-nativePoa pratensis and Smooth Brome Grass (Bromus inermis), although eaten under laboratory conditions, have a mid-summer senescence, or dormancy, making them unsuitable for the larvae of H. ottoe later in the summer and early fall.

Larval development

The eggs of H. ottoe hatch within 12-13 days depending on temperature (Dana 1991). Hesperia ottoe has six to seven instars or larval stages. Each of the first three instars lasts between 7 and 26 days under field conditions. The duration of the fourth instar is between 14 and 36 days and growth often becomes slowed (Dana 1991). Larvae enter an obligatory diapause during the fourth instar, which occurs in late September or October in Minnesota (Dana 1991). During the subsequent spring, the fifth-instar larvae moult shortly after feeding resumes. The duration of the next two instars is 8-13 days and 17-25 days, respectively. Once feeding is completed, the last-instar larvae moult into the pupal stage, which lasts 12-19 days under natural conditions (Dana 1991).

Larval behaviour

Typically, newly eclosed larvae of H. ottoe first eat the chorion before constructing a leaf shelter (Dana 1991). Those on the capitula of E. augustifolia drop to the grass canopy. Newly hatched larvae construct shelters by fastening two or three grass blades together from the points of divergence from the stem of a vegetative shoot of the host, forming a narrow tube with the distal portion of the blades remaining free. The shelters are usually several centimetres above the soil surface. The larvae feed on the free distal portions of the blades forming the shelter and deposit their frass in the lower end (Dana 1991). Feeding takes place both diurnally and nocturnally. The larvae abandon the shelter (which is about 6 cm long) when its lower end fills with frass, and construct a new one. Two to three leaf blade shelters are constructed before constructing a buried shelter in late August or early September in which the larvae diapause (Dana 1991). The buried shelter is at, or entirely below, the soil surface and consists of a steeply angled tubular chamber lined with silk within a clump of one or more of the host grasses (Dana 1991). After diapause, the larvae produce two or three elongated horizontal shelters on the soil surface. These shelters are often concealed by the basal material of the grass clump (Dana 1991). Prior to pupation, the larvae construct a new chamber, which is not waterproof, unlike the waterproof chambers of H. dacotae (Dana 1991).

Larvae in the buried shelters typically leave their chambers, cut off and remove grass blade segments, carry them back to their chambers, and feed on them within the chambers (Dana 1991). They forage on those grass species that are in close proximity to their shelters (Dana 1991).

Natural mortality factors

Few data are available on natural mortality factors for H. ottoe. A small proportion of ova on the capitula of E. augustifolia are parasitized by Trichogramma sp. (Hymenoptera: Trichogrammatidae) and a scelionid wasp (Hymenoptera: Scelionidae) (Dana 1991). Damsel bugs (Hemiptera: Nabidae) may also consume this skipper’s eggs (Dana 1991). An Apanteles wasp (Hymenoptera; Braconidae) was reared from cocoons found in leaf shelters, and a soft-winged flower beetle larva (Coleoptera; Melyridae) was found feeding on a second-instar larva in a leaf blade shelter (Dana 1991).

Dana (1991) hypothesized that H. ottoe’s preference for laying eggs on E. angustifolia capitula might be related to reducing the risk of the ova and early instar larvae being consumed by grazing ungulates, such as bison. He observed that cattle tend to avoid these stems when they have sufficient forage. Echinacea angustifolia may also be an indicator of the presence of good larval host plants (Dana 1991).

Population dynamics

Little information is available on the long-term population trends for H. ottoe in Canada or the United States. It is possible that the species is extirpated from Canada.

Movements/dispersal

Little information is available on the dispersal of H. ottoe in Canada or the United States. In a mark-release-recapture experiment at the Hole-in-the-Mountain preserve in Minnesota, marked adults moved across 200 m of unsuitable habitat between two sections of prairie (Dana 1991). This skipper is a powerful flier and should be capable of covering large distances for a skipper. Anecdotal evidence from Dana (pers. com. 2004) suggests that this species may move at least 3 km over unsuitable habitat. A female was collected in a small (2 hectares) prairie fragment in Minnesota (without a previously known population) which was 3 km from the closest potential source population (the site had appropriate habitat, but had not been surveyed). The closest known population is about 95 km away. It is likely that this observation represents a dispersal event (or the offspring of one).

Because of the long periods between Canadian records and the species’ apparent ability to cross relatively large areas of unsuitable habitat, there is some uncertainty as to whether H. ottoe is a Canadian resident species. Some researchers think that the few scattered occurrences in Canada may represent cases of vagrancy (Lafontaine, pers. com. 2004, Webster, pers. com. 2005). However, there is considerable evidence suggesting that the specimens recorded in Canada were residents: because extant H. ottoe populations in the US are highly local and generally uncommon, they are an unlikely source for vagrants, which tend to come from large populations; the closest known population of H. ottoe is about 200 km to the southwest, too great a distance for a small skipper, even a strong flier, to be likely to cover; all Canadian records occur in the same general area of Manitoba, an unlikely scenario for vagrants, which would be likely to show up in widely separated sites; and vagrants would likely occur as single specimens, but all Canadian records involve at least two individuals. In addition, The Butterflies of Canada (Layberry et al. 1998) and The Butterflies of Manitoba (Klassen et al. 1989) both treat H. ottoe as a Canadian species. In the absence of convincing evidence to the contrary, H. ottoe is considered a Canadian resident, an approach consistent with the Species at Risk Act.

Interspecific interactions

No data are available on interspecific interactions of H. ottoe for Canadian populations. In Minnesota, Dana (1991) observed that interactions with males of other species outnumbered those with conspecifics. However, the interspecific interactions were shorter than those with other males of H. ottoe.

Adaptability

Hesperia ottoe is extremely susceptible to habitat changes and is rarely found in prairie habitats that have been degraded (McCabe and Post 1977, Dana 1991). Although the immature stages and adults can use a variety of plant species for growth and reproduction, they appear to use only species associated with undisturbed prairie habitats. Alteration of this plant community results in the loss of critical resources for the skipper and the loss of the skipper from altered sites. The continued fragmentation of suitable prairie habitats into progressively more widely dispersed remnants make H. ottoe especially susceptible to habitat degradation.