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COSEWIC assessment and status report on the Dakota skipper Hesperia dacotae in Canada

Biology

 

General

Each life history stage of H. dacotae has different resource and microhabitat requirements.

Adult activity period 

H. dacotae has only one generation per year. Adults are active for only about three to five weeks at a given locality (McCabe 1981, Dana 1991).  According to McCabe (1981), males and females of H. dacotae emerge at about the same time.  However, in a study by Dana (1991), H. dacotae males began emerging about 5 days earlier than females under field conditions.  The delay was expected as the duration of post diapause development is longer in female larvae than in males (Dana 1991). 

In Canada, adults have been collected from June 23 to July 29.  Most collection records are between June 27 and July 8 (CNC collection database, Manitoba Conservation, Biological and Conservation Data System data, Hooper, pers. com., 2002).  During 2002, in the inter-lake region near Lundar, the first adults (all freshly emerged males) were observed on July 2. On July 6, 2002, freshly emerged females were present, but the male:female sex ratio (13:5) favored males, suggesting that peak flight had not been reached. Both sexes were common by July 8, but females were still out-numbered by males 3:1, suggesting that protandry occurs in this species in Manitoba.  There is, however, considerable overlap in emergence of the two sexes.  H. dacotae was probably at peak flight at the Griswold site in western Manitoba on July 10, 2002, as a 50:50 sex ratio was observed there.  

Dana (1991) estimated the potential adult life span of H. dacotae in nature to be about three weeks.  One adult was recaptured 19 days after the initial capture in a mark-release-recapture experiment on a Minnesota prairie.  Residency (residence on site before death or emigration) was estimated to be 3 to 10 days (Dana 1991).

Adult food resources

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

In wet-mesic tall-grass prairie sites in Manitoba, H. dacotae were most frequently observed using R. serotina, L. philadelphicum, and C. rotunifolia as nectar sources.  Adults fed from underneath the flower head on L. philadelphicam and were often difficult to observe.  Dogbane, Apocynum sp., was commonly used at two sites near Lundar.  In Saskatchewan, H. dacotae most commonly used Purple Coneflower, E. angustifolia.  One or more of these nectar sources were common to very common at sites where H. dacotae was common.

In North Dakota, a variety of flowers, mostly members of the Family Compositae, are used as nectar sources by H. dacotae.  Among these were Long-headed Coneflower, Ratibida columnifera (Nutt.), Fleabane, Erigeron strigosus Muhl., E. angustifolia, Gaillardia, Gaillardia aristata Pursh, R. serotina, C. rotundifolia and Toothed-leaved Primrose, Oenothera serrulata Nutt. (McCabe 1981).  In a dry-mesic tall-grass-to-mid-grass prairie site in Minnesota, 25 species of flowers were used by H. dacotae (Dana 1991).  Nearly 90% of all flower visitations were, however, to E. angustifolia, Verbena stricta, Astragalus adsurgens and Oxytropis lambertii, with E. angustifolia being the most important nectar source at this site.  H. dacotae is probably opportunistic, foraging on the species of flowers that are most profitable at a given site.

Courtship behaviour

Little data is available on mating behavior of H. dacotae in Canada. However, detailed descriptions of courtship behaviour of this species are provided by McCabe (1981) and Dana (1991) from populations in North Dakota and Minnesota, respectively.  The mating system of H. dacotae appears to be a form of scramble competition polygyny (Dana 1991).  Mate-seeking behaviour dominates the daily flight period of males (Dana 1991).

Courtship is of the waiting-perching-pursuit type.  Males often perch on a high vantage point above the grass canopy, such as the flower heads of composites, and pursue any insect that flies nearby (McCabe 1981, Dana 1991).  In hilly terrain, males often perch on the leeward side of slopes and hills (McCabe 1981, Layberrry et al. 1998).  Often aggregations of up to 100 or more individuals gather in areas on the windward side of these hills, especially where nectar sources are common (McCabe 1981).  In Saskatchewan, males perched on hillsides as well as on the tops of the hills and ridges, often on the flower heads of E. angustifolia.  However, on hill tops where the grasses were often short and sparse, males often perched on bare soil or short grasses. 

The wet-mesic tall-grass prairie sites in Manitoba have little relief.  Within the wet-mesic prairies, males and females were almost exclusively found in the slightly higher, drier areas, usually where the grasses were shorter (10-15 cm high) than in surrounding areas and where nectar sources were more abundant.  Adults were rarely found in the lower, wetter areas of the prairies.  Males often perched on the flowers of R. serotina and Z. elegans, but often also perched on the short grasses and even on bare soil. Adults (both males and females) appeared to be more common in prairies that had extensive areas with shorter grasses on the higher ground than at sites where tall grasses were dominant.  As many as 59 individuals (42 males, 17 females) were counted in a 15-min period at one prairie site east of Lundar with extensive areas of shorter grasses.  In prairies with a late fall mowing regime, more extensive areas with short grasses often occur on the higher ground than in un-mowed sites, and adults of H. dacotae were often common on these mowed prairies.  Adults were also frequently very common in smaller sections of prairie partially surrounded by aspen groves.  These areas may benefit mating behavior because they are more protected from the wind.  More studies are needed to examine the relationship between grass height and H. dacotae abundance in the wet-mesic tall-grass prairies of Manitoba.

When a male H. dacotae encounters another male H. dacotae during the initial pursuit, the pursuit often develops into an aerial engagement with the two whirling about each other at two or three meters above the ground (Dana 1991).  Other males may engage the pair, and then all will separate and each will fly to a nearby perch, often different from the original perch.  There is little evidence that males return to the same perch, as would be typical of territorial behaviour (Dana 1991). 

If the insect the male encounters is a female H. dacotae, 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 crawls alongside her while curving his abdomen with claspers spread toward the abdomen of the female and attempts to copulate with her (McCabe 1981, Dana 1991).  If receptive, the female extends her ovipositor and they mate.  If the female rejects the male, she holds her abdomen between closed wings and periodically jerks her wings forward.  The male may make a few additional attempts to mate, and if unsuccessful, flies to a nectar source and feeds before going to a perch (McCabe 1981, Dana 1991).

Pheromones contained in the androconial particles in the stigmata of males probably play a role in courtship and as a species isolating mechanism (Dana 1991).  Most mating attempts take place during the afternoon between 14:00 and 16:00 h (Dana 1991). Mating pairs remain quiescent within the vegetation, and the duration of copulation is about 45 min.  If disturbed, the pair may take flight and travel several meters in a direct flight pattern.  The female is the carrier in H. dacotae (Dana 1991). 

Females often mate within a day or two of adult eclosion.  Both sexes may mate more than once during their life span, but a single mating is more common for females (based on spermatophore counts) (McCabe 1981, Dana 1991).  When a second mating does occur, it probably takes place shortly after the first mating before the females becomes refractory (Dana 1991).

Oviposition behaviour and fecundity

Females begin to lay eggs shortly after mating and continue ovipositing throughout their life span, which may be up to four weeks (McCabe 1981).  Twenty to thirty eggs are laid daily during the first two days after adult emergence, then daily egg production declines linearly to a few eggs per day two weeks after emergence (Dana 1991).  Approximately 50% of eggs are laid during the first week and 90% by the end of the second week.  Potential maximum life-time fecundity ranges from 180 to 250 eggs per female (Dana 1991). 

Eggs are laid singly to the underside of leaves or the upper surface of erect grass blades, usually one to four centimetres above the soil surface within the grass canopy (Dana 1991).  Females fly slowly above the grass canopy and land on bare spots before crawling into the grasses.  After the female lays an egg, she flies to a new site.  Oviposition occurs throughout the day (Dana 1991).

In the U.S.A., female H. dacotae laid eggs on a wide variety of grasses and forbs (McCabe 1981, Dana 1991).  In a study at the Hole-in-the-Mountain Prairie in Minnesota, females oviposited on five species of grasses and 13 species of forbs (Dana 1991).  The most common grasses used for oviposition, in decreasing order of usage, were A. scoparius, A. gerardii, Bouteloua curtipendula, S. heterolepis and Spear Grass, Stipa spartea Trin.  This contrasts with the findings of McCabe and Post (1977) who reported that eggs were typically laid on leaves of broad-leaved plants in North Dakota.  Dana (1991) suggests that females will lay eggs on any surface as long as it is smooth and wide enough to allow the egg to adhere to it. In some cases, the plants on which eggs are laid are also larval host plants, but in many cases they are not.  However, eggs are usually laid close to larval hosts (Dana 1991).

Larval resources

H. dacotae larvae use a variety of grass species in nature.  Under natural field conditions in Minnesota, larvae fed mostly on A. scoparius, A. gerardii, B. curtipendula, and S. heterolepis.  Secondary hosts were Dichanthelium wilcoxianum, Poa pratensis L. and rarely Carex heliophila (Dana 1991).  Other common grasses, like Koehleria cristata (L.) Pers. and S. spartea, were not eaten in the wild, but were consumed in experimental no-choice conditions (McCabe 1981, Dana 1991).  Larvae generally fed on all grass species close to their larval shelters, except on the avoided species (Dana 1991).

The preferred hosts of H. dacotae are bunch grasses, such as Little Bluestem, A. scoparius and S. heterolepis.  All these grasses 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 these grasses have an architecture that makes them ideal for shelter construction by the larvae and provide 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 for example) or summer senescence (Dana 1991). The non-native P. pratensis and Smooth Brome Grass, Bromus inermis Leyess, for example, have a mid-summer senescence or dormancy, making them unsuitable for the larvae of H. dacotae in the latter part of the summer and in early fall. 

Larval development

The eggs of H. dacotae hatch within 7-20 days (10 days on average), depending on temperature (McCabe 1981, Dana 1991). H. dacotae has six or seven larval instars or stages.  Each of the first three larval stages lasts between 8 and 18 days under field conditions.  The duration of the fourth instar is between 16 and 35 days (Dana 1991).  The larvae enter an obligatory diapause during either the fourth or fifth instar (usually in October).  Most individuals entered diapause in the fifth instar under field conditions in Minnesota (Dana 1991).  In a study in North Dakota, however, the majority of larvae of H. dacotae entered diapause during the fourth instar (McCabe 1981).  Dana (1991) suggests that the difference may be related to the higher latitude of North Dakota where there is a shorter average interval between completion of fourth instar and onset of cold weather.  In Minnesota, larvae complete the fourth instar well before the onset of cool weather and thus have sufficient time to enter the subsequent instar and feed prior to entering diapause.  Presumably, H. dacotae enters diapause in the fourth instar in Manitoba as well.  During the subsequent spring, the fourth or fifth instar larvae molt shortly after feeding resumes.  The next two instars (fifth and sixth or sixth and seventh) last 14-19 days and 15-21 days, respectively.  Once feeding is completed, the last instar larvae enter the pupal stage, which lasts 13 to 19 days under natural conditions (Dana 1991). 

Larval behaviour

Typically, newly eclosed larvae of H. dacotae first eat the chorion, crawl down to the surface of the soil (usually within a clump of one of the bunch grasses, such as A. scoparius) , web small pieces of detritus together at or below the soil surface, and then feed from the shelter.  Second, third, fourth and fifth instar larvae construct steeply angled, tubular chambers within a grass clump at or (more frequently) entirely below the soil surface (Dana 1991).  The chambers are lined with silk and grass stems.  During development, two to three progressively larger shelters are produced.  After diapause, the larvae produce elongated horizontal shelters on the soil surface, often partially concealed by the basal material of the grass clump (Dana 1991).  Pupation occurs in newly constructed chambers.  Fully-grown larvae have a white glandular patch on the ventral portions of abdominal segments seven and eight.  The patch contains a waxy hydrofuge (water repellent) substance.  Prior to pupation, the larvae distribute this waxy material throughout the pupal chamber (McCabe 1981, Dana 1991).  This substance may protect the larvae from the effects of high humidity, which may be an important factor limiting the survival of the skipper (MacNeill 1964). 

In nature, the larvae often forage for food outside their chambers, but feeding takes place within the chambers (Dana 1991).  The larvae leave their chambers, cut off and remove grass blade segments, carry them back to their chambers, and feed on them.  Most feeding may take place at night (McCabe 1981, Dana 1991).  The larvae appear to forage on those species of grasses that are in close proximity to the shelters.

Natural mortality factors

One egg parasitoid, Ooencyrtus sp. (Encyrtidae: Hymenoptera) has been reared from field collected ova of H. dacotae in Minnesota, and ants have also been observed seizing wandering larvae (Dana 1991).  Predation on the Dakota Skipper by ambush bugs, Phymata sp. (Hemiptera: Phymatidae), flower spiders, Misumena vatia (Clerck) and Misumenops carletonicus Dendale & Redner (Aranaea: Thomisidae), and various orb spiders has been observed in Minnesota and North Dakota (McCabe 1981, Dana 1991).  Ambush Bugs and flower spiders are often found on nectar sources frequently used by the Dakota Skipper.  Both are effective predators as they are cryptically colored to match the flowers they rest on and ambush any insects that land on the flowers.  Interestingly, these predators were rarely found on the flowers of one of the main nectar sources (C. rotundifolia) of H. dacotae in North Dakota (McCabe 1981).  Orb weaver spiders were less effective predators of young adult H. dacotae, which can break from the webbing.  Old, worn adults, however, were often less successful in breaking away from the webs (McCabe 1981).  Other potential predators include robber flies (Asilidae), dragonflies and birds.  However, few cases of predation by these taxa on H. dacotae have been observed (McCabe 1981).  Bacterial septicemia may be an important mortality factor for Hesperia (MacNeill 1964). 

Population dynamics

At any given site in North Dakota and Minnesota, population numbers of H. dacotae appeared to be very stable from year to year as long as the habitat remained undisturbed.  No significant year-to-year population fluctuations were reported at sites where populations have been monitored in successive years (McCabe 1981, Dana 1991, 1997), although Dana (1997) suggests that significant year-to-year fluctuations in population size are possible.  No data on long-term population trends are available for any populations of H. dacotae in Manitoba or Saskatchewan.

Movements/dispersal

Little information is available on the dispersal of H. dacotae 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).  Dana (1991) estimated average adult movements of about 300 m over a three- to seven-day period.  Dakota Skipper experts interviewed by Cohrane and Delphey (2002) thought it was unlikely that H. dacotae would move more than one kilometre across non-native prairie habitat (crop fields or pastureland) to another prairie patch.  Royer and Marrone (1992) also suggested that H. dacotae were unlikely to disperse far from their native prairie habitats.  Additional studies are required to examine the potential long-range dispersal capabilities of this species.

Interspecific interactions

At most sites in Manitoba and Saskatchewan, H. dacotae far outnumbered other species of butterflies, and at some sites it was the only species observed. The two most common species of skippers at most H. dacotae sites were Oarisma garita (Reakirt) and P. mystic.  Few interactions between H. dacotae and other species of butterflies were observed at most.  Occasionally, male H. dacotae pursued a skipper of another species, but the pursuits were short.  Most of these interactions were with P. mystic, which often occurred in the adjacent wetter sections of the prairies.  It is unlikely that there is any competition for larval or nectar food resources among these species of butterflies. 

Adaptability

The Dakota Skipper is extremely susceptible to habitat changes and is rarely found in prairie habitats that have been altered (McCabe 1981).  Although the immature stages and adults can use a variety of species of plants for food and reproduction, they appear to be restricted to using species associated with undisturbed prairie habitats.  Alteration of this native plant community results in the loss of critical resources for the skipper, which is unlikely to move to new prairie habitats that are more than one kilometre away from the original habitat (Dana 1991, Royer and Marrone 1992).  The poor dispersal capabilities and dependence on a specific suite of hostplant species make H. dacotae especially susceptible to habitat degradation, particularly when remnant populations are widely dispersed.