Gold-edged gem (Schinia avemensis) COSEWIC assessment and status report: chapter 6

Biology

Life cycle and reproduction

There are few data available on the life history of the Gold-edged Gem. Like all Lepidoptera, they undergo complete metamorphosis with egg, larval, pupal and adult stages. They are univoltine (one brood or cycle per year); adults have been collected in Canada from July 10 to August 20 (Table 1). Observations at Spruce Woods Provincial Park in 2004 showed that adults emerged as the first of the host sunflowers bloomed (August 4), and the flight period had ended while they were still in full bloom (August 26) (G. Anweiler, unpublished data, 2004).

All species of Schina studied by Hardwick (1996) deposit eggs in, or on, the flowering heads of their host plant. The ovipositors of Gold-edged Gems have the hard, knife-like, modified lobes found in species that insert their eggs downward among the florets of flowers in the family Compositae. Hardwick (1996) states unequivocally that in Manitoba Schinia avemensis “feed as larvae in the heads of Helianthus petiolaris…”, but provides no further details.

Schinia eggs hatch within a few days, and the larvae feed on or within the blossom, on the floral parts, and in some instances on the developing seeds of the host plant. Young larvae of some species have very specialized feeding habits, selecting a particular part of the floral structure, such as the anthers or the receptacle. Species feeding on composites usually feed first within the corolla tubes. The larvae of Schina complete development relatively rapidly (within two to four weeks, depending on species). Species feeding on annuals (as do Gold-edged Gems) were observed to develop faster than those feeding on perennials, in some cases in less than 14 days (Hardwick, 1996).

When fully developed, the larvae leave the plant, burrow into the soil, and form a cell in which they pupate. They remain in the pupal stage through the winter and spring, emerging to mate and reproduce when the host plant blooms the following summer (Hardwick, 1996). A number of species of Schina are able to remain in the pupal stage for more than one year, an adaptation to life in xeric habitats where conditions may not be suitable for the host plants to blossom each year (Hardwick, 1996).

All adult Heliothines are apparently short-lived; in laboratory rearings, none lived for more than a week (Hardwick, 1996). There are apparently no closely related species, and there is no evidence, and little likelihood, of hybridization.

Herbivory/predation

There are no data specific to Gold-edged Gems, and the following observations, excerpted from Hardwick 1996, refer to the subfamily Heliothinae in general.

During the early larval stages, both inter- and intra-specific cannibalism is a common phenomenon among species of Schinia feeding on composites. 

Larvae of different species of Schinia have developed a number of behaviours for avoiding parasites and predators, including leaving the host plant and spending daylight hours in a cell created by the larvae at the ground surface, or pulling the rays of the flower over the disc and binding them with silk into a roofed shelter. Larvae of many species greatly resemble the host plant in structure as well as colour and are well camouflaged while resting on the plant. Specific behaviours have evolved for resting on specific parts of the host to minimize visibility, i.e., some species curl around the base of the floral receptacle while others rest aligned along the stem (Hardwick, 1996).

Although there are no specific data, Gold-edged Gems are undoubtedly subject to predation and parasitism by a variety of birds, wasps and other animals during all life stages, as are most Lepidoptera.

Herbivory of the host plant by both wild and domesticated ungulates could result in the destruction of both the host and any larvae feeding therein. The flowering heads of prairie sunflowers are palatable to both wild and domestic ungulates, and significant local mortality of larvae could occur under certain conditions, i.e., during periods of drought when more desirable alternate forage is in short supply, or in situations where livestock is overstocked or confined to a restricted area, resulting in overgrazing (http://www.npwrc.usgs.gov/resource/literatr/wildflwr/species/helipeti.htm).

Physiology

There are no data specific to Gold-edged Gems.

Some species of flower-feeding Schinia are known to be physiologically adapted to have pupal diapause terminated in synchronicity with blossoming of the host plant. In some species, diapause in the pupae may be extended for several years until conditions are right for the host plants to blossom. This adaptation is most common in desert species, where unreliable or erratic rainfall triggers blooming of host plants. (Hardwick, 1996)

Dispersal/migration

Gold-edged Gems have rarely, if ever, been found in places other than in close proximity to colonies of the host plant. However, the adults are small, fast-flying insects and would be very difficult to observe or identify except when nectaring or at rest.

Many species of Schinia are strong fliers and good colonizers and are very adept at colonizing successional habitats or coping with plants that do not flower every year. They often reach high densities and can colonize small patches of plants if there are other source patches in the area. While straying does occur, adults concentrate very near their larval food plant (Schweitzer, 2001 in NatureServe, 2004).

Gold-edged Gems are not known to undergo any regular migrations or dispersal events.

Interspecific interactions

Gold-edged Gems are apparently dependant upon a single species of native annual sunflower for the larval host. Field observations at the Alberta and Manitoba colonies as well as at one site in Colorado indicate that skeletonweed is the primary nectar source for adults (G. Anweiler, unpublished data; B.C. Schmidt, pers. comm., 2004; C. Harp, pers. comm., 2004).

Adaptability

Members of the genus Schinia, and in particular the composite flower-feeding desert species, are capable of remaining in pupal diapause for extended (multi-year) periods, an adaptation to an environment where seasonal cycles of rainfall and plant blossoming can be unpredictable (Hardwick, 1996).

Most species of Schina are apparently relatively easy to rear under laboratory conditions (Hardwick, 1996), and thus it would likely be possible to breed Gold-edged Gems in captivity for release into the wild, should this be desired.

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