Regal Fritillary - Threats

Threats

The greatest threat the regal fritillary faces is habitat destruction (Powell et al. 2006). A number of factors are continually contributing to the loss, fragmentation, and degradation of the butterfly’s habitat. Row crop agriculture, urban developments such as housing and business construction, road construction, and gravel mining, are all activities that contribute to the disappearance and degradation of the prairies that regal fritillaries depend on. Since regal fritillaries require relatively non-degraded native tallgrass and mixed-grass prairies, the alteration of these landscapes have pushed them into a highly vulnerable status (Selby 2007).

Largely due to the loss and degradation of the tallgrass prairie landscape, violet plant density tends to be limited or far reduced in certain areas. This has been shown to have a rather negative impact on regal fritillary populations. A study conducted by Kelly and Debinski (1998) looked at how larval food source limitations may be a factor in the declining regal fritillary populations. In the study, they correlated violet plant density to population size and weights of regal fritillary butterflies. It was found that prairies with significantly lower violet densities had smaller populations of S. idalia. Butterfly weights were also slightly lower in areas with low violet density (Kelly and Debinski 1998). As a result, a number of concerns have been raised regarding the health of the regal fritillary. Areas with few or no violets can be detrimental to female fecundity because there are few suitable places for the eggs to be laid (Kelly and Debinski 1998). Also, smaller fragmented populations are susceptible to restricted gene flow and reduced genetic variability (Williams et al. 2003). This study also showed that habitat quality for the regal fritillary is just as important as the amount of habitat available. Increased violet density and nectar availability are essential to maintaining healthy populations (Kelly and Debinski 1998).

Habitat fragmentation and isolation can have large scale genetic effects on high gene flow species such as the regal fritillary. There is an increased likelihood of population extirpation among high gene flow species experiencing habitat fragmentation (Williams et al. 2003). Williams et al. (2003) conducted a study that compared levels of genetic differentiation and diversity among populations with a relatively continuous habitat to populations in isolated habitat areas. It was found that the isolated and highly fragmented populations had increased differentiation, or divergence from other populations, and decreased genetic diversity in comparison to non-fragmented populations. Restricted gene flow and population bottlenecks likely occur among populations in fragmented habitat areas, causing these phenomena (Williams et al. 2003). As habitat fragmentation continues to increase in much of the regal fritillary’s Midwestern range, genetic problems may become a real threat, disrupting gene flow and increasing the risk of disease.

Prescribed burning is an attractive and widely used conservation tool among land managers today. The historic role that fire played in the prairie landscape is important and can be highly beneficial to many plant species. Prescribed burns have also become a popular low-cost alternative for removing woody vegetation on rural and agricultural lands (Powell et al. 2006). However, there has been some evidence that heavy fire management used on prairie lands can negatively affect the regal fritillary. A study conducted by Powell et al. (2006) examined the effects of prescribed prairie burns by surveying a number of prairies in Kansas. Butterflies were observed on both recently burned and unburned sites to determine the effects of prescribed burning on the populations. Population density of the regal fritillary may tend to vary among sites but was generally quite a bit higher at sites that had not been burned in the past year (Powell et al. 2006). Prescribed burns are usually conducted in the early spring while the first instar larvae remain vulnerable, buried in the leaf litter. Extensive prairie burns kill the overwintering larvae and can have a drastic effect on their population in the following years. One recently burned prairie that was studied used minimal patch burning by burning only small portions of the entire area at a time. This site had by far the highest regal fritillary abundance of any burned site (Powell et al. 2006).

A serious potential threat to the regal fritillary was discovered in a captive breeding study. Wagner (1995) found that disease is a possible mortality factor in some S. idalia populations. In a captive group, nuclear polyhedrosis virus (NPV) caused an 80% loss. The virus is transmitted from females to offspring in eggs or between individuals through excreta (Wagner 1995). NPV could potentially be damaging to populations in the wild (Mason 2007), thus for reintroduction purposes, culturing a virus-free line is critical.

The regal fritillary is highly vulnerable to environmental factors year-round. Extreme weather conditions over a large geographical range can severely influence their populations. First instar larvae are highly sensitive to extreme weather conditions as they overwinter in the leaf litter and as they begin their initial search for food plants in the spring. Hard frosts late in the spring, severe storms, and cool damp conditions have all been shown to negatively impact larvae survival (Selby 2007). Larval development rates tend to be proportional to the temperature. Therefore, unusually cool conditions in the spring can drastically slow larval growth rates, increasing their exposure to mortality factors (Selby 2007). Some environmental factors can limit adult regal fritillary activity as well. Prolonged periods of cooler temperatures, cloudy skies, and rain can encumber normal activities, perhaps limiting reproduction (Selby 2007).

The increased use of pesticides and herbicides can have profound negative effects on the regal fritillary as well. Heavy spraying of herbicides can eliminate nearby larval food plants and nectar sources that they are dependent on (Selby 2007). The indiscriminant use of pesticides also poses a big threat to regal fritillaries and other prairie-specialist butterflies. Pesticides such as the bacterial pathogen Bacillus thuringiensis ("Bt", the agent used in gypsy moth control) is lethal to all Lepidoptera larvae. It is thought that the gypsy moth control programs used in the east along wooded grassland edges may have been a final factor leading to the loss of some populations (Selby 2007). Broadcast spraying of insecticides for pest control on adjacent crop land and rangeland continues to be a direct threat to the regal fritillary.