Summary of findings
Apollo females preferred to oviposit on juniper bushes surrounded by low percentages of fallen wood, while Large Blue females preferred spots with high percentages of host plants and partly opened green buds, and showed a preference for performing this behaviour on red buds over green ones. Both species also showed a slight preference for spots with low percentages of < 5 cm field vegetation. Moreover, the final distance of Apollo larvae to nearby host plants did not differ significantly whether they were released downwind or upwind. However, the larvae showed a significant preference for sunny spots in their environment. Lastly, Marsh Fritillaries were more likely to fly over the interior edges of the study plot and preferred to fly over spots containing high percentages of host plants located far away from the grazed edge.
The finding that host plant cover was not significant for egg occurence in the Apollo was unexpected since the species has been found to be more common in patches containing lots of host plants. On the other hand, the high proportion of host plants at the spots where Large Blue oviposition behaviour occurred was expected since oviposition in this species is restricted to its host plant. The preference for the Large Blue to perform this behaviour on red over green buds in this study goes against previous studies showing the opposite relationship. It is also unclear why fallen wood and < 5 cm field vegetation had a negative relationship with oviposition, although one possible explanation could be that the butterflies prefer spots surrounded by rocks or bare soil because of their heat absorbing properties. However, more research is required before a conclusion can be made. Other factors that may have influenced oviposition for both species are olfactory cues, ambient temperature, microclimate and female motivational state. However, none of these factors were measured in this study.
Since the mean final distance of the Apollo larvae to the host plant clusters did not differ significantly relative to the wind direction, this indicates that olfactory cues are not used for navigation in this species. It is therefore more likely that host plant density is the main factor for host plant location in this species, as has been shown in previous studies. However, there is still a possibility that olfactory cues other than those from the host plants were used by the larvae in this study. For example, earlier studies have shown that Apollo larvae are more likely to remain in spots that smell of soil, possibly because the species host plant grows on patches of soil interspersed by rocky substrate. Because the movement patterns of the Apollo larvae in this study appeared to be mostly random, it is also unlikely that they relied on visual cues for orientation.
The preference of the Apollo larvae to seek out sunny spots was expected because this increases their body temperature, thus facilitating growth. Ambient temperature and microclimate may also have influenced habitat preference since larvae are known to prefer sunny spots at cooler temperatures, but neither factor was measured in this study. It is also possible that the larvae oriented themselves towards the sun as has been shown in other butterfly species, although this was not tested. Each larva was also observed during differnt times of the day, leading to varying levels of sunlight-intensity.
I show that Marsh Fritillaries were more reluctant to cross the edge of the grazed habitat, suggesting that habitat edges may serve as barriers to movements in butterflies, which has also been shown in other studies of butterflies and other insect species. This reluctance to cross over into grazed habitat may also explain why earlier studies of the Marsh Fritillary in the same general area have found fewer larvae in the grazed habitat. Moreover, the species’ host plant was an important predictor of butterfly numbers in this study, which agrees with previous studies. In addition, host plant size and abundance have also been found to be greater in ungrazed habitat, a sign of the importance of preserving this management type.
Conclusions
In this study, I have shown the importance of butterfly behaviour for assessing habitat quality. The reluctance of the Marsh Fritillary to enter grazed habitat shows the importance of ungrazed habitat for the survival of this species and for this reason, grazing should be of low intensity. This study also shows the importance of open habitat for Apollo larvae, signifying the importance of extensive grazing to prevent owergrowth of trees and shrubs, as well as the importance of host plants for both the Large Blue and the Marsh Fritillary. The reason why the Apollo avoided spots with fallen wood and why short field vegetation was selected against by both this species and the Large Blue remains a mystery, but could be explained by a lack of data. For this reason, further studies are required before accurate conclusions can be made.