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Natural Enemies
- Oregon State University Press
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28 life histories of cascadia butterflies lesser fritillaries, and satyrs. Instead of prolonging diapause, these species have the ability to reenter diapause multiple times if environmental stimuli signal the not-too-distant onset of unfavorable conditions. Postdiapause larvae of Euphydryas checkerspots recommence feeding in late winter–early spring on fresh host-plant growth. If host plants are moisture stressed and/or relative humidities are low, indicating the plants may senesce quickly, larvae stop feeding and reenter diapause. Larvae are capable of exiting and reentering diapause multiple times, and individuals may live for 2–3 years with only short periods of development annually. Clearly, this conservative strategy ensures persistence of populations during multiyear droughts, common in many checkerspot localities in eastern basin areas of Cascadia. Spring development of high-elevation species is dependent on timely snowmelt. In late springs, postdiapause larvae of species like the Arctic Fritillary and Pelidne Sulphur may not be able to feed soon enough to enable completion of development for the normal midsummer flight period. It is important for postdiapause larvae of these species to determine whether enough time is available to complete development, and they appear to do this by measuring and responding to daylength. If daylengths are long (more than 12 hours?) once postwintering feeding commences, Arctic Fritillary and Pelidne Sulphur larvae develop through 1 instar, then reenter diapause for a second winter. This strategy may be more common than currently realized and certainly deserves more research, particularly on the precise nature of environmental cues involved for different species. Some high-elevation or northerly latitude species always need two years to complete development and always overwinter twice. These species routinely diapause in two stages, usually early-instar then late-instar larvae (e.g., Great Arctic). Butterflies, like other organisms, have evolved to survive climate-related hardships using an amazing variety of physiological and behavioral strategies. Cascadia butterflies and their immature stages must leave the Pacific Northwest in autumn or have an overwintering strategy that enables good survival under prolonged and harsh winter conditions. Many of the same species also need to deal with limited resources and excessive heat in summer. The result is a butterfly fauna that is extraordinarily tough, resilient, and well adapted to all that nature in the high latitudes can throw at it. Natural Enemies The immature stages of butterflies are prey to an enormous variety of natural enemies. The umbrella term “natural enemies” includes vertebrate and invertebrate predators, parasitoids, and diseases (pathogens). Some natural enemies are “generalists,” feeding Orange-colored dormant Hesperia colorado (Western Branded Skipper) L1, overwintering in grass seed head. Plebejus icarioides (Boisduval’s Blue) L2 stop feeding, turn reddish brown, and are dormant June–March. 29 natural enemies on a wide range of food that may include immature stages of butterflies. Some are “specialists,” specializing on a certain stage or perhaps a certain species or group of species. Eggs, larvae, and pupae of all butterflies are vulnerabletoattackbynaturalenemies.Theappearance, lifestyle, and behavior of all butterfly species are shaped to a large extent by innate strategies to avoid being eaten. Crypsis, distastefulness, gregariousness, web and shelter building, host-plant use, and adult wing patterns have all evolved largely as defenses against natural enemies. From hundreds of eggs laid by each female butterfly, only a handful will reach adulthood, largely as the result of actions of natural enemies. Natural enemies can be a significant obstacle to butterfly rearing. Eggs, larvae, and pupae obtained from the wild may be parasitized and produce parasitic wasps or flies instead of butterflies. Some species are particularly prone to disease: in Cascadia, wildcaught eggs and larvae of the Compton Tortoiseshell (Nymphalis l-album) are often host to a virus that kills larvae before pupation. Host plants obtained from the wild may also harbor natural enemies, particularly predatory insects like Minute Pirate Bugs. While most of these predators can be shaken or washed off, eggs sometimes remain that hatch later, producing tiny but voracious predators. Predators: Predators seek, catch, and consume their prey. Eggs, larvae, and pupae fall prey to many types of predators, from vertebrates like birds, mice, frogs, lizards, and snakes to a whole array of invertebrates, from predatory mites to praying mantids. The impact of predators on populations of specific butterflies is largely unknown, but the relationships are no doubt complex and variable. Very few studies have been conducted on predation of immature stages of butterflies. An exception is the agriculturally important Cabbage White (Pieris rapae), which is targeted by predatory and parasitic insects...