In a study in Illinois, Schemske (1978) found the primary pollinators of Impatiens pallida chasmogamous flowers to be the bumblebees Bombus vagans and B. impatiens, with rare visits by the (non-native) honeybee (Apis mellifera) and Ruby-throated Hummingbird (Archilochus colubris). Bumblebee pollination might be predicted based on the flower's large landing platform, broad sepal sac, and yellow color. Schemske (1978) collected a variety of small halictid and andrenid bees on I. pallida flowers, but viewed these bees as predominantly pollen and nectar thieves rather than effective pollinators because they showed a preference for male flowers, rarely contacted the stigma when foraging for nectar from female flowers, and usually foraged at just a single flower (never more than three) before leaving the area.
In his field studies of I. pallida at several sites in Illinois, Schemske (1978) identified a variety of herbivores feeding on this plant. The chrysomelid beetle Rhabdtopterus praetexus was a major leaf predator, sometimes causing complete defoliation through leaf skeletonizing. This beetle, which was observed mainly from early July to mid-August, fed almost exclusively on I. pallida and I. capensis. The sawfly Aglaostigma semiluteum was common throughout the growing season and probably restricted to I. pallida and I. capensis. Stem-boring larvae of the olethreutid moth Olethreutes agilana were common on both I. pallida and I. capensis (and are apparently restricted to these hosts) from mid-August until the end of the growing season. The arctiid moth Diacrisia virginica is known to be an extreme dietary generalist, but Schemske observed larvae only on I. pallida, on which it was present throught the growing season.
The aphid Dactynotus impatienscolens (probably an Impatiens specialist) was common on the peduncles and pods of I. pallida (and occasional on I. capensis). Galls on leaf midribs formed by larvae of by the cecidomyiid fly Lasioptera impatientofolia and on buds by Cecidomyia impatientis were common at one study site on both I. pallida and I. capensis. Nymphs of the pentatomid bug Acrosternum hilare were major seed predators of both I. pallida and I. capensis. Mortality of chasmogamous buds and flowers was primarily due to feeding by various grasshoppers and katydids.
2n=20 (Gleason and Cronquist 1991).
Quebec and Nova Scotia to Saskatchewan, south to North Carolina, Tennessee, and Oklahoma (Gleason and Cronquist 1991).
Wet woods and meadows (Gleason and Cronquist 1991).
Impatiens pallida is similar to the generally more abundant I. capensis. Impatiens capensis flowers are mostly orange-yellow, usually with reddish brown markings, with a slender spur (7-10 mm) curved gently forward; I. pallida flowers are mostly pale yellow, usually with reddish brown markings, with a short spur (4-6 mm) bent sharply down and forward (Schemske 1978; Gleason and Cronquist 1991; Tabak and von Wettberg 2008).
Impatiens pallida tends to grow on shadier sites than does I. capensis (Gleason and Cronquist 1991). Impatiens capensis is generally restricted to moist habitats adjoining lakes or rivers, but I. pallida is more tolerant of dry sites (Schemske 1978).
Leaves alternate; sepals three (the two upper are smaller than the third lower); flowers in axillary racemes, mostly pale yellow with brown spots, spur bent abruptly down and forward (Gleason and Cronquist 1991).
Impatiens pallida has an unusual mixed mating system with cleistogamous (closed and self-fertlizing) flowers and chasmogamous (open, accessible to pollinators, and outcrossing) flowers on the same individuals (Tabak and von Wettberg 2008).
Chasmogamous flowers are strongly protandrous (i.e., male parts, the anthers, mature first). When the anthers dry, they fall off, exposing the female part, the stigma. Thus, male and female function are separated in time, preventing self-fertlization and promoting outcrossing. Flowers remain in the male phase for about five times as long as in the female phase, making the effective sex ratio of flowers on a plant heavily male biased (Schemske 1978). In cleistogamous flowers, nectaries are absent (because the flowers are closed, pollinators have no access and producing nectar to attract them would be a waste). Sepals, anthers, and the number of pollen grains produced are all reduced. Production of chasmogamous flowers ceases in early fall, but cleistogamous flowers are produced until plant death from frost (Schemske 1978).
The fruit of I. pallida is a 5-valved capsule which dehisces (opens to release seeds) elastically, projecting seeds up to 2 meters from the parent (Schemske 1978). Ovaries of chasmogamous flowers contain 3-5 ovules, cleistogamous flowers 1-5 (usually 2-3). Total development time from bud stage to pod maturation and dehiscence is quite variable, but averages 28-38 days for cleistogamous flowers and 37-48 days for chasmogamous flowers (Schemske 1978).
