The caterpillar of this species, often referred to as the "imported cabbageworm", is a pest to crucifer crops such as cabbage, kale, bok choy and broccoli.

It is not found north of Canadian life zone, nor on Channel Islands off the coast of southern California.

By 1898, the small white had spread to Hawaii; by 1929, it had reached New Zealand[11] and the area around Melbourne, Australia, and found its way to Perth as early as 1943.

In Britain, it has two flight periods, April–May and July–August, but is continuously brooded in North America, being one of the first butterflies to emerge from the chrysalis in the spring and flying until hard freeze in the fall.

The small white will readily lay eggs on both cultivated and wild members of the cabbage family, such as charlock (Sinapis arvensis) and hedge mustard (Sisymbrium officinale).

[8][14] It has been suggested that isothiocyanate compounds in the family Brassicaceae may have been evolved to reduce herbivory by caterpillars of the small white.

[15] However, this suggestion is not generally accepted because the small white has later been shown to be immune to the isothiocyanate forming reaction due to a specific biochemical adaptation.

In contrast, the small white and relatives seem to have evolved as a consequence of this biochemical adaptation to the isothiocyanate-forming glucosinolates.

Traditionally known in the United States as the imported cabbage worm, now more commonly the cabbage white, the caterpillars are bluish-green, with tiny black spots,[16] a black ring around the spiracles, and a lateral row of yellow dashes, and a yellow middorsal line.

[13] In the larval stage, the small white can be a pest on cultivated cabbages, kale, radish, broccoli, and horseradish.

[8] Considered a serious pest, the caterpillar is known to be responsible for annual damage worth hundreds of thousands of dollars.

[19] This dispersal of damage is seen as an adaptive behavior to hide the visual cues from predators that rely on vision.

Even though P. rapae larvae are cryptic, they remain in the sun for the majority of the day, rather than hiding on the underside of the leaf.

Larvae that have previously fed on crucifers will refuse nasturtium leaves to the point of starving to death.

[20] The elevated nutrition level also decreased the fourth instar's consumption rate and increased its food utilization efficiencies.

Larvae on cultivated host plant was observed to have higher growth efficiency than those fed in foliage of wild species.

An unreceptive female may fly vertically or spread her wings and raise the abdomen to reject the male.

leicocarpum), Streptanthus tortuosus, Thlaspi arvense (larvae grow slowly or refuse it); Capparidaceae: Cleome serrulata, Capparis sandwichiana; Tropaeolaceae: Tropaeolum majus; Resedaceae: Reseda odorata.

[8] There are three phases to host selection by the P. rapae adult female butterfly: searching, landing, and contact evaluation.

[26] A gravid female adult will first locate suitable habitats, and then identify patches of vegetation that contain potential host plants.

[28] Females foraging for nectar will readily abandon a linear path; they will show tight turns concentrating on flower patches.

P. rapae in a lab environment showed no significant preference for the shape or size of the oviposition substrate.

Female butterflies preferred the older plants due to the attraction to the darker green color.

Once a gravid female lands on a plant, tactile and contact chemical stimuli are major factors affecting acceptance or rejection of the site for egg deposition.

Once a female lands on a host plant, it will go through a "drumming reaction" or a rapid movement of the forelegs across the surface of a leaf.

A gravid female adult will lay disproportionate number of eggs on peripheral or isolated plants.

[21] Age of butterflies appears to have no effect on their ability to select the source of highest concentration of oviposition stimulant.

The diurnal variation of temperature can be extensive with daily range of more than 20 °C on some sunny days and clear nights.

Studies in Britain showed that birds are a major predator in British town and city environments (such as in gardens) while arthropods had larger influence in rural areas.

[31] It is important to note that only one larva develops per host and the rate of C. rubecula is largely independent of P. rapae population size.