Bombus affinis

All rusty patched bumble bees have black heads, but only workers and males have the rusty reddish patch that gives the species its name.

Size varies a lot between castes: queens are 19 to 23 mm long, workers 9 to 16 mm and males 14 to 17 mm.

This bumble bee historically lived in the eastern and midwestern United States and in southern Canada.

Before the late 1990s it was found across 31 US states and Canadian provinces; since 2000 it has been reported from only 14.

Each colony lasts a single year. Before winter the old queen, workers and males die, and only new queens survive the winter to start new colonies in spring.

Healthy colonies are large and can hold more than a thousand workers.

It is one of the first bumble bees to appear in spring and one of the last to go into hibernation.

Nests are usually 1 to 4 feet underground, in abandoned rodent nests or other mammal burrows.

It gathers pollen and nectar from many kinds of flowers, but its short tongue makes deep, tube-shaped flowers hard to use.

Because it emerges so early, it may depend on spring wildflowers of woodlands.

Since 1999 it has been seen at 103 populations, an 88 percent drop from the number of populations recorded before 2000.

The US Fish and Wildlife Service listed it as endangered in a rule that took effect on February 10, 2017.

Its main threats include habitat loss and degradation, disease-causing organisms, pesticides and the problems of very small populations.

The exact cause of its decline is still unknown; evidence points to an introduced pathogen acting together with insecticide and fungicide exposure.