E. robusta lack strict morphological castes, thus allowing for their plastic social behavior and dominance hierarchies.

It belongs in Apidae family within the order Hymenoptera, which consists of ants, wasps, sawflies, and bees.

In contrast, reused nests have a dark coloration caused by the aging of pollen and nectar from the previous year's brood-rearing activities.

[1] Their nests are constructed in the pithy center of frond stems, and consist of a single, undivided burrow.

These bees typically do not stray far from their birth nest during colonization, leading to a low dispersal rate.

[5] Egg laying occurs during the end of winter and throughout the spring, and new adults emerge from their pupae (eclose) during the summer.

It is important to note that these differences are behavioral, not morphological, meaning that every colony member has the biological capacity to perform any role.

[2] The social structure of E. robusta is polyphenic, meaning different behavioral phenotypes can arise from the same genotype based upon environmental conditions.

Depending on habitat, these bees can exhibit solitary, semisocial, quasisocial, or eusocial colony structure.

The type of sociality found in this species of bee largely depends upon the number of broods produced per season.

However, some heathland populations have been shown to produce two broods per season, allowing for the incidence of sibling rearing and therefore the presence of eusociality.

[1] Essentially, social polyphenism allows E. robusta to respond to changing environments by keeping their behavior plastic.

Once a nest enters its second or later year of use, the first female to eclose takes on a reproductively dominant status resulting in a semisocial organization.

First eclosed females take on a guarding role, which increases their risk of danger in from predators and competitors at the nest entrance.

[5] It has been suggested that dominants control reproduction in nestmates through pheromonal signals that inhibit ovary development in the non-dominant females.

By preventing her nestmates from breeding through nest guarding (or evicting them if they do), she can ensure that her daughters will be the first eclosed and therefore will gain dominant status as she did.

[5] Typically, reproductive skew would be a good indicator of fitness, due to an increase in total offspring being produced.

The total number of daughters produced may not translate into the most genes being spread, because only the production of dominant females significantly increases fitness.

It was found that less than 4% of females nest solitarily, supporting the idea that group living is very important for successful brood rearing.

This supports the observation that nests are founded by multiple females, since no primitively eusocial bee or wasp mates multiply.

[4] The importance of group living in E. robusta helps to explain both the relatedness found within colonies as well as their lack of kin discrimination.

Due to the lack of a rigid caste system in E. robusta, it is easier for I. schwarzi to integrate themselves within the colony.

[8] Due to the communal nature of brood rearing, it is quite easy for I. schwarzi to add their eggs into the common tunnel.