Investigating the role of non-helpers in group living thrips

Behavioural variation among individuals is a hallmark of cooperative
societies, which commonly contain breeders and non-breeders, helpers and
non-helpers. In some cases, labour is divided, with non-breeders
“helping”. Conversely, in some societies, subordinate non-breeders may not
help. These individuals may be (i) an insurance workforce ensuring
continuity of help for breeders when other helpers are lost, (ii)
conserving energy while waiting to breed themselves, or (iii) simply of
too poor physiological quality either to help or breed. In the Australian
Outback, Acacia thrips Dunatothrips aneurae (Thysanoptera) glue
Acacia phyllodes into “domiciles” using silk-like secretions, either alone
or cooperatively. Domicile maintenance is important for humidity, so
repair can be interpreted as helping. I found that not all females helped
to repair damage; some repaired partially or not at all
("non-helpers"). At the same time, some co-foundresses are non-
or partially reproductive ("non-breeders”), and their role is
currently unknown. I first tested the possibility that helping
and breeding are divided, with non-helping females breeding, and
non-breeders helping. In a lab experiment, I rejected this idea.
Experimentally damaged domiciles were typically repaired by reproductive
females, and less so by non- or partially reproductive individuals. To
test whether non-helpers are an insurance workforce, I successively
removed repairing females and found that non-helping females from the same
domicile did not increase effort as a result, rejecting this hypothesis.
Then I tested whether non-helpers were conserving energy waiting to breed.
In a field experiment, I removed all other females, allowing either a
helpful female or a non-helper to “inherit” her domicile. Isolated like
this, non-helpers laid very few eggs compared to helpers or naturally
occurring single foundresses, despite similar ovarian
development. My findings show that labour was not divided:
reproduction and helping covaried positively, probably depending on
individual variation in female quality and intra-domicile competition.
Non-helping females were neither an insurance workforce nor conserving
energy waiting to breed. They are likely simply of poor quality,
freeloading by benefiting from domicile maintenance by others. I
hypothesize they are tolerated because of selection for indiscriminate
communal brood care in the form of domicile repair.