Agriculture Reference
In-Depth Information
Eusociality - advantages and costs
sequently transfer the toxicants to the
developing brood. Finally, the presence of
overlapping generations living in large
groups enhances the fl ow of insecticides
through all castes and developmental
stages, effectively targeting all members of
the colony and helping to achieve complete
colony elimination.
Ants, along with bees, wasps and termites,
are classifi ed as eusocial. Eusocial insects
live in colonies defi ned by three features: (i)
reproductive division of labour - the colony
is composed of one or more reproductive
females and nonreproductive workers that
take care of the colony; (ii) cooperative
brood care - members of the colony share
responsibility for caring for the young; and
(iii) overlapping generations - different
generations (eggs, immatures and adults)
live together. These features have contrib-
uted to the great ecological success of ants,
which are the most diverse of all eusocial
insects, have been present for over 120
million years, and number as many as 10
16
individuals alive at any given time. The
same features, however, also pose risks for
the colony and can be exploited in the
management of ants. Reproductive division
of labour involves extreme task specializ-
ation where one or more reproductive
females lay the eggs and nonreproductive
(sterile) workers perform duties that benefi t
the colony (e.g. foraging, defence and nest
construction).
Task specialization makes colonies
potentially very effi cient in gathering food
resources and this feature can be used to
effectively deliver toxic baits to colonies.
Furthermore, reproductive division of
labour also aids in colony elimination
because the colony contains relatively few
queens capable of laying eggs and is
composed of mostly sterile workers. Killing
the queen(s) typically eliminates the colony
by breaking the reproductive cycle. Queens
are, however, much more diffi cult to kill
using baits so reproductive division is not
necessarily a disadvantage. Cooperative
brood care, where members of the colony
share responsibility for caring for the
developing brood (eggs, larvae and pupae)
can also be utilized in pest management. In
ant colonies, adult workers dedicate a large
proportion of their time to caring for the
brood. This involves cleaning, sorting,
moving and feeding the brood. Workers that
have fed on toxic baits or encountered
residues of spray insecticides may sub-
Polydomy
Ant colonies display enormous diversity in
colony size and spatial organization and
comprise a continuum ranging from
colonies that occupy only one nest
(monodomy) to colonies that occupy
multiple nests (polydomy) (Debout
et al.
,
2007). From a pest management perspective,
polydomy makes ant management more
diffi cult because the colony is dispersed
over a number of nests that may be diffi cult
to locate and treat. Polydomy also acts as a
strong buffer against unfavourable natural
events (e.g. drought, predation, disease and
lack of food) or man made events (e.g.
insecticide applications). If one nest is lost,
the other nests can supply the affected nest
with new queens and workers. Typically,
ant colonies fall into one of four spatial
arrangements: (i) monodomous, (ii)
polydomous, (iii) supercolonial and (iv)
unicolonial. The simplest colony structure
is monodomy where the whole colony
occupies a single nest and individual nests
are typically aggressive towards neighbour-
ing nests of the same species (multi-
coloniality). This confi guration is also the
easiest to control, especially if the nest can
be located and treated directly. However,
relatively few urban pest ants are mono-
domous, and most are highly polydomous,
complicating control efforts.
Colonies of some polydomous species
(e.g. carpenter ants) may be monodomous
during the initial founding stage, before the
colony reaches full size and expands into
additional nests. In polydomous ants, each
colony is made up of a set of neighbouring
nests that are separated spatially yet con-
nected socially by the exchange of indi-
viduals and resources. Substantial variation
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