Increased regulation of chemical pesticides and rapid evolution
of pesticide resistance have increased calls for sustainable pest
management. Biological control offers sustainable pest suppres-
sion, partly because evolution of resistance to predators and
parasitoids is prevented by several factors (e.g., spatial or tempo-
ral refuges from attacks, reciprocal evolution by control agents,
and contrasting selection pressures from other enemy species).
However, evolution of resistance may become more probable
as agricultural intensification reduces the availability of refuges
and diversity of enemy species, or if control agents have genetic
barriers to evolution. Here, we use 21 years of field data from
196 sites across New Zealand to show that parasitism of a key
pasture pest (
Listronotus bonariensis
, Argentine stem weevil) by
an introduced parasitoid (
Microctonus hyperodae
) was initially
nationally successful, but then declined by 44% (leading to pasture
damage of c. NZD
$
160m p.a.). This decline was not attributable to
parasitoid numbers released, elevation or local climatic variables at
sample locations. Rather, in all locations the decline began 7 years
(14 host generations) following parasitoid introduction, despite re-
leases being staggered across locations in different years. Finally,
we demonstrate experimentally that declining parasitism rates
occurred in ryegrass
Lolium perenne
, which is grown nationwide in
high-intensity pastures, but not in adjacent plots of a less-common
pasture grass (
Lolium multiflorum
), indicating that resistance to
parasitism is host-plant dependent. We conclude that low plant
and enemy biodiversity in intensive large-scale agriculture may
facilitate the evolution of host resistance by pests and threaten
the long-term viability of biological control.