Pest management - Ecological Applications: Toward a Sustainable World - page 158

case in point. These birds like to collect caterpillars to feed their young at a time
when caterpillars of winter moths Operophtera brumata and other species are causing
damage in the orchards. Mols and Visser (2002) experimentally manipulated access
of great tits to apple trees using netting. They found that foraging by great tits caused
a small reduction in caterpillar damage (down from 13.8% to 11.2%) but, in terms
of actual fruit yield, the benefi t was much bigger (up from 4.7 to 7.8 kg of apples per
tree). The only cost to the producer was the erection of nest boxes (2 per hectare)
to encourage the birds to breed in their orchards. Conservation biological control
clearly has wide application across urban, cropping, forestry and fruit production
settings.

6.3.3 Inoculation
biological control -
effective in
glasshouses but
rarely in fi e l d crops

Inoculation as a means of biological control is often used to control invertebrate
pests in glasshouses. In this situation, the crop, its pests and their natural enemies
are all removed at the end of the growing season, so there is no opportunity to
establish a pest-enemy equilibrium. Two particularly important natural enemies
used for inoculation are Phytoseiulus persimilis , a mite that preys on the spider mite
Te t ra nychu s urticae , a pest of roses, cucumbers and other vegetables, and Encarsia
formosa , a chalcid parasitoid wasp of the whitefl y Tr i a l e uro d e s va p ora r i or u m , a pest
in particular of tomatoes and cucumbers. Adequate distribution of the natural
enemies is crucial, and this can sometimes be assisted by means of fans or the con-
struction of 'bridges'. Thus, when P. p e r s i m ili s is introduced to control T. ur t ica e on
cut roses, the provision of bridges of plastic tape to enhance predator dispersal leads
to a 50% reduction in leaves infested with the pest (Casey & Parrella, 2005). This
is a modern version of the fi rst recorded use of natural enemies, in China in 324 BCE ,
when artifi cial bridges between citrus tree branches provided ready access to cater-
pillar pests for their natural enemies - ants!
Inoculation may also be used under fi eld conditions, particularly where natural
enemies fail to colonize crops, or arrive too late in the season to provide effective
pest control. However, in their review of studies concerning inoculations of pre-
datory insects and parasitoids into fi eld crops, Collier and Van Steenw yk (2004)
found that inoculation failed to reduce pest populations to target levels in 64% of
cases. This was for a variety of reasons, including unfavorable environmental condi-
tions for the control agent, dispersal of the agent away from the target area and
predation on the agent. These are issues that do not arise under the controlled and
enclosed circumstances of the greenhouse. Moreover, in economic terms, inocula-
tion in fi eld crops was usually more expensive than pesticide application, and in a
few cases actually cost more than all the other crop production costs combined. On
the other hand, augmentation sometimes works well. Thus, when the parasitoid
wasp Aphytis melinus was inoculated at the rate of 50,000 per hectare into US lemon
orchards to control the Californian red scale insect Aonidiella aurantii , the pest was
reduced to target levels at a cost of only US$102 per hectare, a very small percentage
of overall crop production costs (more than $25,000 per hectare; Moreno & Luck,
1992).
Most often insects for inoculation are purchased from a commercial producer, but
plant material infested with dormant pests and natural enemies may provide a local
source of control agents. Kehrli et al. (2005) collected horsechestnut ( Aesculus hip-
pocastanum ) leaf litter with its associated pests (the invasive horsechestnut leaf-
mining moth, Cameraria ohridella ) and natural enemies (parasitoids) and placed all