Pest management - Ecological Applications: Toward a Sustainable World - page 155

Table 6.2 The record of
insects as biological
control agents against
pests and weeds. (After
Waage & Greathead,
1988.)

Insect pests Weeds

Control agent species 563 126

Pest species 292 70

Countries 168 55

Cases where agent has become established 1063 367

Substantial successes 421 113

Successes as a percentage of establishments 40 31

themselves (Section 6.3.4). Here the aim is to kill the pests present at the time of
the release and, by analogy with the use of chemicals, such agents are sometimes
known as biological pesticides .
Insects have been important agents of biological control against both insect pests
(particularly parasitoids that lay their eggs in or on the bodies of their host) and
weeds. Table 6.2 summarizes the extent to which they have been used and the
proportion of cases where the establishment of an agent has greatly reduced or
eliminated the need for other control measures. In the next sections I present some
success stories. Note, however, that most biological control attempts have not been
successful, and some have undesirable effects. I will turn to these in Section 6.3.5.

6.3.1 Importation
biological control - a
question of scale

The most classic example of 'classical' biological control concerns the cottony
cushion scale insect ( Icerya purchasi ), discovered as a pest of Californian citrus
orchards in 1868. By 1886 it had brought the citrus industry to its knees. Species
that colonize a new area may become pests because they have escaped the control
of their natural enemies. Importation of some of these natural enemies is then, in
essence, restoration of the status quo. Ecologists corresponded with colleagues
around the world to try to discover the origin of the scale insect and the identity of
its natural enemies. This requires a high level of taxonomic skill and is by no means
an easy task, particularly if in its native habitat the 'pest' is kept rare by enemies
that are also at low density. The search eventually led to the importation to Califor-
nia of two candidate species. The fi rst was a parasitoid, a two-winged fl y ( Crypto-
chaetum sp.) that laid its eggs on the scale insect, giving rise to a larva that consumed
the pest. Twelve thousand of these were despatched from Australia. The other was
a predatory ladybird beetle ( Rodolia cardinalis ), of which 500 arrived from Australia
and New Zealand. Initially, the parasitoids seemed to have disappeared, but the
predatory beetles underwent such a population explosion that, amazingly, all scale
insect infestations in California were controlled by the end of 1890. Although the
beetles have usually taken the credit, the long-term outcome has been that the
beetles keep the scale insects in check inland, but Cryptochaetum is the main control
near the coast (Flint & van den Bosch, 1981). The economic return on investment
in biological control was very high in California and the ladybird beetles have sub-
sequently been transferred to 50 other countries.
Another invasive scale insect was killing off the national tree of the small South
Atlantic island of St Helena (the last home of another famous invader - Napoleon
Bonaparte). Only 2500 St Helena gumwoods ( Commidendrum robustum ) were left
when, in 1991, the South American scale insect Orthezia insignis was found to be
mounting its attack, killing more than 100 of the trees by 1993. Fowler (2004)