Lecture 24 EFB 502
The Bad (Dark) Side of Biological Control
 Biological control viwed very positively for a 100 years +
 Widely regarded as a safe or ‘green’ alternative to chemical pesticides /
insecticides

Some important caveats
o The lack of obvious non-target effects in most introductions might be
misleading because until recently, few people had looked for them
(absence of evidence is not evidence of absence!!).
o In contrast to chemical approaches, if something becomes an obvious
problem with biological control, you cannot simply remove it from the
market or withdraw its registration. An established biological control
agent, for better or for worse, is functionally a permanent
introduction.
An Awakening
• A seminal paper by Francis Howarth (known as Pandora’s Box)ignited what
became a contentious debate about the relative safety of CBC
• Pitted biocontrol practitioners against conservation biologists
• Critics suggested that many of his examples were notorious vertebrate
introductions (e.g., mongoose, cane toad) and rapacious predators like the
rosy wolf snail that would never be introduced in a modern BC program.
• Conservation biologists lauded the paper as a wakeup call for an industry
that had escaped scrutiny for a long time.
The truth lies somewhere in between and some types of introductions appear to be
much safer (lower risk of non-target effects) than others.
Biological Control of Plants
• Relative to other biological control efforts, non-target effects from agents
introduced for plants have generally had fewer non-target effects.
• Recquiremnts for extensive host-range testing (phylogenetic wheel) and the
restricted diets of many herbivorous insects have led to relatively few
examples of widespread non-target effects.
• There are some egregious examples – Cactoblastis cactorum in North
America, three species of thistle feeding weevils) that have acquired lareg
number of non-targte species. Two of the weevils did go through rigorous
host range testing.
Biological control of insects
• Most biological control programs against insects have utilized parasitoids.
These are organisms that colonize a host (like a parasite) but mostly kill the
host after they finish feeding. Predators and pathogens are also used but to a
much smaller degree.
Parasitoids – largely from several wasp families of the Order Hymenoptera
and the fly family Tachinidae.
• Widely viewed as less risky than predators.
Tend to be specialized on a few species
Often an intimate interaction with host immune systems
Complex host or habitat specific search behaviors (often chemically mediated)
• Idiobionts
Kill or permanently paralyze their hosts at oviposition
Mostly ectoparasitoids
• Koinobionts
Allow host development after oviposition
Mostly endoparasitoids
Narrower host range?
Frequency of Non-Target Effects
• Hawkins and Marino (1997) - non-target attacks of parasitoids released for
classical biological control in North America Overall: 16.3% (50/313)
• Measurement of ‘drift’ , or the movement from intended target species to
others.
• Their paper assesses frequency (does it occur) but not necessarily strength
(how important are the effects) of non-target effects
•
Assessing Strength of Non-Target Effects (Parry 2009: Biological Invasions)
Assessing the importance of non-target parasitoid effects is challenging and
relatively few systems have been looked at. There are several approaches each with
strengths and weaknesses.
• Quantitative Food Webs (look at the connectivity between species and the
importance of those interactions – hard to do for a community of species but
very revealing.
Forest Lepidoptera – Hawaii (revealed big effects and the replacement of many
native parasitoids by exotic ones)
Forest Lepidoptera – New Zealand (a single exotic species became the dominat
source of mortality in multiple non-target species)
• Lifetable Analysis
Koa bug in Hawaii - an exotic fly introduced for control of an agricultural pest
was a significant source of mortality in the endemic Hawaiian native species, the koa
bug, but by itself, was not responsible for the decline in the species
• Experimental Populations
Several different systems, perhaps best developed for North American giant silk
moths (Saturniidae). Revealed that the decline in many of our most iconic (and
largest) moths in New England may be largely due to a parasitoid fly introduced for
gypsy moth control.
Bottomline. Classical biological control is a vital part of the effort to limit the
damage caused by invasive species. When done properly, it is relatively safe.
However, the risk is never zero and not entirely predictable. Thus all introductions
should be carefully considered and the risk evaluated to the best of our ability.