Biological Control and
Invasive Invertebrates
M.L. Henneman
Biological Control (=“biocontrol”):
humans manipulating interactions
between species in order to control a
species considered to be a pest
Examples of Biological Control
Classical Biocontrol
•animal herbivore or pathogen (ex. fungus) introduced
to feed on plant pest
•animal predator or parasite introduced to feed on
animal (usually insect) pest
* If the target pest is native, sometimes the term
“Neoclassical” biocontrol is used.
Types of Biological Control
Conservation Biocontrol
• enhancing conditions under which a natural herbivore,
predator, or parasite of a pest thrives
Types of Biological Control
Augmentation Biocontrol
• artificially rearing above agents to augment natural
numbers
Success with classical biocontrol
(= alien target species)
• Sugarcane leaf hopper and egg predators
• Hibiscus mealy bug (invaded Grenada in 1993) and
parasitic wasp Anagyrus kamali (Encyrtidae)
• Prickly pear cactus and Cactoblastis moth
Sugarcane leaf hopper and egg predators
The sugarcane leafhopper (Perkinsiella saccharicida) is native to
Australia, and was a significant pest in sugarcane-growing regions
throughout the world (though interestingly, it is restricted to Florida
in the Caribbean region).
Biology of sugarcane leafhopper
- Females live a month and lay up to 300
eggs
- Damage is caused by insects sucking the
sap from sugarcane
- Result is dead leaves, and sooty mold
growing from the honeydew produced by
the insects
- It is also a vector of Fiji disease, a virus that
makes tumors in the plant
Sugarcane leaf hopper and egg predators
The egg predator Tytthus mundulus, an Australian true bug from a
family of mostly plant-feeders, produced almost miraculous results
when introduced to sugar-growing areas.
Currently, sugarcane leafhopper is suppressed to the point where it is
no longer an economically damaging pest.
Savings to the sugarcane industry have been estimated to be
US$600 million.
Pink Hibiscus Mealybug
• Originally from India
• Invaded Grenada in 1993
• Extreme generalist – attacks hundreds of species in 200 genera, including
hibiscus, citrus, coffee, sugar cane, annonas, plums, guava, mango, okra, sorrel,
teak, mora, pigeon pea, peanut, grape vines, maize, asparagus, chrysanthemum,
beans, cotton, soybean, cocoa, and many other plants.
• From 1994-1997, damage in Grenada estimated at US$3.5-10 million
Pink Hibiscus Mealybug
PHM has up to 15 generations per year.
As it feeds, it injects toxic saliva that
stunts growth and can kill a plant when
heavily infested.
Pesticides and mechanical control were
not effective in the Caribbean.
Pink Hibiscus Mealybug control
A tiny parasitic wasp, Anagyrus
kamali, became instrumental in control
of pink hibiscus mealybug. Because its life
cycle is half the length of its host, the pest
was quickly overwhelmed wherever the
wasp was introduced.
Prickly pear cactus and Cactoblastis
• Prickly pear from Brazil was introduced to Australia
with the first white settlers in the 1700s in order to
establish a cochineal dye industry
• By the early 1900s, prickly pear had overrun large
parts of Queensland and New South Wales – 30 million
acres in Queensland were completely covered.
• in 1925, the cactus-feeding moth Cactoblastis
cactorum was introduced to Australia with amazing
results
Prickly pear cactus and Cactoblastis
Prickly pear cactus and Cactoblastis
Before
After
Prickly pear cactus and Cactoblastis
“In 1925, prickly pear, the greatest
example known to man of any
noxious plant invasion, infested fifty
million acres of land in Queensland,
of which thirty million represented a
complete coverage… This plaque,
affixed by the Queensland Women's
Historical Association … records the
indebtedness of the people of
Queensland and Dalby in particular,
to the Cactoblastis cactorum and
their gratitude for deliverance from
that scourge.”
Cactoblastis was dispersed around the world
Cactoblastis was dispersed around the world
• Cactoblastis was introduced to Nevis in the 1950s to control native prickly
pear that were considered a nuisance on rangeland. (= Neoclassical
biocontrol)
• It has now spread throughout the Caribbean to Florida, where it is
threatening several species of native prickly pear.
Cactoblastis U.S. distribution
Cactoblastis cactorum detections in the Southeastern United States
SC
MS
AL
GA
Charleston
2009
LA
Mobile
Pensacola
Tallahassee
New Orleans
FL
Farthest Western
Outbreak: Delta
area south of New
Orleans
Tampa
N
0
50 100
2008
2007
2005
2004
2003
2002
2001
2000
1993
1989
200 mi
Miami
Oct. 1989 First Continental US
Detection – Bahia Honda Key, FL
Cactoblastis in the U.S.
• It is being monitored closely, because if it reaches Mexico, it could not only
threaten more species of prickly pear, but it could cause significant economic
damage.
• Biological control using parasitic wasps that attack Cactoblastis is now being
studied.
Disasters in classical biocontrol
• Cane toads and mongooses - many sugarcane growing
regions (birds)
• African land snail and cannibal snail (hundreds of
species of endemic snails)
• Gypsy moth and Compsilura concinnata fly (giant silk
moths)
----> Generalists = non-target interactions!
Specialists can cause problems too
• Cactoblastis is a specialist
• Indirect effects: control of bitou bush weed in
Australia by a seed predator has artificially increased
numbers of a native parasitic wasp species
(Willis and Memmott, 2005)
Bitou control may cause indirect effects
Conclusions
• Success stories in biocontol are well outnumbered by failures – roughly 1 in 6
biocontrol agents are considered successful, and this ignores potential harm
caused.
• Classical biocontrol should be used carefully, and neoclassical biocontrol
should not be practiced. We need to increase monitoring.
• Current practitioners of biocontrol insist that it is now safely practiced; but
we have controlled only the problems we can control.
• We have no idea what impact most invertebrate biocontol agents are having;
because invertebrates are not easily studied, or even noticed, this “absence of
evidence” is clearly not the same as “evidence of absence” of negative effects to
the ecosystem.