Biological control of weeds

Definition of a WEED• “A plant out of place”

– Growing where they are not wanted– A dandelion is a weed in a lawn, BUT grass is

a weed on a dandelion farm! (essential source of nectar for honey bees in early spring)

– A wheat plant in canola field– Mostly, exotic plants outside their native environments (but

bearing suitable climatic conditions) & lacking natural enemies

• Consequences– Significant losses to economic crops– Detrimental effects in natural ecosystem

Weed Management• Mechanical control

– Hand or machine– Burning

• Chemical control– Herbicides

• Biological control(Analogue to biocontrol of insect pests)– Pathogens

• Bacteria; fungi; nematodes… etc.– Host specific phytophagous insects

(plant “predators”)• Classical biological control• Augmentation biological control

Biological control of weeds

BEFORE AFTER

• ExamplesDodder, a parasitic plant, infects cranberry causing yield loss and plant death; controlled with a specified fungus

Waterhyacinth, an introduced invasive aquatic plant, choking a waterway in Mississippi; controlled by insects introduced from the area of weed Origin

Northern jointvetch infests rice fields, lowering yields. Seeds mixed with harvested rice lower crop quality; controlled by aerial applications of the bioherbicide Collego®

Biological control of weeds• Ex.: Purple loosestrife

– Valued wildflower native Eurasian habitats

– Existing in small stands & as isolated plants controlled by a complex of natural enemies

– It was introduced to North America in the late 19th century where it now exists in huge, monotypic stands, choking waterways and crowding out native North American species.

– Traditional methods of weed control, such as cultivation & the use of herbicides, are not practical or desirable for such wide-ranging stands of weeds, and biological control has become an attractive strategy for control of pest weeds.

Biological control of weeds• Started in Canada in 1952

– Release of the defoliating beetles Chrysolinaquadrigemina & C. hyperici to control St. John's wort

• Another examples of successful classical biological control of introduced weeds in North America:– Nodding thistle (Canada; Kansas)– Ragwort (BC, California & Oregon)– Klamath weed (Ontario, California, Oregon &

Washington)

Biological control of weedsEstimated 50 - 75% of problem weeds were either accidental or intentional introductions from other areas

Biological control of weeds• Considerations for species for introduction to control

pest weeds– Specific to one plant species – Has negative impact on host

plant individuals & its population– Is prolific – Thrives and becomes widespread in all habitats & climates

that the pest weed occupies – Is good colonizer– Less overall expense compared to other control tactics– Does not require a high level of technology– Less disruptive ecologically & so natural biodiversity is

maintained

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti – Cactus moth, Cactoblastis cactorumorigins origin

to South America

– Prefers to lay its eggs & feed on prickly pear cacti, Opuntia sp. (also known as Platyopuntia)

– Other related cacti are not thought to be preferred hosts

Classic biological control of weeds: Case studies I

The cactus moth & prickly pear cactia) Before sunrise, female releases sex pheromone signaling to males her readiness to mate; males respond

b) Mating takes placec) After incubation, female deposits egg stick averaging 70-90 eggs

d) Eggs hatch in 25-30 day; gregarious 1st instar larvae bore into cactus pad

e) External damage is characterized by yellowing of plant tissues, with oozing of plant fluids & insect frass

f) Larvae feed on inside tissuesg) Larvae develop internallyh) Feeding results in hollowing out cactus pad; mature larvae exit cactus pad i) Larvae form cocoons j) Pupation is taken place under debris on the ground at the base of the plant. k) After emergence, adult moths disperse to new areas

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti – in the 1920’s, prickly pear cactus spread widely

Queensland, Australia after accidental introduction – Cactus moth introduced from its origin to Australia

to control prickly pear cactus– Achieved control in 18 months; considered the

most effective biological control agent testedBEFORE

AFTER

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti– The success C. cactorum in Australia influenced

its use in other parts of the world

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti

– From biocontrol agent to PEST

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti

– Why pest?!• Agriculture

• United States– Rangeland Grazing

– Nursery Plants and Landscaping

– Fruit and Pad Production in California

• Mexico– Fruit, pads (cladodes), and dye representing 2% of Mexico's

total agricultural production

– Cosmetics & other value added products

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti– Why pest?!

– Conservation• Loss of Biodiversity

– Species richness

– Endangered Species impacted

• Habitat loss – Major plant component of desert ecosystems (Sonoran,

Mojave, Chihuahuan deserts)

– Wildlife habitat and food source

Classic biological control of weeds: Case studies I

• The cactus moth & prickly pear cacti– Why pest?!

– Human Impacts, Values• Recreation

• Tourism

• Hunting

• Indigenous groups use for food, medicine

• Importance to farmers in subsistence areas

• Historically significant in culture and religion

Classic biological control of weeds: Case studies II

• Salvinia and phytophagous weevils–The weed

• Salvinia, aquatic fern, spread by human from SE Brazil to many tropical & subtropical regions

• Prefers warm, nitrogen-rich water• Out of origin area, lacks natural

enemies• Became serious weed problem in

its non-native range– Can completely block waterways, without visible control method– Chocks flow, disrupting livelihood of people depend on

waterways (transportation, Irrigation, fishing…etc.)

Classic biological control of weeds: Case studies II

• Salvinia and phytophagous weevils–The biocontrol agent

• 2 mm long weevil, Cyrtobagous salviniae

• Host specific

• Has high searching efficiency for salvinia

• No immigration with high population

• Adult feeds on salvinia buds

• Larva tunnels through buds & rhizomes& feeds externally on roots

Classic biological control of weeds: Case studies II

• Salvinia and phytophagous weevils

–The control

• Initial biological control of salvinia failed because of

taxonomic mistakes of the plant & weevil

• The taxonomic problems solved in 1978

• The weevils used in many regions in Africa, Asia, US &

Pacific with outstanding successes

Classic biological control of weeds: Case studies II

• Salvinia and phytophagous weevils–The control

• Economic benefits– In Sri Lanks, the return in investment of 53 : 1 in terms of cash– 1678 : 1 in terms of hours of labor– The research team recognized by the UNESCO Science Prize, 1985

BEFORE

AFTER