Read Ch 12 Weed Biological control 1. How do insects affect plants? (see pp#4) 2. How is weed biocontrol affected by components of the biotic community? 3. How do weed and insect biocontrol differ?
Feature #1: The role of plant-plant competition is unique Weed biocontrol agents act by changing the weed s competitiveness with other plants. In insect biological control, insect-insect competition among herbivorous insects is not important for insect biocontrol, which acts solely via antagonism imposed by introduced upper tropic level organisms.
Do not think of the plant-herbivore interaction in isolation. Galerucella pusilla cattail Purple loosestrife Herbivory s job is to reduce the invader s competitive advantage over native plants: plant-plant competition determines the outcome
How strong is cattail-loosestrife competition? How much herbivory will it take to restore the balance? Competition test
Feature #2:Impacts other than death are important to weed BC Weed biocontrol agents act in more ways that just killing plants outright. Other outcomes, might be slower growth, reduced seed set, reduced competitiveness, increased susceptibility to pathogens, shorter lifespans In insect biological control, nearly all impacts are the immediate death of the attacked stage. (In a few cases, reduced fecundity of attacked females is possible)
Weed biocontrol acts through many channels tissue galling seed destruction defoliation Increased susceptibility to pathogens reduced growth altered competitiveness
Feature #3:Reduction of reproduction is a frequent objective in weed biocontrol 1. some plants have long lived seed banks 2. some plants are very long lived (trees) 3. annuals have high annual seed set 4. many plants have vegetative reproduction from parts
How plants reproduce strongly influences approaches to reducing populations via suppression of reproduction Long lived plants Annuals, binenials-big seed banks Vegetative reproduction Waterhyacinth parts float and regrow Mature woody plants difficult to kill Seed banks maintain densities for years Indirect seed reduction by defoliation Purple loosestrife (Lythrum salicariae) in Ontario, Canada, in 1995, One year after first release of the leaf beetles. See the defoliation and lack of flowers Seed head flies eat seeds Direct destruction of seeds Green mat is S. molesta on lake in Australia 1995 Add photos of more species
Effects that act via thresholds example: seed bank reduction Urophora affinis Larinus minutus
Feature #4: Safety to nontarget plants has been a greater concern than impact on insects Platte thistle and R. conicus Ash whitefly and Encarsia inaron Damage to nontarget plants such as native thistles is likely to be noticed and objectionable Damage to nontarget insects, such as native whiteflies would be invisible to all but a tiny handful of specialists and impacts hard to measure
5 10 15 20 25 30 Feature #5: Time needed to conduct a weed biocontrol project is greater than an insect one Weed projects-10-30 years Insect projects-2-10 years Weed biocontrol Insect biocontrol
Feature #6: Requires understanding of plant physiology and plant phylogenetics to assess impact and saftey, respectively Tribe Cardueae Phylogeney of family Asteraceae
Centaureinae Centaurea Cichorioideae Asteroideae Cardueae Carduinae Subfamily Tribe Subtribe Senecio vulgaris Senecio cineraria Helianthus annuus (1) Echinacea purpurea Hemizonia minthornii Eriophyllum stoechadifolium pupae (%) eggs (%) Gnaphalium californicum Liatris punctata Brickellia californica Aster chilensis Artemisia californica Veronieae Lactuceae Mutisieae Echinopsidinae Carlininae Stokesia laevis Stephanomeria cichoriacea Lactuca sativa (2) Agoseris grandiflora Trixis californica Gazania rigens Echinops exaltatus Xeranthemum cylindraceum Silybum marianum Saussurea americana Onopordum acanthium Cynara scolymus (3) Cirsium vulgare Ci. vinaceum Ci. occidentale Ci. loncholepis Ci. hydrophilum Ci. fontinale Ci. cymosum Ci. ciliolatum Ci. brevistylum Carduus pycnocephalus Crupina vulgaris Cnicus benedictus Carthamus tinctorius (4) Acroptilon repens Centaurea americana Ce. rothrockii? Ce. dealbata Ce. cineraria Ce. montana Ce. cyanus Ce. squarrosa Ce. x pratensis Native Commercial Target Ce. maculosa Ce. diffusa Ce. sulphurea Ce. melitensis Ce. calcitrapa Ce. solstitialis 0 20 40 60 80 100 Percentage of trials
Centaureinae Centaurea Subtribe Carduus pycnocephalus Crupina vulgaris Cnicus benedictus safflower Carthamus tinctorius (4) Acroptilon repens pupae % eggs % Centaurea americana Ce. rothrockii Ce. dealbata Ce. cineraria Native Commercial Target? Ce. montana bach. Ce. button cyanus Ce. squarrosa Ce. x pratensis Ce. maculosa Ce. diffusa Ce. sulphurea Ce. melitensis Ce. calcitrapa YST Ce. solstitialis 0 20 40 60 80 100 Percentage of trials
Feature #7:Complexes of agents are seen as necessary to a greater degree for weed biocontrol E. inaron Bangasternus orientalis Eustenopus villosus Larinus curtus Chaetorellia succinea Ash whitefly Yellow starthistle
soil fungus Life Cycle of Yellow Starthistle winter rainy season rosette fly rosette weevil fall seedling seedhead insects seeds in soil seed production seedling mite lacebug adult rosette flea beetle spring rust summer dry season
Feature # 8: Insect x pathogen interactions may be critical The impact of Cactoblastis cactorum on Opuntia is increased because caterpillar feeding opens pads to bacterial infections Pad fed on and diseased
Knapweed biocontrol success
Recovery of vegetation after knapweed biocontrol
CASE 1: Broad-leaved paperbark tree Melaleuca quinquenervia
The Everglades
Melaleuca Open marsh invasion in Florida Invasive stands of melaleuca Sawgrass marsh 22
Species displacement: little native lives here
Increased fire intensity
Soil accretion
How much melaleuca is out there? (Source: South Florida Environmental Report 2007) 1993 200,000 ha 28
The Melaleuca weevil
The Melaleuca Psyllid
The Melaleuca Midge
Natural enemy impacts (tree level): defoliation crown thinning, dieback, mortality Thinned crown Weevil feeding Psyllid infestation Rust-fungus attack Lobate-lac insect infestation 32
Herbivory reduces stump regrowth: Replicated insect exclusion studies: 76% reduction in regrowth 80% mortality of cut stumps Rayamajhi et al. 20
Herbivory decreases reproduction Pratt et
Survival (%) Relative change in height Herbivory decreases recruitment Insect exclusion studies of seedling survival: 1.8 1.6 1.4 b) a Density dependent Growth: shrinking! 1.2 1 0.8 0.6 0.4 b c mortality: 40 to 80% 0.2 0 Zero Low High Psyllid Density c) Zero weevil Low weevil High weevil 100 90 80 70 60 50 40 30 20 10 0 Zero Low High Psyllid density
Melaleuca density (# of stems/ha) Melaleuca density and plant species richness 1.2e+5 1.0e+5 8.0e+4 6.0e+4 4.0e+4 2.0e+4 0.0 Melaleuca density Number of plant species (mean) 1996 1998 2000 2002 2004 2006 2008 2010 2012 Years Pearson's Correlation coefficient = -0.688 P value =0.00000968 N =33 40 36 32 28 24 20 16 12 8 4 0 Plant species richness (mean # of species) 36
Are native plants moving into the declining melaleuca? Number of plant species 60 55 50 45 40 35 30 25 20 15 10 5 0 Total Native Exotic Uncertain Species' origin 1997 2004 2011 37
1996- Florida (before natural enemy introduction) 38
2004-plants stressed by herbivore impact
2011 increased plant species diversity)
Transition is further speeded by manual removal of larger stems 41
Reduction of the size of infestation 1993--------------200,000 ha 2008--------------110,000 ha 90,000 ha (45% reduction infested area in FL in 15 years) 42
The Return of the Natives