|Coetzee, J. - U OF WITSWATERSRAND, S.AF|
|Byrne, M. - U OF WITSWATERSRAND, S.AF|
|Hill, M. - RODES UNIVERSITY, S. AFR|
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 6, 2004
Publication Date: January 1, 2005
Citation: Coetzee, J.A., Center, T.D., Byrne, M.J., and Hill, M.P. 2005. Impact of the biocontrol agent Eccritotarsus catarinensis, a sap-feeding mirid, on the competitive performance of waterhyacinth, Eichhornia crassipes. Biological Control. 32: 90-96. Interpretive Summary: State and Federal agencies spend millions of dollars on herbicides and mechanical harvesters attempting to control water hyacinth, an invasive aquatic plant. Effective biological control could save a large portion of this expense and reduce the environmental hazards associated with the wide-scale application of herbicides into water bodies. A sap-feeding bug released in South Africa to aid in the control of waterhyacinth might also be useful in the United States. Unfortunately, feeding trials conducted under artificial conditions indicated that it might also feed on a native North American plant (pickerelweed), which is related to waterhyacinth. Additional tests conducted under natural conditions indicated that the risk of harm to this plant was minimal. Even though the perceived risk from this insect was negligible relative to the damage to pickerelweed from scouring of drifting waterhyacinth mats and associated herbicide treatments, any risk would be ill advised if the bug did not effect control. Also, the cost associated with implementing an ineffective biological control agent would be wasted. However, effects can oftentimes be quite subtle becoming observable only over prolonged periods. These subtle effects can be magnified and more readily observed when the targeted plant is subjected to competition from other plant species. This study showed that the bug did indeed debilitate waterhyacinth. This was evidenced by a 57% reduction in the competitive ability of waterhyacinth against waterlettuce, a less aggressive floating weed.
Technical Abstract: The mirid, Eccritotarsus catarinensis, having been released in Africa to aid in the control of waterhyacinth, may also be useful in the United States. Unfortunately, it fed and developed on native American pickerelweed during indoor host-specificity trials conducted in South Africa. This perceived risk to North American Pontederiaceae precludes its release in the USA pending evaluation of its efficacy as weighed against possible detrimental effects that it, in the presence of waterhyacinth, may have on pickerelweed. Post-release protocols further require that its impact on the weed in South Africa be quantified, to confirm the value of releasing an additional agent. The subtle feeding damage caused by the mirid is not easily measured. However, interactions with other plant stresses, e.g. competition, often magnify impacts of plant-feeding insects, further elucidating subtle effects. This potential control agent was evaluated using additive series analysis of competition between waterhyacinth and waterlettuce, as influenced by herbivory. Competitive abilities of waterlettuce and waterhyacinth were determined using an inverse linear model with plant weight as the yield variable. Waterhyacinth was 23 times more competitive than waterlettuce in the absence of herbivory, but only 10 times more competitive when exposed to mirid feeding. Waterlettuce was only 0.9 times as aggressive as waterhyacinth in the absence of herbivory, but 1.5 times as aggressive when mirids were impacting waterhyacinth. Most importantly, interspecific competition from waterhyacinth on waterlettuce was insignificant in the presence of herbivory. These results show that the mirid compromises waterhyacinth competitiveness, and validate the introduction of the mirid into South Africa. This, together with evidence that pickerelweed is not used as a host under field conditions, suggests that it should not categorically be ruled out for release in the USA.