Biological control of waterhyacinth in Sinaloa Mexico with the weevils Neochetina eichhorniae and N. bruchi

Water hyacinth (Eichhornia crassipes (Mart.) Solms.)creates severe problems in the irrigationdistricts of Mexico, particularly in westernSinaloa. Therefore water hyacinth weevils(Neochetina eichhorniae Warner and N. bruchi Hustache), imported from the USA in1993, were used to initiate a biologicalcontrol program. Precautionary screeningrevealed that some were infected with amicrosporidian so disease-free colonies wereproduced by eliminating infected breedinglines. To demonstrate effectiveness prior toopen field releases, weevils were firstreleased in cages at field sites. Weevilintensity increased to 6.3 weevils/plant after320 days when the plants were all dead ordying. More than 8,600 N. bruchi and14,500 N. eichhorniae were then releasedat various sites during January 1995 to August1996. Waterhyacinth coverage declined atBatamote reservoir (134 ha) from 95% to <3%by 1997; at the 12-ha Hilda reservoir from100% in May 1995 to 1% by March 1998; at the42.3-ha Arroyo Prieto reservoir from 100% to1% during the same interval; and at theMariquita reservoir (492 ha), the largestreservoir in the Humaya system, from 394 ha(80%)to 98.4 ha (20%).     

The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Competitive Interactions between Hydrilla (Hydrilla verticillata) and Vallisneria (Vallisneria americana) as Influenced by Insect Herbivory

Two experiments (winter and summer) were conducted in outdoor tanks using addition-series methods to evaluate the impact of specialized feeding by two biological control agents,Hydrellia pakistanaeDeonier andBagous hydrillaeO'Brien, on competitive interactions between hydrilla [Hydrilla verticillata(L.f.) Royle] and vallisneria (Vallisneria americanaMichx). Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In the absence of the biocontrol agents, intraspecific competition from hydrilla on itself was 8.3 times stronger than interspecific competition from vallisneria.Hydrellia pakistanaeinterfered with hydrilla canopy formation by removing as much as 80% of the plant biomass in the top 30 cm of the water column. Damage byH. pakistanaealso caused a 43% reduction in hydrilla tuber production during the winter experiment. Similarly,B. hydrillaecaused up to a 48% reduction in hydrilla plant weight in the summer experiment. Neither insect species damaged vallisneria. As a result, there were significant shifts in the competitive balance between hydrilla and vallisneria due to selective insect feedings. In the presence ofH. pakistanae, hydrilla intraspecific competition was nearly equal to interspecific competition from vallisneria, indicating that hydrilla had lost its competitive edge over vallisneria.Bagous hydrillaealso produced similar, but smaller, shifts in the relative competitive abilities of hydrilla and vallisneria. These results indicate that biological control agents can disrupt the competitive balance between plant species in favor of native species, thus adding another element to the weed biological control strategies.