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.     

Reproduction and Development of the Biocontrol AgentHydrellia pakistanae(Diptera: Ephydridae) on Monoecious Hydrilla

Previous investigations of the laboratory biology and host range ofHydrellia pakistanae,a biological control ofHydrilla verticillata(hydrilla), used the dioecious hydrilla biotype common to Florida, Texas, and California. A monoecious biotype that is now spreading throughout the mid-Atlantic states, California, and Washington was not investigated during these original studies. We therefore compared the dioecious and monoecious hydrilla biotypes as hosts forH. pakistanae.FemaleH. pakistanaeaccepted the two biotypes equally as ovipositional substrates. Overall developmental success differed little: 42% of eggs oviposited on monoecious hydrilla produced adults compared to 39% of eggs oviposited on dioecious hydrilla. Fly development required about 33 days on both biotypes (at 22 ± 2°C), but larvae that completed development mined 1.6 times as many leaves on monoecious hydrilla as on dioecious plants. These data suggest thatH. pakistanaewould be a useful biocontrol agent of monoecious hydrilla, should this plant invade areas where it can grow as a perennial.     

Separate and interactive effects of competition and herbivory on the growth, expansion, and tuber formation of Hydrilla verticillata

This study examined the interaction and main-effect impacts of herbivory by the leaf-mining fly Hydrellia pakistanae and plant competition from Vallisneria americana on the growth, expansion and tuber formation of Hydrilla verticillata in a 2 × 2 factorial design experiment. The study was conducted in 14,000-L tanks, over two growing seasons. Each tank represented a single experimental unit and contained 32 1-L pots. At the beginning of the experiment half of these were planted with H. verticillata while the other half were either left empty or planted with V. americana (the competitor). H. pakistanae fly larvae (the herbivore) were added to tanks as appropriate. No significant interactions were identified between herbivory and competition on any parameter of H. verticillata growth analyzed (i.e., total tank biomass accumulation, total number of rooting stems, total tuber number, total tuber mass, and tuber size), indicating that the factors were operating independently and neither antagonism nor synergism was occurring. Both competition and herbivory impacted the growth of H. verticillata. H. verticillata plants grown in the presence of V. americana developed less total biomass, had fewer total basal stems, had fewer tubers and less tuber mass per tank, and produced significantly smaller tubers relative to control plants. Herbivory also suppressed H. verticillata biomass accumulation and tended to suppress the number and total mass of tubers produced in each tank. Both factors showed 30–40% reduction of total H. verticillata biomass, although the mechanism of impact was different. Competition suppressed expansion of H. verticillata into adjoining pots but had little impact on its growth in pots where it was originally planted. Herbivory resulted in a general suppression of growth of H. verticillata in all pots. Although herbivory significantly impacted H. verticillata biomass, it did not result in competitive release for V. americana under the current experimental conditions. We conclude that management activities that promote competition or herbivory will impact the growth and expansion of H. verticillata. Furthermore, since these factors operated independently, the combined use of both factors should be beneficial for suppression of H. verticillata dominance.     

Laboratory biology and host range ofHydrellia balciunasi [Diptera: Ephydridae]

Hydrellia balciunasi Bock, a native of Australia, was evaluated in quarantine in Florida, USA, for its potential as a biocontrol agent of the submersed aquatic weed,Hydrilla verticillata (L.f) Royle. Larvae are leafminers. Mean total development time at 27°C was 22.8 days. Mean duration of the egg stage was 3.0 days, larval was 11.5 days, and puparial was 8.3 days. Mean fecundity was 35.5 eggs. Mean female longevity was 19.7 days, and mean male longevity was 15.6 days. The sex ratio was 1.1∶1 (male: female). Fourteen plant species closely related to hydrilla in 4 families plus rice were tested in no-choice larval development tests and an additional 27 plant species in 16 families were tested in multi-choice tests. Larvae mined in 2 test plant species,Potamogeton pusillus L. andP. crispus L., but developed (1%) only on the introduced weedP. crispus L. Females oviposited on most test plants. Permission to release this fly in the United States was received from federal and state (Florida) officials, and it was released from quarantine on 24 May 1989.      

Foliar nitrogen and larval parasitism as determinants of leafminer distribution patterns on Spartina alterniflora

ABSTRACT, 1. In northwest Florida, Hydrellia valida hew (Diptera: Ephydridae) frequently mines the leaves of salt marsh cord, grass, Spartina alterniflora . Larvae and pupae are more commonly found in shoreline plants within 3 m of the sea than in inland plants.
2. Shoreline plants contain over twice as much total foliar nitrogen as inland plants. In transects from inland to shoreline plants, leafminer density is positively correlated with total leaf nitrogen content.
3. We experimentally increased foliar nitrogen content on inland Spartina patches to levels above those of shoreline plants. Leafminer densities on experimental centres remained far below those on control edges.
4. Larval parasitism by Opius sp. (Hymenoptera: Braconidae) and Pteromalus sp. (Hymenoptera: Pteromalidae) reaches over 90%, with parasitism increasing from shoreline to inland plants. We suggest that gradients in leafminer densities are caused by differences in parasitoid abundance.     

食料条件对稻茎毛眼水蝇种群增长的影响

1994-1995年对稻茎毛眼水蝇在水稻四个生育期(苗期、分蘖期、孕穗期、齐穗期)上的生命参数进行了研究。结果表明：水稻生育期对卵和蛹历期、存活率以及成虫寿命无明显影响,但幼虫取食不同生育期水稻后,其存活率和成虫繁殖力有显的差异,幼虫存活率和成虫繁殖力大小依次为取食孕糖期幼穗>分蘖期叶片>苗期叶片>齐穗期叶片。     

冀东滨海稻区水稻毛眼水蝇的生物学初报

通过鉴定证实冀东滨海稻区稻的主要寄虫潜叶蝇种类应为稻叶毛眼水蝇Hydrellia sinica FanetXia,而非一直沿用的小灰毛眼水蝇H.griseola(Fall啨n)。该虫以1～2代幼虫在水稻生育前期潜叶危害水稻;适当推迟水稻播种和插秧期、浅水灌溉及喷施乐果和阿维菌素等杀虫剂可有效防治该虫。