Michael J. Grodowitz

Impact of herbivory by Hydrellia pakistanae (Diptera: Ephydridae) on growth and photosynthetic potential of Hydrilla verticillata

The impacts of varying levels of herbivory by Hydrellia pakistanae on the dioecious ecotype of Hydrilla verticillata were evaluated by conducting a 10-week growth experiment within mesocosm tanks. The observed leaf damage to H. verticillata stems was highly correlated with the total number of immature H. pakistanae in H. verticillata tissue at the time of harvest (P 0.80). Increasing levels of insect herbivory significantly impacted biomass and growth morphology of H. verticillata. Relative to control tanks, plants under intermediate or high levels of herbivory produced progressively less biomass. Insect herbivory also significantly impacted investment of energy in sexual and asexual reproduction. Plants under an intermediate or high level of herbivory produced fewer than 15% of the number of pistillate flowers produced by plants in control tanks. Furthermore, plants subject to high insect herbivory produced fewer and smaller tubers than control tanks. Finally, herbivory had a strong impact on the photosynthetic potential of stems. With 10–30% leaf damage, the maximum rate of light-saturated photosynthesis was reduced 30–40% relative to undamaged controls. Total daily photosynthetic production in these stems was estimated to balance, just barely, the daily respiratory needs of stems. Photosynthetic rate was reduced by about 60% in stems showing 70–90% leaf damage. This level of photosynthetic reduction would make continued survival of the plants unlikely since they would be unable to meet daily respiratory demands.

Insect herbivores of water stargrass (Heteranthera dubia) in the US

We examined insect herbivores associated with Heteranthera dubia from surveys conducted from 2006 to 2009. Plants were collected, invertebrates were removed, and signs of feeding damage were noted. Herbivores were quantified, and geographic regions were compared based on herbivore density, taxa richness, evenness, and diversity. The greatest density of herbivores occurred at Parker Pond, Washington, which was largely influenced by an abundance of aphids (Rhopalosiphum spp.). Density, richness, and evenness were not significantly different among regions. At least 23 potential insect herbivores were recorded from 15 sites in Texas, Washington, Minnesota, Wisconsin, Vermont, and New York. Of these, five taxa were collected from the order Lepidoptera, one from Coleoptera, six from Diptera, and at least 11 from Trichoptera. The majority of the herbivores were generalists; several had unknown diets. Damage observed to H. dubia included extensive tunneling in the stems of the plant and, in some cases, substantial chewing damage to the leaves.

Herbivorous Insects Associated with Ludwigia peploides (Onagraceae) in the Southern United States

Abstract The wetland plant Ludwigia peploides (floating primrose-willow) often is problematic in the southern USA, as well as elsewhere in its introduced range. Surveys to identify potential herbivorous insects to use as biological controls have focused on South America, but implementing a management program with foreign insects in the USA is difficult and can take years of testing in quarantine. During 2006–2008, populations of floating primrose-willow were surveyed in the southern USA to determine presence and extent of native herbivorous insects. At least nine species were identified, represented mainly by weevils (Curculionidae) and leaf beetles (Chrysomelidae). Of these, two were undescribed species of weevils (Auleutes), and six species, including Tyloderma sphaerocarpae and Chaetocnema, have never been associated with floating primrose-willow.

Efficiency of Sampling to Determine Population Size of Cyrtobagous salviniae (Coleoptera: Curculionidae)

Abstract Salvinia molesta D. S. Mitchell (Salviniales: Salviniaceae), a small floating fern introduced from South America, is causing an increasing number of problems in the US. Increased reliance on the biocontrol agent, Cyrtobagous salviniae, in the US is becoming more commonplace and several mass-rearing facilities have been developed. Because of differences in sampling protocols including sample size, reporting parameters, and numbers released, an investigation into sampling efficiency was initiated. A small pond in southern Louisiana was sampled in an effort to understand what constitutes an adequate sample size and methodologies needed to estimate numbers of weevils. A clumped distribution in the pond was identified, which required a large number of samples to be taken to minimize differences in means and variation. When randomly selecting 10 sets of samples where n = 5 for Sep, means varied from a high of 280 weevils/m2 to a low of only 50 weevils /m2, a difference of nearly 6-fold. However, when randomly selecting 10 sets of samples where n = 20, means were much more consistent and varied from a high of approximately 250 weevils/m2 to a low of 125 weevils/m2, a difference of only 2-fold. Sampling is expensive and to gain the most information based on the number of samples taken it is recommended that 1) the confidence interval be reported, especially when releasing weevils based on an estimation of population size; 2) understand spatial distribution and sample accordingly; and 3) when possible, initiate pilot sampling programs to acquire prior information on sampling biases, sampling errors, and differences in distribution.