Patrick C. Wilson

Toxicity and bioaccumulation of biosolids-borne triclosan in food crops

Triclosan (TCS) is an antimicrobial compound commonly found in biosolids. Thus, plants grown in biosolids-amended soil may be exposed to TCS. We evaluated the plant toxicity and accumulation potential of biosolids-borne TCS in two vegetables (lettuce and radish) and a pasture grass (bahia grass). Vegetables were grown in growth chambers and grass in a greenhouse. Biosolids-amended soil had TCS concentrations of 0.99, 5.9, and 11 mg/kg amended soil. These TCS concentrations represent typical biosolids containing concentrations of 16 mg TCS/kg applied at agronomic rates for 6 to 70 consecutive years, assuming no TCS loss. Plant yields (dry wt) were not reduced at any TCS concentration and the no observed effect concentration was 11 mg TCS/kg soil for all plants. Significantly greater TCS accumulated in the below-ground biomass than in the above-ground biomass. The average bioaccumulation factors (BAFs) were 0.43 ± 0.38 in radish root, 0.04 ± 0.04 in lettuce leaves, 0.004 ± 0.002 in radish leaves, and <0.001 in bahia grass. Soybean (grain) and corn (leaves) grown in our previous field study where soil TCS concentrations were lower (0.04-0.1 mg/kg) had BAF values of 0.06 to 0.16. Based on the data, we suggest a conservative first approximate BAF value of 0.4 for risk assessment in plants.

Host range and searching behaviour of Cricotopus lebetis (Diptera: Chironomidae), a tip miner of Hydrilla verticillata (Hydrocharitaceae)

Abstract A chironomid midge, Cricotopus lebetis Sublette (Diptera: Chironomidae), was discovered feeding on Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) in Crystal River, Citrus, Co., Florida, in the 1990s. Larvae of the midge mine the apical meristems of hydrilla, causing terminal branching and stunting of the plant. We investigated the fundamental host range of the midge by conducting a series of no-choice and paired-choice tests. No-choice developmental tests with neonate larvae revealed that the fundamental host range of C. lebetis included not only on hydrilla but also several other aquatic plants in different families, suggesting that this insect is not a hydrilla specialist. In paired-choice bioassays, larval colonisation of Elodea canadensis Michx. (Hydrocharitaceae) and Najas guadalupensis (Spreng.) Magnus (Najadaceae) was greater than colonisation of H. verticillata. Behavioural bioassays in a Y-tube olfactometer and in Petri dishes suggested that neonate larvae were not able to locate host plant material, whereas older larvae were successful in finding hosts. In paired-choice oviposition tests, adult females discriminated between potential oviposition sites, with greater numbers of eggs laid on E. canadensis and N. guadalupensis than on H. verticillata. This study is the first detailed account of host searching and oviposition behaviour of a phytophagous chironomid midge. The results will be used to assess the potential value of C. lebetis as a biological control agent of hydrilla.

Response of representative cover crops to aluminum toxicity, phosphorus deprivation, and organic amendment

This study aimed to: (1) determine the effect of P depletion and presence of Al on root and shoot growth of representative cover crops, and on their nutrient uptake; (2) characterise the composition of root exudation under P and Al stress in nutrient solution; (3) evaluate the ability of aqueous extracts of composts in reducing Al phytotoxicity. Plants of cowpea (Vigna unguiculata subsp. unguiculata), black oat (Avena strigosa), and lablab (Lablab purpureous) were cultivated in different nutrient solution compositions and concentrations for 3 weeks. It was found that Al at concentration of 20 and 200 µmol/L increased citrate exudation at least 8 and 24 times, respectively, for cowpea and 18 and 36 times, respectively, for lablab, as compared with the blank. However, no release of organic acids occurred due to P deprivation, suggesting that citrate exudation was a specific response to excess Al. No response in organic acid release was observed for black oat under the stress of P deficiency or Al toxicity. Although the presence of Al in solution did not significantly affect chlorophyll content in leaves, it decreased root and shoot weight, as well as root length, surface area, volume, and number of tips. Organic extracts alleviated aluminum toxicity, improving plant growth and ameliorating plant nutrition status. Yard waste extract was more effective in enhancing plant growth than GreenEdge extract in plants under Al stress.

Nontarget deposition and losses of oxamyl in surface runoff from flatwoods citrus production areas

Pesticide export from citrus production areas is a concern in the Indian River Lagoon drainage basin (Florida, USA). These studies evaluated nontarget deposition and losses of the insecticide oxamyl from typical flatwoods citrus production areas in South Florida. Deposition was estimated with the use of Teflon spray targets placed across nontarget water furrows during spray applications. After oxamyl applications, surface water runoff events were generated with an overhead irrigation system to simulate relatively low intensity rainfall. Results indicated that 0.3 to 20.0% of the applied active ingredient landed on nontarget water furrow surfaces and that the majority of oxamyl loss occurs in runoff events shortly following application. Mean concentrations were greater during the first runoff events, ranging from 34.7 to 47.0 microg/L. Total losses during the first several hours of each set of runoff events ranged from 0.1 to 3.8% of the total active ingredient applied. Similar losses can be expected when rainfall occurs shortly after application for other pesticides with similar chemical properties and application methods.

The Diversity of Chironomidae Associated with Hydrilla in Florida, with Special Reference to Cricotopus lebetis (Diptera: Chironomidae)

Hydrilla verticillata (L.f. Royle) (Hydrochari tales: Hydrocharitaceae) hereafter hydrilla is an aquatic plant that is widely distributed in the Old World. A dioecious form of hydrilla was in troduced into Florida in the 1950s, and has since spread across the southern USA (Schmitz et al. 1991; Langeland 1996). Hydrilla is a highly ag gressive weed which is known to displace native vegetation (Haller & Sutton 1975; Hofstra et al. 1999; Van et al. 1999), impede boat traffic and dis rupt water movement (Schmitz et al. 1991). In 1992, the chironomid Cricotopus lebetis Sublette (Diptera: Chironomidae) was discovered in Crystal River, Florida attacking the apical meristems of hydrilla, and may have potential as an augmentative biological control agent (Cuda et al. 2002, 2011). The origin of the midge is un known (Epier et al. 2000), and its distribution in Florida has not been determined. The midge dam ages hydrilla by mining in the apical meristem, which causes tip abscission (Cuda et al., 2002). The objectives of the current study were to exam ine the distribution and abundance of C. lebetis, and more generally of chironomid midges, associ ated with hydrilla in Florida. Six water bodies in Florida were surveyed to examine the distribution and abundance of chi ronomids associated with hydrilla; Lake Rowell (Bradford Co.) and Wacissa Springs (Jefferson Co.) in North Florida, Lake Tohopekaliga (Osecola Co.) and Bulldozer Canal (Brevard Co.) in central Florida and Lake Istokpoga (Highlands Co.) and Lake Okeechobee (Okeechobee Co.) in south-cen tral Florida. Each location was sampled quarterly from Jan, 2011 to Jun, 2012 for a total of 6 sam ples from all locations except Wacissa Springs, which was sampled 5 times. On each sampling oc casion, a 4-pronged steel hook attached to a rope was thrown into the water and dragged along the hydrosoil to collect hydrilla. Hydrilla plants were removed from the hook and placed in plastic bags (46 x 61 cm, -16 liter capacity) partially filled with water from the corresponding sample site. Sufficient hydrilla was collected on each sampling occasion to fill two bags approximately two-thirds full. A separate plastic container was filled with water from the corresponding site. Bags were placed in a cooler and transported to the labo ratory. Apical portions (5-8 cm) of 300 intact hy drilla tips were haphazardly selected from each sample and placed in open plastic containers (34 x 28 x 15 cm, L x W x H) in water collected from the corresponding site. Containers were placed individually in fine mesh emergence cages (50 x 50 x 50 cm) and aerated with an aquarium pump. Cages were monitored daily for 14 days for midge emergence. Adults were collected daily by aspira tor and placed in vials containing 95% ethanol. Specimens were initially sent to J. H. Epler for authoritative identification, but once a reference collection was assembled, most midges were lo cally identified. Species richness at each location was deter mined as the total number of midge species recov ered. Diversity was calculated using the Shannon Index, which combines aspects of species rich ness and evenness (how equally individuals are distributed among species) (Shannon 1948). The Shannon index measures the predictability that an individual selected in a sample will be a given species. Higher values of the Shannon index indi cate that individuals are more evenly distributed among species, whereas low values indicate that some species are much more common than others. The Shannon Index was calculated for each sampling location as follows: