Unoccupied aerial application systems (UAAS) are gaining popularity for weed management to increase applicator safety and to deliver herbicide treatments where treatment sites limit ground-based spray equipment. Several studies have documented UAAS application strategies and procedures for weed control in terrestrial settings, yet literature describing remote spray technology for use in aquatics remains limited. Currently, applicators seek guidance for UAAS deployment for aquatic weed management to overcome site access restrictions, deal with environmental limitations, and improve ground-based applicator safety in hazardous treatment scenarios. In the present case studies, we evaluate a consumer-available UAAS to deliver the herbicide, florpyrauxifen-benzyl, as both foliar and directed in-water spray applications. The first case study showed that the invasive floating-leaved plant, yellow floating heart, was controlled 80% to 99% by 6 wk after treatment (WAT) following UAAS foliar herbicide treatments. The second case study demonstrated that UAAS directed in-water herbicide application reduced variable-leaf watermilfoil visible plant material by 94% at 5 WAT. Likewise, directed in-water applications from UAAS eliminated the need to deploy watercraft, which improved overall operational efficiency. Data from both case studies indicate that UAAS can provide an effective and efficient treatment strategy for floating-leaved and submersed plant control among common herbicide treatment scenarios. Future integration of UAAS in aquatic weed control programs is encouraged, especially among smaller treatment sites (≤4 ha) or where access limits traditional spray operations.

Two species, torpedograss and Southern watergrass, are very difficult to selectively control when they invade desirable turfgrass stands. The purpose of this study was to evaluate selective control of torpedograss and Southern watergrass in ‘Tifway’ bermudagrass turf. Greater than 86% control of torpedograss was observed 4 wk after sequential treatment (WAST) with quinclorac, trifloxysulfuron-sodium, quinclorac and trifloxysulfuron-sodium, sulfentrazone + imazethapyr and quinclorac and trifloxysulfuron-sodium, and quinclorac and trifloxysulfuron-sodium followed by (fb) glyphosate. However, by 8 WAST, control was reduced to <36% for all treatments. Greatest Southern watergrass control was achieved 4 WAST with trifloxysulfuron-sodium (83%), and thiencarbazone-methyl + foramsulfuron + halosulfuron-methyl (75%). Limited control (<30%) was observed with other treatments. By 8 WAST, Southern watergrass control was <12% for all treatments. This study suggests that short-term control/suppression of these two species is possible; however, long-term control is limited with single-year programs. These weeds will probably require multiple applications in successive years to reduce infestations. Future research should continue to screen other herbicides, combinations, and timings for control of these and other perennial grass weeds.

Hydrilla [Hydrilla verticillata (L. f.) Royle ♯ HYLLI] was cultured in flowing water in outside aquaria on rooting media differing in fertility and texture. Nutrient content was determined for rooting media at the beginning of the experiment and for plant tissues at harvest. Growth, as measured by rates of stem elongation, areal coverage of aquaria by plants, and biomass accrual, was highly related to N-, P-, and K-fertility of rooting media. Rooting media were the most important source of phosphorous since water supplies were not adequate to support plant growth. There was evidence of P-deficiencies in plants grown on infertile rooting media. Both N and K were adequately supplied from either water, rooting media, or both. Production of plant biomass was most closely related to P-levels in tissues which were closely related to supplies in rooting media. Pretest soil N (comparable to preestablishment conditions) was best related to biomass production, indicating that growth and establishment of hydrilla may be predictable through assessment of hydrosoil fertility. The need for sampling and analytical procedures to assess fertility is emphasized.

Numerous cations were evaluated as possible substitutes for copper in mixtures with diquat (6,7-dihydrodipyrido[1,2-α:2′,1′-c] pyrazinediium ion) for control of hydrilla [Hydrilla verticillata (L.f.) Royle]. Several cations, including ferrous and ferric iron, when substituted for equal weights of copper in the mixtures, were as effective as copper in enhancing the phytotoxicity of diquat to hydrilla. In most instances, death of the test plants occurred more rapidly when the substitutes were used. Ferric chloride, because of its apparent innocuous behavior in the aquatic environment, is a desirable substitute for the more toxic copper. Bluegill sunfish (Lepomis macrochirus RAf.) were unaffected by week-long exposure to ferric chloride at concentrations as high as 100 ppmw. Since ferric chloride alone exhibited no toxicity to hydrilla in the range of concentrations tested, the greater phytotoxicity of the mixtures probably was due to the increased toxicity of diquat. In evaluations using other species of aquatic weeds, neither the addition of copper nor the addition of other cations increased the phytotoxicity of diquat. In most tests, the mixtures of diquat with other cations reduced the level of activity of diquat on weeds other than hydrilla.

The production of new shoots of American pondweed (Potamogeton nodosus Poir.) and sago pondweed (P. pectinatus L.) was reduced significantly when winterbuds and tubers of these species were planted in dwarf spikerush [Eleocharis coloradoensis (Britt.) Gilly] sod. In most instances, the number of shoots produced was several times greater when winterbuds and tubers were planted in bare soil than when planted in spikerush sod. Similar results were obtained when the pondweeds were grown in separate aquaria, each aquarium being exposed to 500 ml of leachate per day from spikerush sod. The differences in reproduction of pondweeds could not be attributed to altered water quality nor to reduced levels of nutrients. No appreciable allelopathic response was elicited by the pondweed plants originating from planted tubers or winterbuds. The principal response appeared to be reduction in numbers of new shoots produced from the original propagules, although the reduction in biomass of the pondweeds exposed to the effects of spikerush was obvious. Of the two pondweeds investigated, sago pondweed was most sensitive to the influence of spikerush.

Waterhyacinth [Eichhornia crassipes (Mart.) Solms] grown in screen-enclosed, outdoor aquaria were treated with factorial combinations of 0, 0.75, 1.5, or 3 kg/ha of the experimental growth retardant EL-509 [α-(4-chlorophenyl)-α-(1-methylethyl)-5-pyrimidine-methanol] and zero, one, two, or three pairs of waterhyacinth weevils (Neochetina eichhorniae Warner)/plant, with three replicates of each weevil-number by retardant level combination. When plant growth was evaluated over a time course, significant effects of weevils and retardant were observed. After 127 days, however, waterhyacinth was reduced as weevil numbers increased, but the growth retardant was ineffective without weevils. Weevils appeared to be more effective when used in combination with the retardant, and the combined effects were additive.

Hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] was grown for 4 to 16 weeks in pans filled with either an organic muck-sand soil, sand, or sand mixed with Osmocote, Esmigran, and dolomite under outdoor conditions in plastic-lined pools with flowing pond water. Dry weight for plants cultured in sand plus the fertilizers was dependent on the concentration of fertilizer and was from 6 to 14 times that of plants cultured in sand alone. Dry weight was also higher for three treatments of sand amended with fertilizer than for plants cultured in the organic muck-sand soil. Water temperature for different growth periods influenced dry weight of hydrilla cultured with all three root media. Tuber production was independent of three levels of fertilizer for 16 weeks of plant growth, but plant weight was dependent on the concentration of nutrients in the root zone. Of nine plant tissue nutrients measured, only phosphorus in both the shoots and roots was dependent on the level of fertilizer in the root zone. This suggests that growth of hydrilla is controlled by nutrients in the root zone. The use of sand amended with various levels of fertilizers may be a way to simulate fertility levels of sediments as a method to study aquatic sites for their potential to support growth of hydrilla.

Studies in outdoor, circular pools have shown that one shoot tip of hydrilla (Hydrilla verticillata Royle) produced as much plant dry weight during a 16-week period as 16 tips planted under the same conditions. Dry weight of plants 16 weeks after planting was as high as 1,561 times that of the shoot tips planted. Tubers were present within 8 weeks after planting the tips; higher numbers of these vegetative propagules were associated with the higher number of tips planted. Plants under a photoperiod of 11.5 h or less produced more tubers than those exposed to 12 h or more of light.

Invasive watermilfoils, specifically Eurasian watermilfoil and the interspecific hybrid of Eurasian watermilfoil × northern watermilfoil, continue to be problematic for water resource managers. Herbicides are often used to control these nuisance weeds and have been historically successful in controlling Eurasian watermilfoil. A population of hybrid watermilfoil from Townline Lake in Michigan has shown increased tolerance to the herbicide fluridone. The objective of this work is to determine if cross- and multiple tolerance have also developed in this population. Eurasian watermilfoil plants collected from multiple sites and plants from Townline Lake were treated with 0, 5, 10, 20, 40, or 80 µg L−1 of fluridone, norflurazon, or topramezone. Fluridone and norflurazon inhibit phytoene desaturase, whereas topramezone is a 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide. Chlorophyll fluorescence (Fv/Fm) and pigment content was measured at 10 d after treatment. Townline Lake plants responded differently from susceptible plants when treated with fluridone, norflurazon, and topramezone at 40 µg L−1. These results indicate that the Townline population of hybrid watermilfoil has inherent tolerance to multiple herbicide modes of action. These results are especially significant as topramezone has recently been labeled for aquatic use. Screening of additional herbicides to determine potential herbicide tolerance of the Townline Lake population is recommended.

Subterranean turions (tubers) of hydrilla lose viability when desiccated. Experimental data showed that freshly collected tubers had a moisture content between 50 and 60% and more than 90% viability. When desiccated, there was an approximate 2% increase in tuber mortality with each percent decline in moisture content. However under field conditions, the tuber bank within the exposed sediments of a northern Texas reservoir showed no decline in number or tuber viability throughout a 12-mo continuous drawdown. Apparently, the buried tubers were never subject to sufficient dessication to damage them. Finally, an experimental pond with an extensive hydrilla tuber bank was manipulated through six flood/drawdown cycles to determine the effects of short-term drawdowns on tuber survival and quiescence. Initially, the pond had a tuber bank of about 676 and 305 tubers m−2 in the shallow and deep zones, respectively. Although the tuber number was reduced to fewer than 15 to 30 tubers m−2 by these repetitive drawdowns, hydrilla tubers were not eradicated from the pond.

Eurasian watermilfoil, an aquatic invasive weed, has been identified recently at a number of sites in western Nevada and northeastern California, including Lake Tahoe. Because Eurasian watermilfoil is easily spread by fragments, transport on boats and boating equipment plays a key role in contaminating new water bodies. This is an important means of the potential spread of this weed throughout key recreational and agricultural areas surrounding Lake Tahoe. Unless the weed is controlled, significant alterations of aquatic ecosystems, with associated degradation of natural resources and economic damages to human uses of those resources, may occur. This research uses the economic valuation approach known as benefits transfer to estimate the value of a portion of the recreational service flows that society currently enjoys in the Truckee River watershed below Lake Tahoe. The lower-bound estimates of baseline water-based recreation value at a subset of sites in the watershed range from $30 to $45 million/yr. Impacts from the continued spread of Eurasian watermilfoil in the watershed could be significant; for example, even a 1% decrease in recreation values would correspond to roughly $500,000/yr as a lower bound.

Weeds are the major biotic constraint to rice production. Field observations have suggested that certain fertilizer regimes could enhance infestations of particular weed species emerging with rice. The study objective was to determine the effect of surface-applied calcium phosphate on weed growth in flooded California rice systems. In field and pot studies, triple superphosphate (TSP) applied to the soil surface increased weed emergence. Surface-applied TSP increased the number of sedge and broadleaf weeds, including smallflower umbrella sedge, blue-flowered ducksalad, redstem, ricefield bulrush, waterhyssop, and California arrowhead. A laboratory study measured germination of smallflower umbrella sedge and ricefield bulrush in response to the application of phosphorus (P) and calcium (Ca), which comprise 20 and 15% of TSP, respectively. Calcium stimulated smallflower umbrella sedge germination and had no effect on ricefield bulrush germination. Phosphorus did not stimulate either smallflower umbrella sedge or ricefield bulrush germination. Results indicate that surface applications of calcium phosphate increase the growth of certain weed species and that Ca may stimulate germination of smallflower umbrella sedge. By incorporating preplant applications of calcium phosphate into the soil profile, growers can reduce weed pressure from certain species. Alternatively, surface applications of calcium phosphate may be useful to stimulate weed emergence in stale-seedbed management.