The Aquatic Plant Control Research Program supports studies in four technology areas (Biological Control, Chemical Control, Ecological, and Simulation Technologies) to better manage populations of Eurasian watermilfoil. The Biological Control technology area is investigating fungal pathogens and insects as potential Biocontrol agents for managing Eurasian watermilfoil populations. The Chemical Control technology area examines concentration/exposure time relationships for aquatic herbicides and evaluates the applicability of these relationships on an operational level. The Ecological technology area examines the environmental parameters that determine the distribution and abundance of Eurasian watermilfoil. The Simulation technology area has been developing growth models for Eurasian watermilfoil, as well as predictive models for specific control techniques used on this species, such as herbicides, harvesting and grass carp.
The study of phenological weak points in the life strategies of target aquatic plants will indicate the potential timing of management tactics from all technology areas. The objective of examining phenological cycles in nuisance aquatic plants is to more effectively manage these species by providing a mechanistic approach for optimizing management successes.
One mechanism to optimize the timing of management tactics is to examine the storage of carbohydrate reserves by plants, and apply control tactics when reserves are at their lowest in storage organs. Low points in the storage of carbohydrate reserves can then be correlated to observable phenological phenomena, such as the onset of flowering.
To illustrate, a hypothetical curve of total nonstructural carbohydrates (TNC) in a plant storage organ is presented as Figure 1. Stored carbohydrates are utilized by the plant from the onset of regrowth in late February until late-June, when storage concentrations no longer decrease, but begin to increase. Carbohydrate production is in excess of the metabolic requirements of the plant, and excess production is stored. The late June time indicated in Figure 1. is a primary control point. Additional observations also indicated that this control point coincided with the initiation of flowering. Therefore, the optimal time to apply a management tactic would be the onset of flowering.
 |
|
Figure 1 |
This approach was successfully demonstrated in cattail (
Typha glauca) by Linde, Janisch and Smith (1976), in which a late-June control point was identified by low carbohydrate reserves in the rhizome, which coincided with the appearance of staminate (pollen-producing) inflorescences. A similar approach has been used to successfully control terrestrial weeds, such as quackgrass (Schirman and Buchholtz 1966). These approaches have only recently been investigated for aquatic plants by researchers at the Waterways Experiment Station (WES).
Waterhyacinth (
Eichhornia crassipes), hydrilla (
Hydrilla verticillata), Eurasian watermilfoil, and alligatorweed (
Alternanthera philoxeroides) are considered by the Corps of Engineers to be the major nuisance aquatic plants in the United States. The goal is to perform phenological control point studies on all four species.
Small-scale studies of waterhyacinth, conducted at WES, concluded that control points based on the phenology of the plant and carbohydrate storage patterns occurred in early spring and in mid-September to early October, before substantial storage had occurred in stembases (Luu and Getsinger 1988, 1990). These studies were verified by pond-scale studies at the Lewisville Aquatic Ecosystem Research Facility (LAERF) in Lewisville, Texas (
Photo 3), in which the same two control points were observed for waterhyacinth. In addition, the developmental stage of the population was noted as an important factor. That is, waterhyacinth populations should be controlled before large mature plants can develop (Madsen, Luu, and Getsinger 1993). This result substantiates the current practice of maintenance management.
|
|
Photo 3
|
Carbohydrate Allocation
Many aquatic plants have well-defined storage organs for carbohydrate reserves, such as the tubers of sago pondweed (Potamogeton pectinatus) and hydrilla, the turions in curly-leaf pondweed (P. crispus) and American pondweed (P. nodosus), the rhizomes in cattail and yellow pond-lily (Nuphar advena), and the stembase of waterhyacinth (Madsen 1991). However, Eurasian watermilfoil lacks any distinct, specialized anatomical structure for storage. Instead, carbohydrates are stored in the lower nonphotosynthetic shoots and the root crown, which is a mass of stems at the sediment-water interface that initiates new shoot growth. In Figure 2, the carbohydrate production and storage "compartments" are diagrammed. The upper photosynthetic shoots produce excess carbohydrates, which are translocated as sugars to the lower stems and root crown. In the storage compartment (lower shoots and root crown), sugars are converted to starch for long-term storage. When plant carbohydrate requirements exceed plant production capability, carbohydrates in the storage compartment are remobilized and exported to the tissues requiring additional carbohydrates ("usage" in Figure 2).
 |
|
Figure 2 |
During a hypothetical growing season, new spring growth will consume stored carbohydrates (Figure 3). Carbohydrate storage is high, and is being exported to overlying shoots. At an undetermined point, plant production exceeds plant requirements, and carbohydrates are now transported to the storage compartment. Since stored carbohydrates are low, this time period is the point at which control should be optimal. Carbohydrate production and storage continues in autumn, with the carbohydrate content of the storage compartment increasing to higher levels. Carbohydrate stores remain high throughout the winter, with little used by the plant during a dormancy period, indicated by no net exchange in the diagram.
 |
|
Figure 3 |
Several studies substantiate the potential for a control point in Eurasian watermilfoil. Data from Perkins and Sytsma (1987) for a population in Lake Washington, in west-central Washington state, showed a late-May control point, with stored TNC falling to 7 percent. Studies of carbohydrate storage in Eurasian watermilfoil in Lake Wingra, Wisconsin, indicated potential control points in late June or early July of one year, and in mid-May in a subsequent year (Titus and Adams 1979). Because these studies were not performed to determine phenological control points, neither of them examined potential phenological indicators to correlate to the carbohydrate minima.
Conclusions
Studies of phenological control points based on the depletion of
carbohydrate reserves have demonstrated that excellent potential exists for
improving the management of Eurasian watermilfoil through the timing of control
tactic applications. Studies of several northern tier populations by other
investigators have indicated that a single control point occurs from mid-May
through mid-July, but these studies did not correlate carbohydrate levels to
other phenological events. Preliminary studies conducted at the LAERF indicate
two potential control points, following the two peaks in population biomass. The
first of these coincides with the end of the first inflorescence event, and the
onset of senescence and initiation of autofragmentation. The second coincides
with the second series of seed set and the peak in autofragment production.
Continued studies on carbohydrate production and phenological events will refine
this relationship. Studies of northern tier populations will assist in
developing these relationships across the continent.
Future Studies
Future studies will further define the relative importance of season, water temperature, and population age and development to the observed patterns of biomass and carbohydrate allocation in Eurasian watermilfoil populations. These studies will help to explain the successes and failures of past Eurasian watermilfoil control efforts. More importantly, this research will enable managers to better predict control results in the future.
Demonstration projects will be designed to apply the concept of phenological control points to the management of Eurasian watermilfoil. These demonstration projects will focus on chemical technology areas, but similar control point applications could be used with other management technologies.
Experimental Approach for Eurasian Watermilfoil
Research being conducted at the LAERF to investigate the phenology of Eurasian watermilfoil is focused on five areas of study: 1) seasonal growth and allocation studies; 2) growth rate and seasonal colonization events; 3) temperature-dependent allocation patterns; 4) seasonal spread dynamics; and 5) seed production and germination requirements (Photo 4).
 |
|
Photo 4 |
Seasonal Growth and Allocation Studies
Seasonal growth and allocation of biomass and carbohydrates is being examined through whole-pond studies (Photo 5). Samples taken throughout the growing season are separated into lower and upper shoots, root crowns, inflorescences and autofragments. These studies will provide the best indication of a control point based on carbohydrate storage.
 |
|
Photo 5 |
Growth and Colonization Trends
Growth rate and seasonal colonization trends are being examined in an extensive pond study in which Eurasian watermilfoil was planted in 6-liter sediment-filled pots in spring, summer, fall, and winter, to determine when colonization and growth would be most successful. Establishment and colonization are important phases of the Eurasian watermilfoil invasion process that have not been studied.
These studies, carried through two growing seasons, will indicate if there is a lag time in the development of dense beds that can be exploited for management of new colonies.
Temperature-dependent Allocation Patterns
Temperature-dependence of biomass and carbohydrate allocation will be examined in greenhouse tanks with controlled temperatures (Photo 6). These studies will indicate if patterns observed seasonally are in direct response to water temperatures, as well as indicating the difference in growth and production based on water temperature alone.
 |
|
Photo 6 |
Seasonal Spread Dynamics
Seasonal control of the spread of Eurasian watermilfoil is being examined in a series of pond experiments using a grid system which allows the mapping of all rooted plants and fragments. The grid systems are examined regularly, which allows the computation of seasonal expansion and decline by both runners and fragments (Madsen, Eichler, and Boylen 1988).
Seed Production and Germination
The importance of seeds to the overwintering and spread of Eurasian watermilfoil is still largely unknown (Madsen and Boylen 1989). Seed production and flowering is being monitored in Eurasian watermilfoil ponds. In addition, the effect of seed drying on subsequent seed germination is being investigated.
Preliminary Results
Carbohydrate Storage and Control Point Identification
Eurasian watermilfoil carbohydrate concentrations reached their maximum at 20 percent in both roots and lower stems (Figure 4). Two control points occurred in this growing season, which is consistent with the bimodal biomass pattern observed. The first control point (mid-July) occurred simultaneously with the onset of autofragment production and after completion of the first seed production episode (Figure 5). The second control point (October) occurred at the peak of autofragment formation, and simultaneously with the peak of inflorescence production and onset of seed formation during the second biomass peak. Both control points can be exploited for management purposes-- the first, to prevent excessive summer growth, and the second, to prevent successful overwintering. Different populations may exhibit different patterns, and populations may also vary between years.
 |
 |
|
Figure 4 |
Figure 5 |
Biomass Allocation Patterns
Eurasian watermilfoil biomass was predominantly allocated to the upper stems, a strategy that is consistent with maximizing photosynthetic and production capabilities (Figure 6). Lower stem and root crown allocation is consistent across season. In both 1991 and 1992, a strong peak in allocation to autofragment production in late summer and fall was noted. This was the chief form of vegetative propagation, and used up to 20 percent of total plant biomass. Autofragment formation may be linked to the peak of flowering and seed set, which is also related to the length of growing season and water temperature (Figure 5
above). Autofragments can survive independently in the water column, and stored carbohydrates may allow them to establish new colonies (Kimbel 1982, Madsen, Eichler, and Boylen 1988).
 |
|
Figure 6 |
Literature Cited
Aiken, S.G., Newroth, P.R., and Wile, I. 1979. The biology of Canadian weeds. 34. Myriophyllum spicatum L. Canadian Journal of Plant Sciences 59:201-215.
Couch, R. and Nelson, E. 1985. Myriophyllum spicatum in North America. Proceedings of the First International Symposium on Watermilfoil (Myriophyllum spicatum) and Related Haloragaceae Species; 23-24 July 1985, Vancouver, BC, Canada. Aquatic Plant Management Society, Vicksburg, MS.
Kimbel, J.C. 1982. Factors influencing potential intralake colonization by Myriophyllum spicatum. Aquatic Botany 14:295-307.
Linde, A.F., Janisch, T. and Smith, D. 1976. Cattail - The significance of its growth, phenology and carbohydrate storage to its control and management. Technical Bulletin 94, Wisconsin Department of Natural Resources, Madison, WI.
Luu, K.T., and Getsinger, K.D. 1988. Control points in the growth cycle of waterhyacinth. Aquatic Plant Control Research Program, Vol A-88-2, US Army Engineer Waterways Experiment Station, Vicksburg, MS.
Luu, K.T., and Getsinger, K.D. 1990. Seasonal biomass and carbohydrate distribution in waterhyacinth: Small-scale evaluation. Technical Report A-90-1, US Army Engineer Waterways Experiment Station, Vicksburg, MS.
Madsen, J.D. 1991. Resource allocation at the individual plant level. Aquatic Botany 41:67-86.
Madsen, J.D. and Boylen, C.W. 1989. Eurasian watermilfoil seed ecology from an oligotrophic and eutrophic lake. Journal of Aquatic Plant Management 27:119-121.
Madsen, J.D., Eichler, L.W., and Boylen, C.W. 1988. Vegetative spread of Eurasian watermilfoil in Lake George, New York. Journal of Aquatic Plant Management 26:47-50.
Madsen, J.D., Luu, K.T., and Getsinger, K.D. 1993. Allocation of biomass and carbohydrates in waterhyacinth (
Eichhornia crassipes): Pond-scale verification. Technical Report A-93-3, US Army Engineer Waterways Experiment Station, Vicksburg, MS.
Madsen, J.D., Sutherland, J.W., Bloomfield, J.A., Eichler, L.W., and Boylen, C.W. 1991. The decline of native vegetation under dense Eurasian watermilfoil canopies. Journal of Aquatic Plant Management 29:94-99.
Perkins, M.A., and Sytsma, M.D. 1987. Harvesting and carbohydrate accumulation in Eurasian watermilfoil. Journal of Aquatic Plant Management 25:57-62.
Schirman, R., and Buchholtz, K.P. 1966. Influence of atrazine on control and rhizome carbohydrate reserves of quackgrass. Weeds 14:233-236.
Titus, J.E., and Adams, M.S. 1979. Comparative carbohydrate storage and utilization patterns in the submersed macrophytes, Myriophyllum spicatum and Vallisneria americana. American Midland Naturalist 102:263-272.