The management of exotic aquatic plants with insect biological control agents officially began in 1959 and is an active and growing management alternative. From 1959 to the present, twelve insect Biocontrol species were introduced and released on four introduced aquatic plant species at a variety of locations across the United States. In addition to the introduced species, many native insects also feed upon and damage these same problem plants.
The increasing diversity of introduced and native insects that impact aquatic plants is causing complications for operational and other nontechnical personnel. This occurs because they are now faced with a large complex of organisms (both introduced and native) that offer potential as Biocontrol agents instead of only minimal numbers commonly found just a few years ago.
Biological control can play a significant role in aquatic plant management at federal, state, and local levels if proper knowledge and information on the use of various organisms are available. Increasing the awareness of operational personnel to the potential of this management technique was the driving force behind the development of this expert system and associated short course.
The ability to rapidly and effectively disseminate information on the use of Biocontrol led to the development of this information system, which allows for rapid identification of the Biocontrol agents and their associated damage. The expert system also provides detailed textual information through hyper-linked textual information concerning general concepts of insect Biocontrol in aquatic plant management, as well as supplying information on population biology, ecology, identification, and descriptions and use of selected insect Biocontrol agents.
The textual information is roughly organized as follows:
general concepts of biological control, including historical, ecological, and philosophical aspects of Biocontrol
an overview of the problems posed by specific exotic aquatic plants
the impact of various herbivores on aquatic plants
descriptions, life histories, and plant damage caused by specific organisms (discrete, as well as overall damage to the plant population)
collection techniques for Biocontrol agents associated with aquatic plants; and
a brief overview of computer-based information systems including a detailed use guide.
Each section is self-explanatory, but some general information is worth considering.
Basic ecological concepts illustrate why introduced aquatic plants become problematic rapidly after their introduction. Similarly, ecological concepts illustrate the manner by which biological control agents are able to effectively suppress target populations as well as address the practicality of using Biocontrol management techniques as part of an overall aquatic plant management strategy.
To effectively assess the potential and probable impact caused by various agents it is important to be able to recognize each species in all life stages and, more importantly, to understand their potential impact. Knowing the proper collection techniques for specific Biocontrol agents allows operational personnel to begin to "actively" use Biocontrol procedures. Finally, familiarity with the computer-based expert system permits operational personnel without formal training in plant and insect taxonomy, to quickly identify insect species and their associated damage with a high degree of accuracy.
History of Biocontrols
How and why did man begin to use biological control? What prompted man to begin to utilize 'natural' control measures to manage nuisance populations?
Unfortunately, little direct information is available on the early practice of biological control techniques. Perhaps the earliest use of biological control began with the recognition that many species of organisms feed and hence control other more noxious animals and plants. No doubt, early farmers observed that certain kinds of birds were attracted to the large numbers of insects drawn to their crops. Mice and other rodents must also have been attracted to the abundance of food present in the fields of these early farms. Observance of certain small felines feeding on these rodents most likely led to the domestication of the house cat by the ancient Egyptians.
This may have been the first attempt at using another organism for the biological control of a pest species.
As early as 350 BC Aristotle and others apparently recognized that certain diseases of bees were brought on by other insect species. By 900 AD the early Chinese were introducing certain species of predatory ants into their citrus groves to protect oranges against "wormy" fruit. In 1762 the first documented example of an introduction of a biological agent for the purpose of management of a pest species took place when the mynah bird was transported to the island of Mauritius for the management of locusts.
The use of biological control was becoming rapidly accepted as a viable means of pest management by the early to middle nineteenth century. By this time Western Europe was already deeply entrenched in the use of biological agents such as ichneumonid wasps, dragonflies, spiders, and ladybird beetles to control a variety of pests. By the middle to late nineteenth century, international transport of a species of mite from America to France occurred to control grape phylloxera. The basic concepts and understanding of biological control arose from these early attempts to manage pests (mainly arthropod) that attack crops and ornamental plants.
However, just prior to the twentieth century, the stage was set for a new era in the use of biological control. At this point, the newly developed citrus industry in California was undergoing hard times because of the widespread infection of citrus trees with the plant-feeding insect, cottony cushion scale. Cottony cushion scale most likely originated in Australia and was accidentally introduced in California with the citrus trees. It was proposed that an entomologist be sent to survey for potential enemies of cottony cushion scale in its supposed site of origin. A "natural enemy" was found, the vedalia beetle, a close cousin to our ladybird beetle. The insect was introduced into the California citrus groves, and within a few months cottony cushion scale infestations were severely curtailed.
This seems to be the first real widespread recognition of the "natural" approach to biological control. This is the idea of recreating population equilibrium by "reintroducing" regulatory factors into the new environment, i.e., the natural enemies of an "exotic" pest species.
So far we have been discussing the history of biological control in a general way, and you have probably noted that there has been no mention of exotic aquatic plants. The use of biological control for the management of exotic plants was longer in coming.
Aquatic Plant Biocontrol History
Waterhyacinth became a serious problem not long after its introduction at the Cotton Exposition in New Orleans, Louisiana. As early as the late 1890's waterhyacinth was beginning to block navigable waters in several southern states. In 1899, the US Congress authorized the US Army Corps of Engineers to remove waterhyacinth from navigable waters in Florida and Louisiana. The persistence of waterhyacinth was such that the removal of it from waterways became an ongoing function of the US Army Corps of Engineers.
In 1945, shortly after the development of 2,4-D and other herbicides, chemical management techniques were first instituted to supplement conventional hand and mechanical techniques for the removal of waterhyacinth. Although modest success was attained using mechanical and chemical management techniques, efforts were made to find viable alternative strategies for waterhyacinth management.
In 1958, the US Congress authorized the Expanded Project for Aquatic Plant Control. This project instructed the US Army Corps of Engineers to research the control and eradication of aquatic plants in waterways in the Gulf and Atlantic states. As a result of this project, biological control began to be seriously considered as a viable option for the management of introduced aquatic plants.
Formal use of biological control for management of exotic aquatic plants began in 1959 when the US Army Corps of Engineers and the US Department of Agriculture entered into a cooperative agreement to study the possibility of using biological control for management purposes. The first two plant species studied were alligatorweed and waterhyacinth, and researchers were sent to South America to survey for possible insect Biocontrol agents of these plant species.
Since that initial cooperative agreement the use of biological control as an integral management technology for aquatic plants has grown considerably.
For example, 12 insect agents have been released on 4 problem plants (i.e., alligatorweed, waterhyacinth, waterlettuce, and hydrilla) and the agents have established populations in 11 US states. A variety of effective organisms have been released including the alligatorweed flea beetle, two species of waterhyacinth weevil, the waterlettuce weevil, and two species of hydrilla leaf-mining flies, among others. Also, the program is expanding by considering the use of other types of organisms. This includes the potential use of plant pathogens as aquatic plant mycoherbicides. Not only has the US benefited from these Biocontrol efforts, but the technology developed in this country has been applied directly to aquatic plant problems in 11 other countries.
The scope of the program is tremendous. For example, the program has close cooperative ties to 4 major Federal agencies, 5 state agencies, and numerous local and academic institutions. In addition, the magnitude of the insect release program is immense. From 1987 to the present, over 4 million individuals of the 4 species of insect Biocontrol agents of hydrilla have been released in 6 states. The released individuals were obtained from rearing programs maintained at 4 Federal facilities; the USDA, ARS in Fort Lauderdale, FL; TVA in Muscle Shoals, AL; the U.S. Army Engineers Lewisville Aquatic Ecology Research Facility in Lewisville, TX, and the US Army Engineer Research and Development Center in Vicksburg, MS.
Biological control offers an exciting and viable alternative method for the management of aquatic plants. When used in conjunction with more traditional management techniques, Biocontrol can be effective for long-term, environmentally compatible aquatic plant management.
Benefits/Disadvantages to Biocontrol
There are many advantages as well as disadvantages to the use of biological control as part of an overall aquatic plant management program.
Advantages include longer term control relative to other technologies, lower overall costs, as well as plant-specific control that lead to enhanced environmental compatibility.
On the other hand there are several disadvantages to consider, including effect (or control) times of years instead of weeks, agents available for only a limited number of target plants, and relatively strict environmental conditions for success.
Therefore, it is important to consider all aspects of the plant problem before deciding on the use of one or more management strategies.
Probably the most important advantage to the use of biological control is that it typically offers longer term management than the more traditional technology areas. Longer term control is achieved because Biocontrol agents act as if a host-specific control method is continually present and impacting the target plant.
For example, once an agent is released and well established, insect population levels cycle proportionately with the population of the plant. That is, when plant population levels are high, there will be a corresponding increase in the population level of the Biocontrol agents. When plant levels decrease, there is a corresponding decrease in the numbers of the Biocontrol agents. In other words, small population levels of the insect agents persist and continually exert controlling or regulatory pressure on the target plant.
Another advantage is that the cost for control is typically lower relative to more traditional control procedures. Typically, Biocontrol agents are released in relatively low numbers for only a short time in the beginning of the program unlike more traditional methods of control which are used continually over many years. After the releases are discontinued, the agent population increases, if successful, and begins to damage the target population. Only in rare circumstances are the agents released continually. A lower cost for control is the result, because the initial high cost of introduction, release, and establishment is offset by the value of achieving and maintaining control for many years after initial release of the organisms; i.e., benefits accrue over many years.
Alligatorweed is an excellent example. Presently, few, if any, chemical applications are used to control the aquatic form of alligatorweed in most of its southern U.S. range. This situation has occurred because the three insect Biocontrol agents released in the 1960's have been extremely effective in maintaining population levels of alligatorweed below problematic levels. The insect Biocontrol agents for alligatorweed are normally present at all times; however they may be present only in small numbers when the density of alligatorweed is low. When alligatorweed populations increase, there is a subsequent increase in the populations of the Biocontrol agents. After increases occur, the combined effects of insect feeding and various abiotic factors cause a subsequent decline in alligatorweed density.
With the use of Biocontrol techniques for the management of alligatorweed there is a substantial savings in the cost for control on a per-acre basis. Comparing the costs per acre for alligatorweed control, the expenditures for mechanical and chemical control are 192 and 32 times higher per acre, respectively, than the costs associated with the use of biological control. Hence, control of alligatorweed using insects resulted in a lasting control that represented many dollars in cost savings.
Additionally, Biocontrol techniques using insects are specific to the target plant. Such host- specificity ensures that there will be no impact by the Biocontrol agents on desirable native plant species or crop plants.
However, it is important to realize that an effective management plan for aquatic plants includes not only insect Biocontrol but also all of the other available control technologies. The choice of what management technique or multiple techniques are used is dependent on the specific scenario. In many instances Biocontrol techniques may not offer the best overall management, and other procedures must be used. For example, Biocontrol typically takes between 5 and 10 years from the initial releases to the time effective control is achieved. This is compared to only weeks for chemical applications and days for mechanical harvesting.
In addition, insect agents for some plants may be effective only under certain highly specific environmental conditions.
And finally, some problem plant species may not have agents available for use at the present time.
All of these factors, on a site-by-site basis, must be considered before deciding on the use of insect Biocontrol. Strategies for deciding when to use Biocontrol alone or in conjunction with other control technologies will be discussed in greater detail in later sections.
Using Biocontrol Agents More Actively
For more information see "An Active Approach to the Use of Insect Biological Control for the Management of Non-Native Aquatic Plants" published in Journal of Aquatic Plant Management (vol. 36; pp 57-61)
Currently, Biocontrol technology is a rather passive procedure that requires little, if any, participation at the operational level. Biocontrol agents are released at many areas across the United States, usually by US Army Corps of Engineers researchers, in the hope that the agents will become permanently established, develop large and damaging populations, and thereby reduce the level of nuisance aquatic plant infestations to below problem levels. In many cases such an approach has proven to be highly adequate and several outstanding successes have been observed.
However, by utilizing existing Biocontrol technologies more actively, with greater participation at the operational level and in knowledgeable conjunction with more traditional control procedures, a higher degree of plant suppression may be realized. While this will require more effort on the part of operational personnel, the rewards in greater overall management and in reduction of the use of chemicals may be worth the added effort.
Important questions to ask are: What will it take to begin actively using Biocontrol technology, how is the active use of Biocontrol going to increase my already tremendous workload, and will it be worth the added effort?
To begin with, four major steps toward more actively using Biocontrol technology have been identified. These steps include: 1) knowledge, 2) survey, 3) supplement, and 4) integrate.
The most important step to take in an effort to increase your participation in the use of Biocontrol is to gain a thorough and practical knowledge of Biocontrol technology. This expert system represents an effort on the part of researchers to transfer information on Biocontrol technology in a manner that will be useful to operational personnel.
Important information includes 1) familiarity with the introduced as well as the native organisms that feed on nuisance aquatic plants in terms of their life history, associated damage, and identification 2) basic understanding of the general tenets of Biocontrol including ecological theories, population biology, etc. (see also - General Concepts of Biocontrol, 3) impact caused by the feeding action of a complex of herbivores (see also - Overall Herbivore Effects ), 4) the benefits and disadvantages to the use of Biocontrol (see also - Benefits/Disadvantages of Biocontrol ), 5) collection techniques (see also - Overview of Collection Techniques ), and 6) a working knowledge of how the introduced agents are discovered in their country of origin and finally released in the United States (see also - Process of Biocontrol ).
While much of this information may seem superfluous, most if not all will allow you to better utilize existing Biocontrol technology and thereby increase your active use of Biocontrol technology. This will become clearer with discussions on other aspects on the use of Biocontrol technology.
The second step toward increasing the active use of Biocontrol is to develop a systematic plan to survey aquatic plant infestations. Surveys are important to estimate population sizes (mainly for the introduced agents) and their potential impact. By estimating population numbers and impacts, a clearer picture of the existing Biocontrol technology will be available, and you can better judge the steps necessary to preserve or enhance the existing agents.
An example of the importance of making periodic surveys can be taken from a site in south-central Texas. During a recent trip to Lake Corpus Christi with Texas Parks and Wildlife (TPWD) personnel Waterways Experiment Station personnel noted the relative absence of insect Biocontrol agents of waterhyacinth throughout the lake. The waterhyacinth infestation was expanding rapidly, and the plants were robust, 3 to 4 ft tall, with tremendous flowering. Such growth characteristics are typical of waterhyacinth grown in the absence of insect herbivores. From discussions with TPWD personnel it was discovered that the insect agents were apparently absent from this site for several years. While reasons for the absence of the insects are unknown, they may be related to periods of drought that occurred during the late 1980's and/or the wholesale herbicide applications commonly made during this same time. Whatever the reasons, periodic surveys would have revealed the absence of the agents, and steps, such as augmenting or supplementing the population could have been made at an earlier date to help rectify the situation.
There are several ways to correctly survey plant infestations for Biocontrol agents. All of these deal with determining the kinds and numbers of agents present as well as estimating impact. Such information is not easy to collect; however, some survey methods are more time and cost effective.
The easiest method of surveying is to note at each site the presence or absence of the agents, both native and introduced. This can be done easily by visual observations noting relative abundance based on feeding damage or actual observation of the agents. Such information is important, although at best, it is a crude method of determining agent numbers and impact.
A better method is to actually count the agents on either a per plant or per unit area basis. While this method is more involved, it too can be accomplished relatively easily. For example, for waterhyacinth you can count the numbers of agents on 25 to 50 plants taken from several areas around the site and form an average number per plant based on the sample. Another way would be to sample a 0.25-square meter area at several locations. All the plants contained within the area would be counted and the number of agents determined. While such sampling is still fairly crude, repeated samplings taken over several growing seasons at the same time of year would provide valuable insight into population numbers and potential past and present impact.
The third step to actively using Biocontrol technology is to supplement the agent's populations when survey information reveals that the agent numbers are low, nonexistent, or that impact is negligible. Recent research conducted by Dr. T. Center of the USDA, ARS, in Fort Lauderdale, FL, demonstrated that supplementing waterhyacinth weevils at sites on the St. Marks River significantly increased the control of waterhyacinth in relatively short time frames.
Supplementing the agents may sound far-fetched, but in most cases it is relatively easy especially considering the variety of ways available to obtain the insects.
The best method to obtain the agents would be to contact C. Ashton of the Aquatic Plant Operations Support Center located at the Jacksonville District (phone (904) 232-2219). The center has shipped both alligatorweed and waterhyacinth insects to interested parties in the past.
Another method for supplementing the agents would be to collect them yourself from nearby sites. Personnel from TPWD collected over 9000 waterhyacinth weevils from a site in Wallisville, TX, to supplement populations on Lake Corpus Christi. The insects were collected by a team of seven people in less than 6 hours.
The third method for obtaining the agents would be to purchase them from a private source. Recently, ERDC purchased 10,000 weevils from a dealer in Florida for $1,500, an excellent price considering the costs associated with the collection process.
All in all, the practice of surveying sites, not only for plant infestations but also for the presence of Biocontrol agents, will pay off in the long run with an intimate knowledge of the site's past and present status, increased awareness of the practical benefits of using Biocontrol technology, and a collection of useful information on seasonal changes in the plant's status and associated herbivores.
The final method of increasing your active use of Biocontrol is the integration of all technologies into a fully compatible management procedure. Research at ERDC and other locations has indicated the potential of herbicides to indirectly impact Biocontrol agents. This occurs through the herbicides' rapid action on the plants, causing degradation and decomposition. Since the agents, mostly the immatures are tied to the plant for food and shelter, any change in the plant's status will severely impact the agents. Hence, at sites where it is desirable to maintain a high level of Biocontrol activity, precautions should be taken to minimize such impact from herbicide application.
Minimizing the impact of chemical applications on Biocontrol agents is easy and is accomplished mainly by leaving a portion of the plants unsprayed to act as harborage areas or conservation areas for the agents. The bottom line; Spray only those areas that must be controlled immediately, that is, high priority areas. While leaving plants unsprayed goes against all common sense, such a practice allows Biocontrol agent populations to increase unimpeded and subsequently form damaging population levels. Such harborage areas must be monitored periodically to make sure they are not contributing to overall plant infestation. However, the agents, with time, typically keep the plants in the harborage areas at minimal levels by their combined feeding action. In addition, these areas act as insect nursery areas, for later recolonization of plants in nearby sites.
Overview of Collection/Sampling Techniques
This section is designed to acquaint the user with general procedures used to collect the organisms (mainly insects) that feed on and hence damage the four nuisance aquatic plants included in this system. In this section only an overview will be given, with more plant and insect specific information given in the sections concerning insect and damage descriptions.
Basically four types of procedures are used to collect insects and related organisms that feed on and damage the nuisance plants contained within this system. These procedures include 1) hand collection, 2) sweep netting, 3) extraction techniques, and 4) infested plant removal.
Knowing the correct and most time efficient collection procedures will allow personnel at the operational level to accurately and rapidly determine population levels of the agents. Information on population level is important in determining potential impact on the plant infestation by the agents as well as for supplementing the agents in areas where their population numbers are low or nonexistent. Determining numbers of agents in a systematic manner is an important first step in utilizing insect Biocontrol technology more actively in existing aquatic plant management programs (see also: Using Biocontrol Agents More Actively).
As the name implies, hand collection techniques are nothing more than simply collecting the agents by hand directly off the plant. For many insect species this is the best and most efficient manner to collect large numbers of organisms. While this technique sounds easy it is best to understand a little about the life history of the organism before attempting to collect large numbers.
For example, hand collection techniques are an efficient way to collect large numbers of both species of adult waterhyacinth weevils (see also: Neochetina eichhorniae - "Mottled Waterhyacinth Weevil" and Neochetina bruchi - "Chevroned Waterhyacinth Weevil"). However, the ability to collect large numbers is dependent on knowing where on the plant to look for the adults. Both of these species tend to reside in the unfurled wrapper leaves and within the leaf sheaths or ligules. They are not found within the plant or simply crawling along the exposed leaf surfaces.
In most cases the organisms can be simply removed from the plant by hand. However, in certain cases additional equipment will be necessary because of the small size and delicate nature of the organisms. Such equipment may include forceps or small battery-powered vacuum cleaners for picking up the insects.
The use of an insect net is a common collecting technique used by entomologists to gather certain species of insects. In the case of the organisms contained within this system, sweep netting is probably one of the least efficient. One species that can be collected using this technique is adults of the alligatorweed flea beetle (see also: Agasicles hygrophila - "Alligatorweed flea beetle" ). Large numbers of adults can be collected relatively rapidly using this technique.
The use of a sweep net is not difficult but usually requires practice to become efficient. To correctly use a sweep net, hold the net with both hands with the net handle directed forward and the net opening pointed sideways. Swish the net from side to side in a small arc in front of your body as you walk slowly forward. As the net reaches on the side of your body, begin to return the net to your opposite side. Be careful when switching directions since insects can escape at this time. Therefore, quickly flick the net over to close the bag against the net frame when changing directions.
Similarly, when ready to remove the insects, close the opening to the net by allowing the end of the bag to drape over the circular net frame. Since the insects will be alive (of course), carefully remove the entire contents to a plastic bag for later sorting. The presence of bees and wasps is a distinct possibility, so remove the organisms with great care.
Since a majority of the organisms feed within the plant tissues, a more efficient and less time-consuming procedure must be used to collect high enough numbers. In this case a passive technique using specialized equipment to extract the organisms directly from infested plant material is used. The most commonly used equipment is the Berlese funnel.
As the name implies, the Berlese funnel is a large metal container shaped like a funnel that uses heat and light to desiccate or dry infested plant material contained within the funnel. The process of plant drying forces the insects to seek wetter plant material until they eventually fall into a collecting container located at the bottom of the funnel.
Berlese funnels can be purchased from a variety of sources, but with ingenuity one can be built with little effort. Imagine a funnel held upright by a large stand. Within the funnel is a piece of hardware cloth or screen upon which rests the infested plant material. A 60- to 75-watt fixture is placed on top of the funnel. The light fixture covers but does not completely seal the funnel off. It is important for air to circulate within the apparatus for slow and even drying of the plant material. At the bottom of the funnel is a small glass jar (Mason jars work great) filled with a small quantity of the appropriate plant material. As the plant material within the funnel dries from the top down, the insects are forced to seek moister material until they eventually fall into the collecting container.
Large amounts of plant material can be processed very quickly using this method.
Removal of Infested Plant Material
In many cases, especially when insect populations are high, there is no need to extract the organisms from the plant material. Instead, the plant material can be moved directly to the site where the agent levels are low. This is a highly efficient technique that requires little effort, but only works when population levels are high. If levels are low, large amounts of biomass must be moved to allow for significant increases in population levels at the new site.