THE ROLE OF NEOCHETINA EICHHORNIAE AND NEOCHETINA BRUCHI ON BIOLOGICAL CONTROL OF WATER HYACINTH IN SRI LANKA
By
ANUSHA P. KASIGE
THIS THESIS WAS SUBMITED TO THE DEPARTMENT OF CIVIL ENGINEERING OF THE UNIVERSITY OF MORATUWA IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE
DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF MORATUWA SRI LANKA DECLARATION
I hereby declare that the work included in the thesis in part or whole has not been submitted in any form for any other academic qualification of any institution.
Miss. A. P. Kasige
Certified by
^g^X^r^
Dr. N.P.D. Gamage
Supervisor
Dr. Mahesh Jayaweera
Supervisor. ABSTRACT
One of the major scourges accompanying water resource development in Sri Lanka is the explosive proliferation of water hyacinths {Eichhornia crassipes). A better sustainable solution to manage the infestations seems to be biological control and the main biological control agent used in many parts of the world including Sri Lanka is reported to be the weevil [{Neochetina eichhorniae Warner) (Coleoptera; Curculionidae)]. Fernando and
Room used the weevil N. eichhorniae for the first time in Sri Lanka in 1988. Although some 15 years have elapsed since the first release, infestation in the areas in which the weevil was released is as high as in the areas in which N.eichhorniae was not released.
The present study therefore focuses on the evaluation of the role of N. eichhorniae and
N.bruchi on controlling water hyacinth and is designed to evaluate the optimum weevil densities required to cause significant damage to the plants. Healthy plants of height 21 cm ±1 were cultured in 6 and 4 fiberglass tanks respectively for a period of 8 weeks to complete one life cycle of weevil. Different weevil densities were used, varied from 1, 3,
6,10 and 15 weevils per plant, and the control with no weevils. In case of N.bruchi the first 3 treatment levels were tested with the control. Field monitoring carried out in eight locations within the Western province and showed the average maximum weevil density in natural conditions as 2 per plant. The success of biological control using N. eichhorniae will ultimately rely on host plant quality and the habitat conditions to establish a healthy population of weevil densities. Results showed that the treatments with weevil numbers less than 3 of N.eichhorniae per plant did not significantly change (p>0.01) the water hyacinth stands, but 3 weevils per plant of N.bruchi was the best option in sustainable management. Densities of 10 of N.eichhorniae and 6 of N.bruchi were subjected to complete eradication of the plant. ACKNOWLEDGEMENT
First, I would pay my heartfelt gratitude to my supervisors, Dr. N. P. D. Gamage,
Dr. M. W. Jayaweera and Dr. (Mrs) S.D. de Silva for guiding me in all aspects throughout the research. 1 wish to offer my cordial thanks to Professor (Mrs) N.
Rathnayake, Director of Post Graduate Studies and to the Chancellor of University of Moratuwa, Dr. Ray Wijeywardena for their helping hands stretched out to me at turn.
Special thank are offered to Dr. I.V.S. Fernando and Dr. Lalith Gunasekera for their advise concerning this project.
It is with grateful appreciation that I acknowledge the contribution made by the Environmental Engineering Laboratory staff of the University of Moratuwa, namely Ms. Priyanka Dissanayake, Mrs. Nilanthi Gunathilake and Mr. Justin Silva by way of technical assistance.
I wish to offer my sincere thanks to my loving parents, my heartiest friend Akalanka and all the others including my friends, who supported and encouraged me in numerous ways to make my effort a success.
Finally I gratefully acknowledge funding from the Asian Development Bank and
Saitama University, Japan for this project. TABLE OF CONTENTS
Page
List of figures i
List of tables ii
Chapter 1: Introduction
1.1 Water hyacinth - as a troublesome weed 1
1.2 Control strategies in Sri Lanka 2
1.3 Introduction to water hyacinth weevils 4
1.4 Scope of the study 7
1.4.1 Study objectives 8
Chapter 2: Literature review
2.1 Introduction 10
2.2 Water hyacinth 11
2.2.1 Description 11
2.2.2 Growth and life cycle 12
2.2.3 Distribution and habitat 13
2.2.4 Nature of damage 14
2.2.5 Utilization 16
2.3 Management of water hyacinth 16
2.3.1 Mechanical control 17 Pages
2.3.2 Chemical control 17
2.3.3 Biological control 18
2.3.3.1 Exploration for natural enemies 19
2.3.3.2 Agents currently being studied 22
2.4 The water hyacinth weevils 22
2.5 Host range testing and results for 25
Neochetina species
2.6 History of introduction 26
2.6.1 Case studies reported 26
Chapter 3: Methodology
3.1 Field study-phase 1 30
3.2 Impact of Neochetina eichhorniae on host plant 31
3.3 Impact of Neochetina bruchi on host plant 34
3.4 Statistical analysis 35
Chapter 4: Results and Discussion
4.1 Results of field study - phase 1 36
4.2 Impact of Neochetina eichhorniae density 39
on host plant
4.2.1 Nature of damage 39 Pages
4.2.2 Impact of Neochetina eichhorniae 44
on plant growth
4.2.3 Impact of Neochetina eichhorniae 45
on leaf dynamics
4.3 Impact of Neochetina bruchi density on host 47
plant-phase III
4.3.1 Nature of the damage and impact 47
on plant reproduction
4.3.2. Impact of Neochetina bruchi on plant growth 50
4.3.2 Impact of Neochetina bruchi on plant 50
Leaf dynamics
4.4 Comparison of the effectiveness of the two weevils 52
4.5 Effect of weevil herbivory on plant N content 53
Chapter 5: Conclusion and Recommendations
5.1 Conclusion 55
5.2 Drawbacks of the study 56
5.3 Recommendations for future work 57
Appendix 1: Study sites in the field a-b LIST OF FIGURES
Pages
Figure 1 Water hyacinth infestation in Diyawanna Oya 1
Figure 2 Manual removal of water hyacinth 3
Figure 3 Neochetina eichhorniae 4
Figure 4 Neochetina bruchi 4
Figure 5 General life cycle of Neochetina species on 5
water hyacinth host plant
Figure 6 Water hyacinth growth in Sedawatta area 10
Figure 7 Water hyacinth plant external morphology 12
Figure 8 Highly infested water bodies in western province 14
Figure 9 Quadrate sampling 30
Figure 10 Initial water hyacinth colonies 32
Figure 11 A close view of the tanks containing water 32 hyacinth infested with Neochetina eichhorniae
Figure 12 Weevil introduction 33
Figure 13 Mass rearing of Neochetina bruchi in Open 34
University premises Pages
Figure 14 Weevil densities distributed within the field 37
Figure 15 Relationship between shoot length and weevil 38
density in the field
Figure 16 Percentage damage leaf area for different weevil 40
Densities
Figure 17 Total number of plants for different weevil 41
Densities
Figure 18 Damage done by Neochetina eichhorniae after 4 weeks 42
Figure 19 Damage done by Neochetina eichhorniae after 8 weeks 43
Figure 20 Variation of plant height with different weevil 44 Densities
Figure 21 Variation of total number of leaves per plant in 45 different weevil densities
Figure 22 Variation of total leaf area in different weevil densities 46
Figure 23 Variation of 3rd leaf lengths in different weevil densities 47
Figure 24 Damage done by Neochetina bruchi after 4 weeks of 48 introduction
Figure 25 Damage done by Neochetina bruchi after 8 weeks of 49
Introduction
Figure 26 Variation of number of plants in different weevil 49
Densities
ii Pages
Figure 27 Variation of average plant heights in different weevil 50
Densities
Figure 28 Variation of number of leaves per plant in 51
different weevil densities
Figure 29 Variation of total leaf area in different weevil 51
Densities
Figure 30 Comparison of percentage damage made by two 52
Weevils
Figure 31 Effect on leaf and shoot N content by Neochetina bruchi 53
Figure 32 Effect on leaf and shoot N content by 53
Neochetina eichhorniae
iii LIST OF TABLES
Pages Table 1 Areas in which N.eichhorniae were released in 4 Sri Lanka
Table 2 General introduction to Neochetina eichhorniae and Neochetina bruchi
Table 3 Key differences between two weevils
Table 4 Some of the studies conducted on biological control of 20 water hyacinth
Table 5 Main three groups of biological control agents on 21 water hyacinth
Table 6 Neochetina eichhorniae status of releases for each country 23
Table 7 Neochetina bruchi status of releases for each country 24
Table 8 Development durations for each life cycle stage 25 and fecundities for both Neochetina species
Table 9 Nutrient composition of the water hyacinth culture medium 31
Table 10 Average measurements of water depth, water quality 36
parameters and plant characteristics in each study site
IV Chapter 1 Introduction
Chapter 1 Introduction
1.1 Water hyacinth -a troublesome weed
Enrichment of aquatic eco-systems with N and P as a result of human interferences
creates favorable conditions for the spreading and growth of water hyacinth (Eichhornia
crassipes [Mart] Solms.). This weed seems to be an aggressive floating aquatic weed
with high intrinsic growth rates in the tropics and subtropics. Water hyacinth is said to
have ranked among the top ten of nuisance aquatic plants in the world (Holm et al.,
1977). This plant was first introduced in Sri Lanka to the Botanical Gardens in Colombo
in 1904 and by 1909 it had caused sufficient problems so that the proclamation of "Water
Hyacinth Ordinance" resulted in (Room and Fernando, 1992; Gopal, 1987). Despite
several expensive eradication campaigns, the weed has been found throughout the
lowlands by 1922 and 338 infestations had been reported in 1933. Infestations were said
to have remained widespread, numerous and flourishing throughout the 1980s (Room and
Fernando, 1992). Both published and unpublished data and observations in Sri Lanka
revealed that the infestations have been widespread even now in the Northwest, Central
and Southern provinces (Fig. 1) (Fernando. 1996 and personal observations).
Figure 01: Water hyacinth infestation western province
• 1 Chapter 1 Introduction
One of the major scourges accompanying water resource development in Sri Lanka is the explosive proliferation of water hyacinth. The largest river, the Mahaweli Ganga, carries substantial runoff from the central mountains to the north east coast and, during the 1980s
US$ 4 billion had been spent to build a series of dams, irrigation systems and hydropower generators. Water hyacinth had posed serious threats to the viability of this investment
(Room and Fernando, 1992).
The term "weed" is used for water hyacinth as it interferes with the utilization of a particular natural resource or has other adverse impacts on man (Pantilu, 1984). They add to oxygen demand through their decomposition and by transpiring into the atmosphere, the member plants do not contribute photosynthetically produced oxygen to the water body. Furthermore by forming a surface cover the suds inhibit considerably the photosynthetic activity of algae. Their very presence reduces wind action, which hinders mixing, and resultant oxygenation in the lake. The resultant oxygen depletion can cause fish mortalities on a large scale. Furthermore, these plants can interfere with the operation of fishing nets, cause hazards to navigation, clog water intakes, and provide a good habitat for aquatic vectors of human diseases (Pantilu, 1984). Increase of evapotranspiration is another issue to be dealt with. It has been estimated that the evapotranspiration by water hyacinth is more than six times the evapotranspiration rate from open water bodies (Gopal, 1987). This can dramatically increase the rate of water loss from a water body, imposing higher operational costs on water supply schemes and threatening their viability in dry seasons (Mien, Grifths and Wright, 1999).
1.2 Control strategies in Sri Lanka
It is clear from the foregoing that water hyacinth is a weed that should be controlled within Sri Lankan freshwater ecosystems. Three methods have been used in various parts of the country namely chemical control, mechanical control and biological control. The widely used measure of control is the manual or mechanical removal (Fig 2). But manual Chapter I Introduction
removal does not seems to be an adequate measure in controlling the weed since the
weed can reinfest a water body even when a single plant is left behind within a very short
period of time.
Figure 2: Manual removal of Water hyacinth
Except for several eradication campaigns launched in early 1920s along with the proclamation of "Water Hyacinth Ordinance", no other control project has been developed till Room and Fernando implemented (1988) the biological control programme as an adjunct of the Salvinia management project. Once it became clear that Salvinia could be controlled, another weevil, Neochetina eichhorniae, host specific to water hyacinth has been introduced with the aim of stopping water hyacinth from occupying space vacated by Salvinia (Fernando, 1996; Room and Fernando, 1992). 874 individuals of this South American weevil have been imported from Australia and have been reared • at the University of Kelaniya, Sri Lanka. The successfully multiplied adults have been
released into the following infested sites in particular numbers as in Table 1 (Room and Fernando, 1992).
In October 1989 (i.e. 10 months after the release of the weevils) water hyacinth leaves at
the Peliyagoda site bore feeding scars caused by adult weevils and new leaves had been
rarely appeared. At Wellampitiya, 50% of leaves examined had feeding scars and plants
apparently killed by larvae, indicating the onset of a significant reduction in water
hyacinth population density. In late 1990 at Peliyagoda, though Neochetina eichhorniae
had continued to be abundant but there are no reductions in water hyacinth infestation.
3 *
Chapter 1 Introduction
All the other sites other than Peliyagoda had not been inspected more recently since
October 1989 (Room and Fernando, 1992).
Table 01: Areas in which Neochetina eichhorniae were first released in Sri Lanka
(Source: Fernando and Room, 1992)
Year of Site Number of
Released Weevils
1988 Wellampitiya 250
Peliyagoda
1989 Eragama (Ampara) 16
1989 1 lungama (Hambantota) 100
1989 Aluthgama 100
1990 Chilaw Unknown
Lunuwila
1.3 Introduction to water hyacinth weevils
Figure 3: Neochetina eichhorniae Figure 4: Neochetina bruchi
The two types of water hyacinth weevils, Neochetina eichhorniae, the Mottled water
hyacinth weevil (Fig. 3) and Neochetina bruchi, the Chevroned water hyacinth weevil
(Fig. 4) belong to order Coleoptera and family, Curculionidae.
4 Chapter 1 Introduction
Figure 05: General life cycle of Neochetina species on water hyacinth host plant
(hit of the six members of the genus Neochetina, these two weevils are native to South
America. The two weevils are semi aquatic and more or less similar in appearance. Both
weevils have a common life cycle with slight variations of generation time (Fig. 05).
• Appearance and the biology of both weevils and the damage done to water hyacinth
plants have described in Table 2. Chapter 1 Introduction * :— Table 02: General introduction to Neochetina eichhorniae and Neochetina bruchi
Neocheina eichhorniae Neochetina bruchi
Appearance Gray and brown colour Brown colour, tan
bands on the elytra
Biology Small whitish eggs (0.75mm in length) are more Similar to N.
slender and softer, laid singly beneath the eichhorniae except
epidermis of the leaves, petioles and ligules. Eggs generation time
hatch into larvae of white
colour with a yellow orange head, with no legs or
pro legs in about 10 days. 1st instar larva is about
2mm length and the 3 rd instar larva is 8 to 9 mm in
length. Pupa is white and about 5mm, enclosed in
a cocoon attached to a root below water surface.
Damage Adult make feeding scars on leaf both upper and Much similar to
lower epidermis. When numerous, debilitate the N. eichhorniae. The
plant by removing extensive portions of epidermal efficiency of damage
tissue thus increasing water loss and exposing to depends on habitat
attack by pathogens. Extensive feeding around characteristics and
upper petiole may girdle the petiole and kill the climatic conditions
lamina above. Larval tunneling in the lower
petiole and crown damages tissues and buds,
initially preventing flowering. As damage
increases, plant growth rate is reduced and the
production of the new leaves and new stolons is
reduced. Plant size (height, weight, size of leaves,
size of stolons) declines. Internal damage to plant
tissue result in rotting of the lower petiole, water
logging of the crown and gradual sinking of the
plant and death take place
6 Chapter 1 Introduction
Table 03: Key differences between two species
Character Neochetina bruchi Neochetina.eichhorniae
Elytra markings Short and located mid way along Long and extending forward on
elytra elytra.
Two markings are dark colour Generally of unequal in length.
equal in length.
Elytra furrows Broader furrows with shallow Narrow furrows with strong
curvature curvature.
Elytra patterning Scale colouration forms a No chevron or V pattern
Chevron or V shape across entire
elytra.
Most obvious in newly emerged
ones
(Julien, Grifths anc wright, 1999)
1.4 Scope of the study
Neochetina eichhorniae can now be regarded as an established member of the Sri Lankan
water hyacinth fauna. The preliminary observations carried out in Colombo, Kalutara,
Gampaha and Hambantota districts suggested that the species has established in many
parts of Sri Lanka. Despites its spreading in other areas, hyacinth infestation is as high as
that in the areas without any species of Neochetina eichhorniae. The number of weevils
existing in the field and the habitat conditions are the critical factors to be maintained in
effective control of the weed. Therefore my study was carried out in three phases. From
the first phase I tried to identify the density of weevils in local field conditions and then
the factors affecting on the survival and reproduction of Neochetina eichhorniae in the
field Usually the plant, water hyacinth, and the weevil follow a cyclic change typical of
predator prey interactions. Therefore collapse of the water hyacinth population is closely
followed by an inevitable collapse of the weevil population. However if the plant Chapter 1 Introduction
population recovers rapidly and the weevils only slow, then dramatic re-growth can occur
(Wright and Center, 1984). Maintenance of optimum weevil density over an initial period of time is essential to achieve a successful biological control for water hyacinth as water hyacinth has a rapid growth rate compare to other aquatic weeds. Therefore the phase I of the study focused on the field aspects to identify the existing weevil densities under natural conditions and the possible factors that limit the increase of N. eichhorniae population to a threshold density.
Followed by the results of phase I, it was evident that there are no adequate weevil densities to do a significant damage to water hyacinth in the infested areas. Our field observations made in six sites clearly indicated that the control of water hyacinth is
mainly dependent on the average number of weevils established per plant. It was observed that the maximum colonization was 6 weevils per plant, with the minimum
being 0.5 per plant. Therefore phase II was carried out to find the optimum weevil
density of Neochetina eichhorniae.
Phase 3 was to analyze the efficacy of Neochetina bruchi with compared to Neochetina
eichhorniae since it is important to introduce a weevil population which maintain the
ecological balance avoiding inter specific competition. Several other countries have
exhibited successful results in controlling water hyacinth by using Neochetina bruchi.
Therefore our next scope of importing and studying of Neochetina bruchi was to identify
the suitable and best combination of weevil species to achieve a better control of the plant
in Sri Lanka.
1.4.1 Study objectives
1. Investigate the Neochetina eichhorniae weevil densities in different sites and to
identify the factors related to weevil colonization
2. Identify the optimum weevil density of N. eichhorniae and Neochetina bruchi
(imported from Ivory Coast) followed by a mass rearing of the weevil after under
8 Chapter 1 Introduction
Sri Lankan climatic conditions.Study the effect on leaf and petiole N levels due to
herbivory attack by the two weevils.
4. Identify the suitability of the species depending on the rate of control of the water
hyacinth with a minimum weevil density. Chapter 2 Literature Review
Chapter 2 Literature Review
2.1 Introduction
Water hyacinth, Eichhornia crassipes (Mart.) Solms-Laubach (Fig 6), is one of the world's worst weeds (Center et al., 2002), invading lakes, ponds, canals and rivers. It was introduced into many countries during the late 19th and early 20th centuries, after which it spread and degraded aquatic ecosystems. The plant is said to have reached Botanical
Gardens in Colombo in 1904 purely because of the beauty of its purple coloured flower
(Gopal, 1987). Water hyacinth has a multitude of direct and indirect effects on almost all aspects of human life once a water body on which man so much depends is invaded and covered by the weed mats (Makhanu, 1997): fisheries, water supply, hydroelectric power generation, human health, agriculture, transport, biodiversity, evapo-transpiration and increased cost of water treatment are some of the adverse effects.
Figure 6: Water hyacinth growth in Sedawatta area
Three main methods in controlling water hyacinth have been discussed and adapted in various regions in the world: mechanical, chemical and biological removal. Manual or mechanical removal has been employed in most of the infested areas in Sri Lanka, but the practice is neither economic nor effective. The use of machines to destroy or remove
10 Chapter 2 Literature Review
water hyacinth has limitations, including their inability to move around a large lake
(Mailu, 2000). Increasing concern about the financial and environment costs associated with herbicidal control measures and their limited effectiveness has led to growing interest in the use of biological control agents which have been identified and researched since the 1960s.
Biological control of water hyacinth offers sustainable, environmentally friendly, long- term control, and is a only feasible method to provide some level of control to those infestations. Several biological control agents have now been introduced into countries having heavy water hyacinth infestations. The species most widely used are Neochetina eichhorniae, the mottled water hyacinth weevil, and the Neochetina bruchi, the
Chevroned water hyacinth weevil. These have been introduced in more than 30 countries and are contributing to control the weed in many areas. Those thoroughly researched, proven biological control agents are readily available for introduction into countries where water hyacinth is a problem. Neochetina eichhorniae weevil has already become a member of Sri Lankan insect fauna. Needless to say, further research on factors affecting the efficacy of bio-control using these two weevil species with respect to local climatic and habitat conditions is needed without delay.
2.2 Water hyacinth
2.2.1 Description
The mature water hyacinth plant consists of roots, rhizomes, stolons, leaves, inflorescences and fruit clusters (Fig. 7). The roots are fibrous, unbranched and with a conspicuous root cap. They are purplish in exposed situations but white when in darkness or when rooted in the soil. The vegetative stem consists of an axis with short internodes, which produces the roots, leaves, offshoots, and inflorescence of the plant. This portion is known to be the rhizome. Long internodes, which are usually, nearly horizontal produce
11 Chapter 2 Literature Review
in open conditions whereas short, which are nearly vertical internodes, found within
dense mats. In either case they produce new off shoots at their distal ends. These
elongated internodes known as stolons grow horizontally to produce daughter plants
(Penfound & Earle, 1948). The buoyant leaves vary in size and morphology. The short,
bulbous leaf petioles produced in un-crowded conditions provide a stable platform for
vertical growth. Plants in crowded conditions form elongate (up to 1.5 m) petioles
(Center et al., 2002). Leaves are arranged in whorls of 6 to 10 and individual plants
develop into clones of attached rossets. The bisexual flowers are bluish purple with a
yellow center and are produced on single spikes to 60 cm in length.
Lamina
Petiole
Older flower stalk
Seed capsule Daughter plant Roots
Figure 7: Water hyacinth plant external morphology
<^ 2.2.2 Growth and life cycle
Water hyacinth grows from seed and through vegetative propagation. The flowers can get self - fertilized. The 14-day flowering cycle concludes when the flower stalk bends, positioning the spike below the water surface where seeds are released. Seeds capsules normally contain 50 seeds each. Each inflorescence can produce more than 3000 seeds and a single rosette can produce several inflorescences each year. The small long-lived seeds sink and remain viable in sediments for 15 to 20 years (Gopal, 1987). Seeds germinate on moist sediments or in warm shallow water and flowering can occur 10 to 15 weeks thereafter. Lack of germination sites limits seedling recruitment except during
12 Chapter 2 Literature Review
drought, on decaying mats after herbicide applications, or at the margins of water bodies.
Population increases mainly by vegetative propagation as the result of differentiation of apical or axillary meristems (Center et al., 2002). The single apical meristems on each stem tip can be vegetative, producing leaves with axillary buds, or producing flowers. If an inflorescence develops, termination of the apical meristems halts leaf production. In this event, the axillary bud immediately below the inflorescence differentiates into a continuation shoot. This produces a new apical meristem that allows leaf production to proceed. If the axillary bud doesn't form a continuation shoot, then it produces a stolon.
Elongation of the stolon internode moves the axillary bud apex away from the parent rosette. It then produces short internodes that grow vertically into a new rosette (Center et al., 2002). Under favourable conditions the population of water hyacinth doubles between 5-15 days. If completely undisturbed its biomass weighs 25kg per square meter or 400 tones per hectare (Makhanu, 1997).
2.2.3 Distribution and habitat
The place of origin of water hyacinth is believed to be Amazonia, Brazil, with natural spread throughout Brazil and to other Central and South American countries (Julien et al.,
1999; Penfound and Earle, 1948). Water hyacinth apparently has become a problem from late 1800s spreading through the tropics of the continents including USA and now reaches around the world and north and south as far as the 40° latitudes. The spread of water hyacinth into new areas said to be commenced in the 1880s with its deliberate introduction into the USA as an attractive pond ornamental (Penfound and Earle, 1948).
Thereafter plants continued to be spread around the USA and eventually around the world. More recently it spread into the many waterways of Africa and has expanded rapidly, probably in response to high nutrient conditions, to cause serious problems
(Jullien, 2000).
The first introduction of water hyacinth in Asia appears to have been into Japan independently in 1890s. Water hyacinth spreads in water environments such as bays and inlets with quiescent water, shallow depths (<6 m), bed surface covered with deposited
13 Chapter 2 Literature Review sediments rich in organic matter and availability of key mineral elements namely N and P in the nutrients (Makhanu, 1997). The plant grows best in neutral pH waters with warm temperatures (28° to 30° C), and high light intensities. It tolerates pH levels from 4.0 to
10.0, but not more than 20 to 25 % salinity (Center et al., 2002). The plants survive frost if the rhizomes do not freeze, even though emergent portions may succumb. Prolonged cold kills the plants, but the reinfestalion from the seed follows during later warmer periods. Growth is inhibited at water temperatures above 33° C. Plants stranded on moist sediments can survive several months (Penfound and Earle, 1948; Center et al., 2002).
2.2.4 Nature of Damage
Majority of local human lives and their activities are largely associated with the fresh water bodies. Human interferences with fresh water resources are bound through agricultural purposes, fishing and other daily needs of water. One of the major scourges accompanying water resource development is the explosive proliferation of water hyacinth vegetation (Pantilu, 1984). Water hyacinth grows rapidly forming expansive colonies of tall, interwoven floating plants (Fig. 8) and leads to economic, ecological and social damages (Penfound and Earle, 1948).
Figure 8: Examples of highly infested water bodies in Western province
(A- Papiliyana; B- Angulana)
14 Chapter 2 Literature Review
Economic damage
Water hyacinth blankets large water bodies, creating impenetrable barriers and obstructing navigation. Floating mats blocks drainage, causing flooding or preventing subsidence of floodwaters. Large rafts accumulate where impeding water flow, by clogging irrigation pumps, and by interfering with weirs (Penfound and Earle, 1948).
Multimillion- dollar flood control and water supply projects can be rendered useless by water hyacinth infestations (Center, 2002). Infestations further block access to recreational areas and decrease waterfront property values, often harming the communities that depend upon fishing and water sports for revenue. Shifting water hyacinth mats sometimes prevent boats from reaching shore, trapping the occupants and exposing them to environmental hazards (Center, 2002). Furthermore these water hyacinth drifting masses can clog pen stock intakes to hydroelectric turbines, displace significant quantities of live storage in reservoirs and considerably increase evapo- transpiration losses in man made lakes. This affects in reducing water levels particularly in dry zone water tanks. They also interfere with the operation of pumps in irrigation schemes and in irrigation systems by clogging filters and water intakes and thus increasing maintenance expenses (Pantilu, 1984).
Ecological damage
Dense mats reduce light to submerged plants, thus depleting oxygen in aquatic communities. This results in lesser phytoplankton biomass which them alters the composition of invertebrate communities, ultimately affecting fisheries. Drifting mats scour vegetation, destroying native plants and wildlife habitats. Water hyacinth also competes with other plants, often displacing wildlife forage and habitat. Higher sediment
loading occurs under water hyacinth mats due to increased detrital production and siltation. The plants absorption of heavy metals causes secondary water pollution problems after they die and sink. In rural areas, after control by harvesting, of plants are
always heaped together along the banks of rivers and allowed to decay, which them
greatly affects the environmental quality (Jianqing et al., 2000).
15 Chapter 2 Literature Review
Social damage
Water hyacinth has caused a series of problems to local society. People have difficulty in their daily lives as it covers their rivers, ponds and lakes. The health of local people is threatened as water hyacinth provides a habitat for mosquitoes and flies.
2.2.5 Utilization
It has been observed that water hyacinth can be used markedly to reduce the pollutant
levels of rubber, paper and oil palm industry effluent (John, 1984). Several literature discuss about the utilization capacity of water hyacinth in anaerobic compost formation, manufacture of paper, board, handicraft, furniture, ropes, preparation of active carbon, as
a raw material in bio gas production and in the management of water quality by treating
wastewater (Behera et al., 1984; Vedanayagam et al., 1984; Mantabuddin, 1984). Water
hyacinth comprises 95% water and only 5%> dry matter of which 50%> is silica, 30%
Poatsium, 15% Nitrogen and 5%> protein. From the unique chemical content of the water
hyacinth, its beneficial uses are limited. The water hyacinth cannot be used as a livestock
feed because it contains too much of silica, calcium oxalate, potassium and too little
protein. It cannot be directly used as a fertilizer because its C: N ratio is too high
necessitating addition of N fertilizer. Because its fiber length is too short, it is poor raw
material for paper, mats or ceiling boards. A few beneficial uses have been identified but
the large-scale production is uneconomical when compared with the negative effects
attributed to the water hyacinth field. Such beneficial uses include biogas production and
removal of heavy metals from industrial pollution when water passes and sieved by the
water hyacinth fabric (Makhanu, 1997).
2.3 Management of water hyacinth
The gravity of the problems discussed above highlights the need for adequate and timely
measures to manage the problem. The control has been achieved in three main ways,
which are manual or mechanical removal, chemical or herbicidal removal and biological
16 Chapter 2 Literature Review
removal. The complete removal of water hyacinth is impossible for most areas (Julien,
Grifths and Wright, 1999). Eradication efforts of an infestation usually last for a short term, where reinfestation can occur. Currents, boats fishing nets and possibly animals and birds, readily transport plants or seeds and only one or few plants can result in reinfestation. Therefore the aim of any control programme is to manage, rather than eradicate, the weed. In many situations management extends only to maintain open water around critical sites depending on the usages by human livelihood (Julien, Grifths and
Wright, 1999).
2.3.1 Mechanical control
Water hyacinth removal by hand or machine is a practical control method often used for small areas or when plant density is low. This is historically the most widely used form of control specially among the rural communities. In urban areas mechanical harvesters have been developed which speed the physical removal of water hyacinth. Floating booms and barriers are used to maintain areas free of weed and to reduce the downstream spread of an infestation. In Philippines, weed removal has been done chiefly with the help of hand tools and simple devices like draglines and floating booms (Gopal, 1987).
Generally, pre rainy season period is more suitable for manual removal because then the weed is restricted to smaller water areas. The rate of growth and invasion by water hyacinth usually exceeds the rate at which it can be cleared. Therefore the removal must be repeated and continuous. The material removed from the site must be transported away from the site and should be disposed of appropriately in order to prevent reinfestation. Therefore manual or mechanical removal cannot be considered as a cost effective and timely measure of management of the weed (Wright and Center, 1984).
2.3.2 Chemical control
Several herbicides are effective against E. crassipes. The herbicides most commonly used are diquat, glyphosate, amitriole, and the amine and acid formulations of 2,4-D, applied as foliar sprays. (Julien, Grifths and Wright, 1999). One key factor affecting the efficacy
17 Chapter 2 Literature Review
of herbicides is translocation from stolons to other parts of the plant, particularly the roots. Younger plants translocate these substances faster. However older plants and flowering plants may be more susceptible to stress from treatment. Warmer temperatures result in more rapid translocation of some herbicides. Mats consisting of older plants take longer time to sink after herbicide application than mats with younger plants (Sculthorpe,
1985). All chemical treatments face a difficulty of not sinking the sprayed and killed plants. This is due to the integrated and chopped nature of mats and it will take at least 6 weeks for the dead plant masses to sink. Large masses of the rotting weed will use the oxygen in the water and potentially lead to fish and wild life kills (Penfound and Earle,
1948). Although the chemical control has the advantages of being quick and effective, direct and indirect impacts of the herbicides on the environment are significant enough to invite caution in their use. Firstly, the rapid kill of a large thick mat of the weed adds a huge quantity of organic matter to the water body. Generally the detritus form after chemical control cannot be removed and its decay releases large amounts of nutrients.
This results in rapid degradation of water quality, development of algal blooms and other changes commonly associated with eutrophication. More often the organic matter creates anoxic conditions in shallow water bodies, resulting in large-scale death of fish and other aquatic organisms (Gopal, 1988).
2.3.3 Biological control
Biological control is a technique that is affordable, environmentally friendly and sustainable and continues to show promise, which can be developed further depending on the gravity of the infestation and the habitat specifications (Wright and Center, 1984).
18 Chapter 2 Literature Review
Definitions on "Biological Control" of water hyacinth:
Biological control:
"The use of natural enemies of water hyacinth to suppress the population of the weed"
Natural enemies:
"Organisms that attack on the water hyacinth plant in its native range and thus contribute to the maintenance of the population levels of the weed"
Classical biological control of water hyacinth:
"The use of the target-specific natural enemies of the weed to suppress population oj the weed in its exotic range "
Biological control agents:
"Natural enemies (usually insects, but also mites, fungi, nematodes, fish) that have been released to control the weed. They have normally undergone the range of plants that they are capable of damaging and are only released if they do not pose a threat to other plants and organisms "
(Julien, Grifths and Wright, 1999)
2.3.3.1 Exploration for natural enemies
Surveys for natural enemies of water hyacinth for use as biological control agents began in 1962 and have continued until recently. The range of surveys provided lists of fauna related to the weed. From these lists Arthropods and pathogens have been selected for further studies. The host ranges of those selected are observed in the field and studied in the laboratories. Those showing a narrow host range are then subjected to host-specificity tests to determine the safety of releasing them in the exotic range of the weed (Julien,
2000).Table shows some of the work carried out by Entomologists in different countries
82546
19 Chapter 2 Literature Review- <
Table 04: Some of the studies conducted on biological control of water hyacinth
Year Country Work description
1962-1965 Uruguwe A. Silveria found the moth Xubida (Acigona)
infusellus, two-weevil species Neochetina
eichhorniae and Neochetina bruchi, the mite
Orthogalumna terebrantis and the grasshopper
Cornops aquaticum
1962-1968 USDA Biology and host range studies were conducted on a
number of agents at the ARS laboratory at Buenos
41 Aires. This laboratory was setup to work on alligator
weed but then studies focused largely on water
hyacinth.
1968 Guyana, Surinam F. Bennett and H. Z. wolfer of CIBC, now CABI
and Brazil they added the petiole -tunneling moth Biosciences,
conducted surveys. To the list of species
Niphograpta (Samoedes) albigutalis, the petiole -
boring flies Thrypticus spp., and an undamaged
mirid bug.
1960 West Indies, Belize Bennett surveyed and found 0. terebrantis and the
and Florida- USA stem-boring moth Belura densa.
1981 West Indies, Belize Bennett surveyed Mexico, finding X. infusalis N.
and Florida USA eichhorniae, C. aquaticum and 0. terebrantis.
1989 St. Catarina State, Stephen Neser, PPRI, collected the mirid
Brazil Eccritortarsus catarinsis This may have been the
bug recorded by Bennett Zwolfer during their 1968
surveys in Guyana, Surinam and Brazil.
1999 South Africa M.Hill, PPRI, H. Cordo and T. Center, USDA-ARS,
and H. Evans and D. Djeddour, CABI Biosciences
conducted a survey, into the upper reaches of the
Amazon River in Peru.
20 Chapter 2 Literature Review
In its native range of water hyacinth, there are six biological control agents belongs to phylum Arthropoda have been released around the world; five are insects (two weevils, two moths and a sucking bug), and one is a mite. The most recent list was presented to the International Organization for Biological Control (IOBC) Water hyacinth workshop in 1968 and the controlling agents had been classified into three groups (Table X2);.
Table 5: Main three groups of biological control agents on water hyacinth
Group Species Type of agent Type of damage
Group 1 Neochetina eichhorniae Weevil Adults feed on foliage and
petioles, larvae tunnels in
petioles and crown
Neochetina bruchi -do- -do-
Niphograpta albigutallis Moth Larvae tunnel petioles and
buds
Orthogalumna terebrantis Mite Immatures tunnel in lamnae
Group 2 Eccritotarsus catarinensis Sucking bug Adults and nymphs suck
cellular or intercellular flid
from leaves
Xubida infusellus Moth Larvae tunnel petioles and
buds
Cornops aquaticum Grass hopper Under investigation
Bellura densa moth Stem boring
Parades (Palustria) tenuis Moth Under investigation
Thrypticus species Fly -do-
Group 3 Currently under studying Insects & Mite -do-
21 Chapter 2 Literature Review
2.3.3.2 Agents Currently being studied
Three native North American species, which sometimes severely affect water
hyacinth populations, are now under searching.
• The moth B. densa Walker has been rejected because it attacks taro, Colocasia
esculenta (L.) Schott.
• The moth Parades tenuis has been rejected as it developed on a range of
plants over several families.
• The grasshopper Cornopos aquaticum was under study in South Africa to
clarify its host range and considered it to be specific, however concerns for
pickerelweed precluded further considerations for release in the United States
(Julien, 2000;Center et al., 2002).
• Thrypticus species (Flies) are being studied in Argentina. Until recently this
group of flies was thought to have low priority because of suspected wide host
acceptance. Current studies have identified a number of species within the
group, one or more apparently specific to water hyacinth (Julien, 2000).
• The moth Xubida infusellus has been rejected for release in the United States
because it is clearly a threat to pickerelweed (Center et al., 2002).
• The mirid E. catarinensis is still under consideration for release as the risk to
pickerelweed seems minimal under field conditions, but information on its
efficacy is needed for a proper risk-benefit analysis (Center et al., 2002).
2.4 The water hyacinth weevils
One or more natural enemies have established in most of the countries in which they have
been released, and their impact on water hyacinth has been significant in some areas. The
two Neochetina species are the most widely distributed of the water hyacinth biological
control agents, and to date are the most successful according to literature. The Tables X3
and X4 discuss the status of impacts made on controlling water hyacinth throughout the
world (Julien, Grifths and Wright, 1999).
22 Chapter 2 Literature Review
Table 6: Neochetina eichhorniae status of releases for each country
Country Year Established Control
Released No Unknown Studying Yes No Unknown Studying Yes
Philippines 1992 •
Taiwan 1992 •
Vietnam 1984
Congo 1999
Egypt 2000 •J
Rwanda 2000 •
Fiji 1977 • •
Indonesia 1979 •
Mexico 1970 • •
Sri Lanka 1988 •
Australia 1975 • •
Benin 1991 • •
India 1983 • •
Kenya 1993 •
Nigeria 1993 •
PNG 1986
S. Africa 1974 • •
Sudan 1978 •
Tanzania 1995 •
Thailand 1979 •
Uganda 1993
USA 1972 •
Zimbabwe 1971 •
China 1996
Malawi 1995 • •
Ghana 1994 •J
Honduras 1990
Malaysia 1983
Mosambique 1972
Myanmar 1980 • •
Solomon Is: 1988 •
Zambia 1996
23 Chapter 2 Literature Review
Table 7: Neochetina bruchi status of releases for each country
Country Year Established Control
Released No Unknown Studying Yes No Unknown Studying
Panama 1977
Philippies 1992 •
Taiwan 1993 •
Zambia 1997
Congo 1999 •
Egypt 2000 •
Rwanda 2000
Malaysia 1992 •
Benin 1992 • •J
Australia 1990 •
India 1984 •
Kenya 1995 •
PNG 1993
Sudan 1979
Tanzania 1995
Thailand 1991 • •
Uganda 1993 •
USA 1974 %/
Zimbabwe 1996
China 1996 •
Malawi 1995 •
Mexico 1995 •
S Africa 1989 •
Cuba 1995 •
Ghana 1994 • •
Honduras 1989 •
Mozambique 1972 •
Nigeria 1995 •
Indonesia 1996
Vietnam 1996
24 Chapter 2 Literature Review
The genus Neochetina is known to have six species, all of South American origin and all restricted in their feeding to the family Ponterderaceae to which the water hyacinth belongs (Julien, Grifths and Center, 1999). The two Neochetina species in use as biological control agents are Neochetina bruchi, the chevroned water hyacinth weevil, and Neochetina eichhorniae, the mottled water hyacinth weevil which has been described in detail in Chapter l.The development durations of the different life stages and the average fecundities are detailed in Table 2 referred to Julien, Grifths and Wright (1999).
Table 8: Development durations for each life cycle stage and fecundities for both Neochetina species (Julien, Grifths, Wright, 1999)
Development stage Approximate ( uration (days) Neochetina bruchi Neochetina eichhorniae
Egg 7 10 Larva I Instar 10 II Instar 14 III Instar 6 Total 32 75-90 Pre-pupa 7 Pupa 23 Pre-pupa + Pupa 30 14-20
Generation time 96 120 Adult longevity 89 142 Fecundity Total 682 891 Daily maximum 8.5 26
2.5 Host range testing and results for Neochetina species
The host range and host specificity for feeding and breeding are to be investigated with great care. Plants of similar habitat requirements, economically or otherwise important plants are to be included in the tests in field and in the laboratory. The organisms totally dependent on the concerned weed for their feeding or completion of the life cycle are considered suitable bio control agents—Polyphagous organisms and those infesting
25 Chapter 2 Literature Review
economically important plants are not worthy for further consideration, Feeding habits are determined by multiple choice tests (offering several food plants at the same time)
and starvation tests (starving the organism before providing a selected food plant). The
extent of damage, time taken in causing the damage, and the reproductive rate of the
biocontrol agent are important considerations. It is obvious that the damage is at a rate
greater than the growth rate of water hyacinth. Water hyacinth in particular has a highest
growth rate among the aquatic weeds (Gopal, 1984).
The two weevil species (Neochetina eichhorniae and Neochetina bruchi) have been
released on water hyacinth in 30 and 27 countries, respectively. Both have subjected to
extensive screening. They have been tested against 274 plant species in 77 families
worldwide (Center et al., 2002).
2.6 History of introduction
Based on the results from the various host range trials, these Neochetina species have
been widely released throughout the distribution of water hyacinth. Tables 6 and 7 clearly
show the extent of weevil introduction and status of their success and establishment in the
field.
2.6.1 Case studies reported
There are several case studies found among literature related to biological control of
water hyacinth using two Neochetina species. Most of the literature discusses about the
successful suppression of the target weed while some deal with the establishment and
spread of the weevils under different ecological and environmental conditions. The
following are the highlights of key research and introduction campaigns of two
Neochetina species used in the bio-control of water hyacinth.
26 Chapter 2 Literature Review
• Neochetina eichhorniae had been released in Southern Florida in 1972, using eggs
from 2,479 adults sent from Argentina during August 1972 to March 1973. Adults
removed from founder colonies were then redistributed by numerous agencies. As
a result, Neochetina eichhorniae was released at 1999 sites in Florida, 492 sites in
Lousiana, one site in Texas, and four sites in California (Center et al., 2002)
• Neochetina bruchi became available, but there was no similar dissemination
campaign as to Neochetina eichhorniae. As a result it was released at only 40
sites: 21 in Florida, 10 in Lousiana, 5 in Texas and 4 in California (Center et al.,
2002).
• Among the numerous field studies conducted in decline of water hyacinth diverse
areas of the United States, the reduction campaign conducted to reduce water
hyacinth up to one-third of its former acreage in the Gulf Coast states is
important. This reduction resulted from both direct plant mortality and reduced
regrowth after introduction of Neochetina eichhorniae in 1974 and N. bruchi in
1975 (Center et al., 2002).
• Uganda made the first introductions of Neochetina eichhorniae and Neochetina
bruchi in 1995,followed by Kenya and Tanzania in 1997. A significant reduction
in the extent of the weed on the Ugandan shore had been evident by November
1998, with many of the mats having sunk. These results had been later repeated
on the Kenyan and Tanzanian shores. An estimated 75% of the mats on the
Kenyan side had sunk by December 1999 (Center et al., 2002).
• Successful results had been obtained in Sialoa, Mexico where the release of
Neochetina eichhorniae and N. bruchi during 1995 to 1996 reduced 3,041 ha of
water hyacinth distributed over seven impoundments by 62% (to 1,180 ha) by
1998 (Center et al., 2002).
27 Chapter 2 Literature Review
• Water hyacinth had become a bio disaster in Wenhzou located in the southeastern
province of China. Chemical and manual control failed to control the weed. In
cooperation with Biological Control Institute, Chinese Academy of Agricultural
sciences had introduced Neochetina eichhorniae and released at 4 sites in 1996.
The area of each site is said to be about 1000 m2 and 1000 adult weevils had been
released. From 1996 to 1999, the area and height of plants corded to be reduced
greatly (Lu Xujan et al., 2000).
• Since 1996, KARI had been introducing Neochetina weevil from Australia, South
Africa and Uganda as a part of a biological control program for water hyacinth.
Community based lake side rearing facilities have produced over 142,000 mostly
adult weevils, which released in to the lake at 30 sites in 8 districts bordering
Lake Victoria (Mailu, 2000).
• First biological control agent the weevil, Neochetina eichhorniae, ad introduced
into Nseleni/Mposa rivers system in South Africa during 1985-86. By the end of
1986 beetle activity as estimated by adult feeding scars and had shown successful
establishment (Jones, 2000).
• Integrated weed management strategies have shown a significant impact on water
hyacinth control in Tanzania. Water hyacinth control had been achieved mainly
through biological control by two weevils, Neochetina eichhorniae and
Neochetina bruchi. Weevils had established with adult population up to 30 per
plant. There had been a significant reduction in water hyacinth plant population
density from 45 to 7 plants per 0.5 m2 and large reductions in surface area covered
and the biomass (Mallya et al., 2000).
• Neochetina weevils had been released in Lake Kyoga, Uganda in 1993 and in
Lake Victoria in 1996. Visual observations and regular monitoring recorded the
establishment and spread of the weevil. Within 2 years weevils were established
28 Chapter 2 Literature Review
at 55% of release sites and were being recovered 50km from release sites. The
critical threshold of 5 weevils per plant had attained within 2-3 years of the initial
release (Ochiel et al, 2000).
• Successful biocontrol was achieved in a relatively short time frame of 4years on
lake Victoria in Uganda and in Papua New Guinea using only two insect agents,
Neochetina eichhorniae and N. bruchi (Hill and Olckers, 2000).
• Study carried out to investigate the critical weevil densities; the number of
weevils had varied (0 to 4000, in increment of 1000 weevils) on to 10m mats in
North and South Florida of United States, during 1992 and 1993, respectively.
Results were more striking at the Northern site. Weevil infestation at both sites
produced smaller, less intertwined and impeded the growth of the plants (Center
et al., 1999).
29 Chapter 3 Methodology
Chapter 3 Methodology
The entire study was carried out in three phases.
3.1 Field study - Phase I
Field monitoring studies were carried out in eight study sites namely Angulana, Boralasgamuwa, Bolgoda, Rajagiriya, Papiliyana, Wellampitiya-Sedawatta, Kelaniya. and Peliyagoda within the Western Province of Sri Lanka (Appendix 1). The sites were selected with different hydrological features including some of the sites where Room and Fernando (1992) had made their first introduction of Neochetina eichhorniae. This study was conducted in the dry season from January to February 2003. During the field study host plant measurements of shoot length, weevil density of each plant and the habitat characteristics such as pH, water depth and Total N (TN) levels of water were recorded. Total N was measured using Pursulphate Digession method (APHA, 1995 and Larry and
Andy, 1996). Field data were collected using three random quadrat samples of lm2 areas in each habitat (Fig. 9).
Figure 9: Quadrat sampling
30 Chapter 3 Methodology
3.2 Impact of Neochetina eichhorniae on host plant- Phase II
The study was conducted at the University of Moratuwa, Sri Lanka using 6 fiberglass tanks each having dimensions of 0.93 m depth and 1.05 m diameter. Water was artificially fertilized with 1/3- fold concentration of a nutrient medium prepared from the
recipe given as 1- fold concentration (i.e. 3.5 mg/1 NH4-N; 24.5 mg/1 NO3-N and 7.7 mg/1
P) by Sato and Kondo (1981) (Table 03).
Table 09: Nutrition composition of the water
hyacinth culture medium
Compound 1 fold 1/3 fold
(mg/1) (g/500 1)
NH4H2PO4 28.8 4.8
KNO3 75.8 12.6
MgS04.7H20 184.9 30.8
CaCl2.7H20 73.5 12.25
NaN03 85.0 14.16
EDTA-Fe 18.0 3.00
H3BO3 1.2 0.2
(Source: Sato and Kondo, 1981)
However our field observations showed that the medium in which the water hyacinth grows constitutes on average 2-fold concentration (i.e. 56 mg/1 TN and 15.4 mg/1 TP) of nutrients. Nevertheless higher nutrient concentrations were not tried since such high concentrations favoured the growth of algae, which was difficult to be curtailed. However with the lower concentrations, algal growth was completely eliminated and hence the impacts of algae on water hyacinth growth was minimized. In view of this fact it was decided to set the water with a nutrient solution concentration of 1/3-fold. This concentration was selected since the field observations revealed that plants inhabiting more eutrophic water bodies were less palatable to Neochetina eichhorniae.
31 Chapter 3 Methodology
The tanks were stocked with healthy water hyacinth plants with no insects or pathogens, obtained from the Bolgoda Lake located near the premises of the University of
Moratuwa, Sri Lanka (Fig 10). For the study 25 rosettes, each having an average of 6 leaves with average plant height ranging from 20.33-21.2 cm was used.
Figure 10: Initial water hyacinth colonies
All the plants were allowed to acclimatize in the introduced nutrient medium for a period of 3 weeks prior to the start up study. The tanks were covered with a green plastic, insect proof mesh to prevent other insects from infesting the hyacinths as well as to prevent the introduced weevils from escaping the tanks (Fig. 11).
Figure 11: A close view of the tanks containing water hyacinth infested with Neochetina eichhorniae
Adult weevils of Neochetina eichhorniae for this study were collected from their natural habitats beside the Ratmalana-Papiliyana Road after making several field visits commencing from April 30th 2002. These weevils were stored in transparent plastic containers filled with a 1/3-fold concentrated nutrient medium with 1-2 fresh water hyacinth plants. Observing the shiny black mark restricted to the tip of the snout separated the males. The females were identified from the shiny black mark running to some extent along the snout as described by De Loaches (1975).
32 Chapter 3 Methodology
In the start of the study, weevils were introduced into the tanks on 28 June 2002 at the
following densities: I per plant, 3 per plant, 6 per plant. 10 per plant and 15 per plant
with 1:1 sex ratio (Fig 12). This weevil density having a range of 1-15 was selected by
taking into account that the maximum density of weevils observed in the field was 6,
while the minimum was 1 and also to evaluate the impact of higher densities on water
hyacinth proliferation. The control was maintained by giving similar conditions as for
other tanks but without any weevil. r
Figure 12: Weevil introduction
Evaluation of the 6 tanks began on July 05 , 2002 and continued up to 8 weeks so as to
ensure the herbivory attack was only from the lsl generation (Heard and Winteron, 2000).
All measurements (i.e. total number of plants in each tank; average plant height; average
number of leaves; damaged area % of leaves; average third leaf lengths; TN of plant
leaves and shoots) were taken each week while making observations on the nature of
feeding such as larval tunneling and other habitat descriptions. Five plants were randomly
^ selected from each tank and returned to the tanks after taking the measurements. When
estimating plant density plants in each tank were counted in situ as single rosettes. Plant
height was measured by holding measuring stick vertically, from the water level to the tip
of the leaf along 3 longest petioles and the average was taken as the height of the selected
plant. We also measured the length (from petiole base to apex of lamina) of the 3rd leaf
(from apex) from each of the 5 plants selected for measurements. The total number of
leaves per plant was also counted weekly. The damaged area as a percentage was
calculated from the total damaged area with scars / total leaf area of selected five plants.
The damaged leaf area and the total leaf areas were measured with planimeter. Total N
33 Chapter 3 Methodology was measured using Pursulphate Digession method (APHA. 1995 and Larry and Andy,
1996).
3.3 Impact of Neochetina bruchi density on host plant - Phase III
For phase III of the study 150 adult weevils of Neochetina bruchi were imported from the
Republique De Cotte D'I voire on September 10th 2002 and mass rearing was carried out at the Open University premises in Nawala. Sri Lanka, conforming to quarantine regulations (Fig. 13).
Figure 13: Mass rearing of N.hruchi at Open University premises
This study was then conducted at the premises of the University of Moratuwa. Sri Lanka by using four out-door fiber- glass tanks having dimensions of 0.93 m depth and 1.05 m diameter. Water was artificially fertilized with the nutrient medium of 1/3-fold given Table 03. Initially the tanks were stocked with the nutrient medium (medium was changed in every 14 days) and stocked with healthy, uninfected (no insects or pathogens) water hyacinth plants collected from Bolgoda Lake. 10 plants with an average number of 6 leaves (± 0.12) and an average height ranging from 19.5 - 19.8 cm (± 0.21) were cultured in 4 tanks under similar conditions as in phase II. All plants were allowed to stand in the tanks for 3 weeks (i.e. prior to the introduction of weevils) in order to acclimatize the plants to the nutrient medium and each tank was enclosed in a cage covered with green insect proof plastic mesh.
34 Chapter 3 Methodology
In view of the fact that Neochetina bruchi has a long generation time and due to time constraints, the phase III of the study was conducted using only three weevil densities of
1, 3, and 6, per plant with 1:1 sex ratio. Control was maintained under similar conditions but without any weevil. The measurements were taken as for Phase II.
3.4 Statistical analysis
A statistical analysis was done with a computer software excel package. Significant differences in results pertaining to variations were estimated using two-sample t-test in
Minitab Version 12 (Johnson and Bhattacharya, 1996). Significant differences were set at
PO.05.
35 Chapter 4 Results and Discussion
Chapter 4 Results and Discussion
4.1 Results of field study - Phase I
The eight field study sites showed a considerable variation in plant population density
£ and plant morphology (Table 10). Most of these water hyacinth infested sites except
Bolgoda Lake and Boralasgamuwa Lake area, are highly urbanized and associated with
low in come houses, metal workshops, garages, solid waste dumping sites etc. Dumping
of domestic, industrial wastes and release of effluents were in general observed. Each
study site had high infestation of water hyacinth plants except at Bolgoda Lake,
Boralasgamuwa Lake and Rajagiriya (Table 10) due to the continuous flow of water
(Appendix 1). Leaves of most of the water hyacinth plants showed the presence of scars
induced by Neochetina eichhorniae, but larval tunneling inside petioles was rarely
observed except in young plants.
Table 10: Average measurements of water depth, water quality parameters and
plant characteristics in each study site.
Study site Habitat Plant characteristics Water quality characteristic parameters Water depth Plant density Shoot length PH TN (cm) (m2) (cm) (mg/1)
Angulana 77.6±0.3 ** 71.6±0.3 ** 29.2±0.1 ** 6.33 0.26
Bolgoda 62.0±1.03 47.3±l.8 44.7±l.5 6.10 0.06
Boralasgamuwa 64.3±1.2 48.0±3 40.4±2.8 6.07 0.07
Kelaniya 62.3±l.4 93.0±0.5 ** 37.5±0.5 6.44 0.33
Papiliyana 62.0±4.5 52.0±0.5 49.8±4.5 6.15 0.39
Peliyagoda 47.6±9.3 84.6±5.4 ** 37.6±0.8 6.44 0.32
Rajagiriya 72.2±1.3 ** 44.3±1.6 28.3±1.3 ** 6.15 0.19
Wellampitiya I3.1±8 87.0±15 37.3±8 6.43 0.43 ** P< 0.05 from 2-sample t-test compared with rest of the sites
36 4 Chapter 4 Results and Discussion
Average values of habitat characteristics, water quality parameters and plant
characteristics recorded during the field visits are summarized in Table 10 The sites of
Angulana, Kelaniya and Peliyagoda with relatively high total nitrogen (TN) values
showed a significant difference in average plant densities at 95% confidence limits
compared with the rest of the sites.
Figure 14 depicts the distribution of average weevil, Neochetina eichhorniae per plant in
each study site. Average number of weevils per plant or the average weevil density
showed variations in each of the sites. It was evident that the highest weevil density
recorded was 2.04 ± 0.36 weevils / plant. These results indicated that if a successful
population of weevils arises and exists for a sufficient period of time, undoubtedly an effective level of control could be achieved as also demonstrated in other studies (Center et al., 1999; Hill et. al., 1990: Hill & Olckers, 2001,Moorhouse, Agaba & McNabb.
2001)
Water hyacinth Infested Sites
• Angulana S Horalasgamuwa gkclanisa BKajaginva gPapilivana B Wellampitiya B Peliyagoda B Bolgoda Figure 14: Weevil densities distributed within the field
The results of lower weevil densities of around 0.5 per plant in average obtained from the
field survey revealed that the factors involved in low population growth of Neochetina
eichhorniae could have been due to direct and indirect effects appeared from the habitat
itself. Among the factors identified in constraining the survival and growth of weevils,
variability of hydrological features and the instability of the hyacinth mats seemed to be
important. When the mat is stable for long periods, insect numbers could build up to a
:>7 •
Chapter 4 Results and Discussion
threshold density. The sites visited are subject to removal of hyacinth mats by periodic
and annual floods hence the life cycle of weevils having a long generation time of % to
120 days (Jullien, 2000) tend to get disrupted thereby delaying the start of the next
generation. In some places such as Bolgoda Lake and Boralasgamuwa Lake, the action of
wind and waves assisted the rate of damage and sinking of the mats. Also saline water
intrusion at habitats such as Bolgoda Lake has been known to cause complete eradication
of hyacinths thereby resulting in the death of weevils due to lack of food.
The average shoot length of the host plant at Angulana and Rajagiriya where the weevil
densities were higher showed a significant difference (P < 0.05) compared with the rest
of the sites (Figs. 15 and 16). Size of the plant as measured by shoot length is said to be
the most important parameter for the establishment of a population of Neochetina
eichhorniae (Kannan and Kathiresan, 1997). Plant sizes could be categorized into 2
different ranges from the field measurements: (i) moderately sized plants with 20 - 30
cm shoot lengths and (ii) larger plants with a shoot length of greater than 35 cm. The
plants falling into moderately sized plants were abundant with higher weevil densities
whereas the sites with matured plants were found to have lower weevil densities
suggesting that matured plants are less palatable to Neochetina eichhorniae.
Figure 15: Relationship between shoot length and the weevil density in the field
38 Chapter 4 Results and Discussion
It has been documented that biochemical constituents such as cellulose, Ca and Mg increases and
thereby making them less palatable to weevils (Kannan and Kathiresan, 1997). Further, the
special ability of water hyacinth to remove heavy metals through phytoextraction in water
hyacinth is known to increase with plant age (Kannan and Kathiresan, 1997). Since most of
the study sites except Bolgoda and Boralasgamuwa, are associated with highly polluted
areas with dumping sites and industrial effluent pumps, there was a possibility of
matured plants showing toxicity to weevils. It has been found that the uptake of heavy
metals by the water hyacinth reduces the fecundity of the weevils (Julien, 2000).
The next most obligatory factor is the highly eutrophic waters in which the weed thrives.
The significantly lower weevil densities (P < 0.05) observed at Kelaniya, Peliyagoda and
Wellampitiya (i.e. sites having higher TN levels ranging from 0.3-0.5 mg/1) could be
explained as follows (Fig. 14; Table 3); Plants growing nutrient rich or highly eutrophic
habitats [i.e. TN levels exceeding 0.3 mg/l(Salisbury and Ross, 1990)] generally contains
higher N levels in plant tissues. These higher N levels known to reduce the palatability of
leaves to weevils Higher N levels in plants growing eutrophic waters reduce the
palatability of leaves to weevils (Heard and Winteron, 2000).
4.2 Impact of Neochetina eichhorniae density on host plant - Phase II % 4.2.1 Nature of the Damage
Figure 16 shows the relative percentage of damaged area of leaves caused by the weevil
at different densities. The results showed that tanks having more than 03 weevils per
plant yielded significant damage (P < 0.01) compared with other combinations. The one
with 15 per plant showed a damage of almost 100% by the end of 2nd week while those of
10 per plant and 06 per plant had percentage damaged leaf areas of 62% and 58%,
respectively.
39 ft. ft
Chapter 4 Results and Discussion
A large number of case studies have been reported describing the effects of mottled water
hyacinth weevil in controlling water hyacinth. Among them, the first reported collapse of
water hyacinth was in 1978 following severe damage by the weevil at an infestation at
Rock Hampton (Wright, 1984).
Figure 16: Percentage of damaged leaf area for different weevil densities
In this study, a high correlation was obtained between the estimates of the number of feeding scars per lamina caused by adults and the percentage of water hyacinth collapsed. It has been documented that the death and subsequent collapse of plants could occur when populations of Neochetina eichhorniae reached a level where adults produced 750 feeding scars per lamina, which is equivalent to an area of feeding of a medium sized lamina of 60 cm, representing a 57% of damage on the leaf upper surface upper surface area per plant (Wright, 1984). These results supported this study since the damage area
amounted to 57.7% at the end of 8th week in the tank having 6 weevils per plant. Weevil densities of equal or less than 3 per plant did not show any significant change (P > 0.05) with respect to total percentage damage of lamina area. In such tanks the leaf area damage was below 15% throughout the study. In contrast to combinations having weevils less than 3 per plant, other tanks with weevil densities 6 and above showed substantially showed different morphological changes as a consequence of herbivory attack.
40 Chapter 4 Results and Discussion
It is a well-known fact that the substantial damage to leaves suppresses metabolic activities, which are essential for the sustenance of the plant; hence lower production rate of photosynthesis. Consequently this phenomenon helps to reduce the plant height, leaf area and leaf length etc. Even though the damage due to adult feeding on lamina area, adults feeding on upper petiole region with subsequent leaf detachment may have caused to significant impacts on plant growth.
Figure 17: Total number of plants for different weevil densities
Figure 17 shows the variation of the number of plants over time. Number of plants perfectly correlated with the percentage damage on the leaf area (Figs 16 and 17). It was observed that when the number of weevils was increased feeding scars were substantially high, and consequently lower number of plants resulted in. Extremely high competition among weevils may have driven the adults to feed on the stem and the crown. It was further observed that the larval tunneling and feeding on petioles and crown particularly in high weevil densities of more than 10 per plant caused the exposure of petioles and rhizome cores in which secondary infestations by fungi, ants and aphids began and continued. Plant numbers decreased with the higher densities with the gradual sinking particularly after the 4th week (Fig 18). In tanks having higher weevil densities lower number of plants predominated mainly due to the little or no stolon production. Tanks having weevil densities of 10 and 15 per plant had no new plant production but a
41 i
Chapter 4 Results and Discussion
reduction inflicted by the heavy damage on both leaves and petioles. In the case of the
tank having 15 weevils per plant, a complete eradication of hyacinths was noticed after 4
weeks (Fig 18).
4
Figure 18: Damage done by Neochetina eichhorniae after 4 weeks
42 ( hapter 4 Results and Discussion
In contrast to the tanks having weevil densities of 10 and 15 the number of plants in the
control tank drastically increased up to 55 (5th week) from 25 then remained almost at 55
until the 7th week possibly due to limited space and then the number of plants slightly
reduced suggesting that plant senescence due to aging was occurring than reproduction.
Similar phenomenon of plant reproduction and later senescing was observed in tanks
having I and 3 weevils per plant. In the case of the tank having 6 weevils /plant, a slow
reproduction of plants was evident up to 4 weeks. Thereafter a slight reduction in the
plant numbers was noticed and the numbers were maintained at around 30-32 probably
due to significant damage done by Neochetina eichhorniae (Fig 17). Figures 18 and 19
depict the damage inflicted by different weevil densities on the plant in each of the
treatments. At the end of the 4th week the weevil was capable of destroying all the plants
in the tank with 15 weevils per plant from the system, which does not agree with
management strategies of the weed. The same trend was also observed after 8 weeks in
the tank having a weevil density of 10 per plant. The figures 18 and 19 further show the
stunted appearance of the plants with the increasing pressure of the weevil damage.
0/Plant 1/Plant 3 / Plant
6/Plant 10 /Plant 15/Plant
Figure 19: Damage done by Neochetina eichhorniae after 8 weeks
43 14
Chapter 4 Results and Discussion
4.2.2 Impact of Neochetina eichhorniae on Plant Growth
Figure 20 shows the variation of average plant heights for different weevil densities.
Growth inhibition was mainly reflected by the variation of average plant heights. Weevil
densities less than 6 per plant showed a rapid increase in plant height particularly since
the 4th week. The physiological basis behind increasing plant height in the case of control
and tanks having a weevil densities of 1 and 3 was attributed to the growth or activation
of axillary buds cutting off from the rhizome and the cell division taking place within the
emerged leaf petioles (Salisbury and Ross, 1990). Once the larvae tunnel in to the base of
the petiole bud initiation gets retarded, hence the plants become short and stunted. All
other densities exceeding 3 weevils per plant showed a significant difference (P < 0.05)
in the average plant height in comparison with the control with no weevils.
• Control • 1 Weevil /plant • 3 Weevils / plant • 6 We evils plain • 10 Weevils / plant • 15 Weevils / plant
Figure 20: Variation of average plant heights with different weevil densities
In contrast the control attained the highest height of 50.9 cm ± 0.55 while a density of 10
weevils per plant achieved negative growth with a maximum height of 13.7 cm ± 0.4 at
the end of 8th week. The one with 6 weevils per plant had an average plant height in the
range of 21 - 24 cm throughout the study period.
The reduction in plant density mainly occurred due to the suppression of stolon
production by larval tunneling of the rhizome. Moreover overall deterioration of plant
44 Chapter 4 Results and Discussion growth by restricting photosynthesis and other metabolic activities may have contributed to minimizing daughter plant production.
4.2.3 Impact of Neochetina eichhorniae on leaf dynamics
Q 1 2 3 4 5 6 7 8! | Time (Weeks)
—O— Control —Q— l\*feevil/Plant —0—3 Weevils/plant —O— 6 Weevil/Plant —Q- B Weevils/Plant —O—15 Weevils/Plant
Figure 21: Variation of total number of leaves per plant in different weevil densities
Figure 21 shows the variation of the number of leaves per plant with the time for different weevil densities. During the 1st week of the study all six water hyacinth cultured tanks showed an increase in the average number of leaves per plant. From 1st week to 4th week treatment tank with more than 10 weevils / plant showed a substantial reduction and after the 4th week no more leaves were found in the tank with 15 weevils / plant since a complete eradication of water hyacinth occurred in this tank (Figs 18 and 19). It was observed that in the control and in the plants exposed to a weevil density of 1,3 and 6 per plant maintained more or less the same number of leaves per plant throughout the study.
45 Chapter 4 Results and Discussion
•DOO
41k Time (Weeks) —0—0 weevils/plant —Q—1weevi/plant —6—3 weevils/plan! —&—6 weevils/plant —Q—t) weevils/plant —O—15 weevils/plant
Figure 22: Variation of total leaf area in different weevil densities
Although the leaf area is mainly determined by the genetic composition of the plant,
external environment factors such as herbivory attack may change the plant physiology,
which is reflected, by a change in leaf dynamics. The average total leaf area with a weevil
density of 6 per plant remained at around 211 ± 1 cm2. In contrast the average total leaf
area in the control and tanks having weevil densities of 10 and 15 remained at around 103
± I cm and 26 ± 0.7 cm respectively (Fig 22).
Most of the adult weevils in the experimental tanks, except in the control were found
within the spaces where the 3rd leaf is rolled around the stem bases and either made scars
on the leaf or entirely fed on the leaf tissue as it made round shaped holes of about 1-2
mm in diameter. The figure 23 shows the variations of 3rd leaf length with time for
different weevil densities. This followed a similar trend as in the variation of leaf area, in
each tank of weevils except in the tank with 3 weevils per plant. In this particular
treatment tank the total leaf area remained the same after the 4th week, but the 3rd leaf
length increased (Figs 22 and 23) rapidly. This can be explained as an adaptation to over
crowding where the leaf width reduced and elongated in search of more sunlight for
photosynthesis.
46 Chapter 4 Results and Discussion
0 12345678 Time (Weeks) —0—Control —Cl—1 Weevil/plant —£i—3 Weevils /plant —| )—6 Weevils / plant —Q—K) Weevils / plant —O—15 Weevils / plant
Figure 23: Variation of 3rd leaf lengths in different weevil densities
4.3 Impact of Neochetina bruchi density on host plant - Phase III
4.3.1 Nature of the damage and impact on plant reproduction
The nature of the damage inflicted by Neochetina bruchi during the study is clearly demonstrated in figures 24 and 25. The stunted appearance of the plants more prominent in water hyacinth exposed to higher densities of Neochetina bruchi where as the control tank with no weevils continued its usual growth pattern.
Neochetina bruchi profoundly influenced plant colony development by suppressing mat expansion in all tanks except in the control (Fig. 26). The total number of plants in all four tanks increased during the first two weeks of this study. Total number of plants at each weevil density level a showed a correlation with the stress imposed by the weevils on the plant population and this will ultimately decide the level of control. Plant abundance declined considerably after 2nd week in the highest density treatment, whereas high numbers persisted in the untreated tank during the remainder of the study.
47 Chapter 4 Results and Discussion
Initial
m
V 0 / Plant 1 / Plant
3 / Plant 6 /Plant
Figure 24: Damage done by Neochetina bruchi after 4 weeks of introduction
48 Chapter 4 Results and Discussion
m
A 0/Plant 1 / Plant
3/ Plant 6 / Plant
Figure 25: Damage done by Neochetina bruchi after 8 weeks of introduction
...milt
In.lul Week"! Week.0 *tl(H Wet 104 WcekiK WtekO* Wtek07 VWeklM • 0 Weevfe/ptant TlBt (WrcktlH il plant • 3 Weevfc/plant •6mevk/pbni Figure 26: Variation of number of plants in different weevil densities
49 Chapter 4 Results and Discussion
4.3.2 Impact of Neochetina bruchi on pliant growth
The changes in plant height at different weevil densities mirrored the growth rate and the
water hyacinth colonies stocked with higher weevil densities of 3 and 6 weevils per plant
limited its height increment at 20.4 cm ± 0.3 and 14.9 cm ± 0.7, respectively by the 5th
week and showed about 90% height reduction towards the end of eighth week (figure
27).
Initial Week Week Week Week Week Week Week Week 01 02 03 04 05 06 07 08 'lime (weeks)
—O— Control —•—01 Weevils/Pbnt —6—03 Weevils/Plant —Q-06 Weevils/Plant Figure 27: Variation of average plant heights in different weevil densities
The average plant height closely correlated the measurements of total leaf area, which is
V the parameter we consider on evaluating the damage percentage mad by the weevils.
Reduction of the plant height mainly occurs due to the deactivation of axillary buds
cutting of from the rhizome of the plant by larval tunneling the rhizome via leaf petiole.
4.3.3 Impact of Neochetina bruchi on plant leaf dynamics
Water hyacinth rosettes normally retain only a few leaves so turnover is obviously rapid. This is reflected in the analysis, as there was no significance (P< 0.05) in cumulative leaf numbers compared to the control tank. The leaf mortality is strongly influenced by the size of the weevil population over time (Fig. 28). The average number of leaves was
50 Chapter 4 Results and Discussion primarily affected by the length of time that plant expose to the stress of the weevils.
Plants exposed to higher number of weevils died more quickly and thereby produced lesser leaves.
hhial Week Week Week Week Week Week Week Week 01 02 03 04 05 06 07 08 Time (Weeks )
-Control -01 Weevil/Plant -03 Weevils/Plant -fJ-06 Weevils/Plant Figure 28 : Variation of number of leaves per plant in
different weevil densities
Plant density also has an effect on reducing the number of leaves. Lesser number of leaves in the control tank may be due to its crowdness. Greater damage on plants is reflected by the reduction of lamina area of the plant. This is an obvious mechanism adapted by plant, the lamina being the of prime target part of the plant in weevil feeding.
900 800 700 600 500 400 •5 2 5 5 5 £ 300 200 100 Initial Week 01 Wcck02 ttfcck03Wcck0 4 WcckOS Wcck06 Wcck07 VUjckOB Timt (Wrrfc.) -Control -01 Weevils /Plant -03 Weevils/Plant -06 weevils/P lant Figure 29: Variation of total leaf area in different weevil densities
51 Chapter 4 Results and Discussion
Figure 29 describe the variation of total leaf are in each of the treatment with respect to
Neochetina bruchi.
4.4 Comparison of the effectiveness of two weevils
Initial Week Week Week Week Week Week Week Week 01 02 03 04 05 06 07 08 Weeks •I0/P lani (bruchi) JOl'Plant (bruchi) Wtrn 03/P lam (bruchi) ]06/Plant (bruchi) -*—0/P bint (eichhorniae) 01/Plant (eichhorniae) -*—03/Pbnt (eichhorniae) 06/Plani (eichhorniae) Figure 30: Comparison of % damage made by two Neochetina species
Figure 30 compares the percentage damage made on lamina area by both Neochetina species in the three weevil densities of 1, 3 and 6 weevils per plant. This graph clearly describes that at the end of eighth week Neochetna. bruchi has done a damage of 99.47% while the weevil Neochetina eichhorniae has done a damage of 57.7% with 6 weevils per plant.. Therefore undoubtedly the weevil Neochetina bruchi could be more effective in controlling water hyacinth on water hyacinth infestations with a lesser number of weevils within a shorter the period of time than Neochetina eichhorniae does.
52 Chapter 4 Results and Discussion
4.5 Effect of weevil herbivory on plant N content
Some literature has shown that plant density and weevil numbers interactively altered
plant tissue N levels (Center and Van, 1989). Accordingly, in our results with respect to
Neochetina bruchi gives clear evidences to prove that water hyacinth plant tissue N levels
( both in shoots and roots separately), increased with higher weevil densities (Fig. 31).
Int iol ^fybcR WG&R w©€k W©GK w©©k wc©K w©€k 01 02 03 04 OS 06 07 08 Wa«kt ••Control (shoot) MHBVPIant (Shoot) •••3/Plant (Shoot) ••6/Plant (Shoot) • Control (Leaf) * f Plant (Leaf) B-3/Plant (Leaf) —«—6/Plant (Leaf)
Figure 31: Effect on leaf and shoot N content by N. bruchi
Mi
Figure 32 : Effect on leaf and shoot N content by N.eichhorniae Chapter 4 Results and Discussion
As illustrated in figure 31, total tissue N levels increased proportionately to the weevil density or the level of herbivory inflicted by the weevil. Though the same trend followed by the highest Neochetina eichhorniae weevil density of 15 weevils per plant (figure 32), the rest of the tanks did not show the same trend
54 Chapter 5 Conclusion and Recommendations
Chapter 5
Conclusion and Recommendations
5.1 Conclusion
In the density dependent study, Neochetina.eichhorniae with a density less than 3 per
plant showed a slight inhibition in plant growth. However there is no significant difference (P < 0.05) in plant growth in comparison with the control. The study elucidated that a density of weevils per plant could be recommended as the effective
weevil density to achieve a successful control of the plant suppressing excessive plant growth, vegetative propagation and sexual reproduction to some extent with debilitating
flower initiation. Higher weevil densities more than 10 per plant were capable in total
destruction of the plants, loosing its buoyancy by damaging internal tissues and leading to
sinking. For complete eradication of water hyacinth stands, weevil densities more than 10
per plant could be used but the introduction of higher to the field (aquatic ecosystems) is
impractical since adult weevils tend to migrate by developing flight muscles to overcome
the interspecies competition.
Highest weevil density was observed at Angulana (2.04 ± 0.36) and the lowest at Kelaniya and Wellampitiya (0.15 ± 0.05) sites where the average shoot lengths ranged between 20- 30 cm. 6 weevils of N eichhorniae recommended as the optimum weevil density from the density dependent study of Neochetina eichhorniae were not found in practice including the sites where Fernando and Room first released the weevil. Disturbances in microclimates, mechanical removal of plants and several hydrological features notably the size of water body affected the degree of biological control. Therefore the success of biological control using Neochetina eichhorniae will ultimately rely on host plant quality and the habitat conditions to establish a healthy population of weevil densities for proper management of water hyacinth in Sri Lanka.
55 Chapter 5 Conclusion and Recommendations
Mass rearing was successful with the imported weevil, Neochetina bruchi and the study revealed that Neochetina bruchi could be successfully used to control water hyacinth.
Treatment with 3 Neochetina . bruchi weevils per plant showed greater inhibition of plant growth and reproduction with a significant difference (P < 0.05) when compared with the control (P<0.05). Therefore a density of 3 weevils per plant could be recommended in an effective control of the plant. Higher weevil densities more than 6 per plant leads to complete eradication.
The damage caused by the two weevil species at a density of 6 weevils per plant had a significant difference (P < 0.05), amounting 60% of damage by Neochetina eichhorniae while Neochetina bruchi caused a 99.9% damage on the water hyacinth stands after 8 weeks. Hence this weevil could be considered as an adaptive natural enemy of water hyacinth colonies growing under local climatic and environmental conditions. All these observations confirmed that Neochetina bruchi could substantially control hyacinth
infestations within shorter periods of time.
6.2 Drawbacks of the study
In the density dependent study, treatments were unable to replicate due to the practical constraints like limited time period allocated for the research. At the same time there are problems in obtaining weevils to repeat the study. Since the study was conducted under artificial chambers and containers, the weevils' activity may have been retarded or accelerated. Therefore pilot scale field trials are essential before mass introductions in the environment.
56 Chapter 5 Conclusion and Recommendations
6.3 Recommendations for future work
• The weevil densities required on control can be differing depending on the growth
stage of the plant. Therefore, some work has to be conducted to evaluate the
appropriate weevil densities needed for different shoot lengths.
• The identified weevil densities need to be used in a field trial to identify the
control mechanisms in their natural conditions
• Re-infestation resulted by improper management practices can be avoided
conducting instant dispersion of the weevils with their maximum densities to a
selected enclosed location in the field. A radial dispersion must be continued one
after the other till the entire infestation is gone. This simply calls active dispersion
of bombardment of the agent.
• Other than the particular research, further work on the biological control of water
hyacinth is needed in the following areas:
1. Evaluation of available biological species (i.e. Neochetina eichhorniae) 2. Search for other natural enemies where existing controlling agents do not give the desired level of control
3. More active approaches to biological control such as mass or supplemental releases and serial releases should be examined 4. Search for better ways to integrate biological control with other control options
5. The factors that accelerate success or limit control need to be further investigated BIBLIOGRAPHY
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c II
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d Appendix 1 Study sites in the field
Bolgoda Wellampitiya - Sedawatta