S. LIFE CYCLES AND MORPHOLOGY OF WATER

HYACINTH WEEVILS

The potential biocontrol agents of water hyacinth are Neochetinabruchi and Neochetina eichhorinae. The present chapter discusses the life

cycle, morphology and biology of these weevils.

5.1. METHODOLOGY

5.1.1. Life Cycle

Eggs and larvae used for the study were obtained by placing adult

female weevils in plastic containers containing leaves and petioles of water

hyacinth for feeding and oviposition. The jars were placed in fabricated

growth chambers at constant temperatures (Plate 4). Plants were changed

every 2-3 days. When the eggs were 2-3 days old, they were removed from

the laminae and petioles with forceps under a dissecting microscope. The

eggs were then transferred to petriplates containing wet filter paper surface

sterilized with hypochlorite. The eggs were left undisturbed by placing them

in chambers at constant temperatures and were examined periodically for

hatching. Newly hatched larvae were collected from the petridishes and

transferred to punctures made in the petioles of water hyacinth. The leaves

of water hyacinth were placed in plastic containers with water, in such a way,

only the base of the petioles extended into water. These experimental

containers were placed in fabricated growth chambers at constant

A- Eggs in the petiole B-Larva

C-Pupa D-Adult

2-Stages of Water Hyacinth Weevil

FW L I

2E-Neochetina eichhorniae

2F-Neoc/zetina brzichi

ltnp,i

Antenna

Rostrum

p1pn

Antenna

Rostrum

lp1

2G-Neoclzetina bruchi- Male

2H-Neochetina bru clii- Female

An

Tibia IClaw

21-Weevil - Head

F I0•25 P111*1

2J-Weevil - Leg

Coxa

Trochanter

Femur

Tarsus

3A-Water hyacinth leaves scrapped by weevils

At

3B- Linear galleries formed b mite

3C-Pond snail feeding on water hyacinth

All

.$

(

d4

4rx-

1

a

/

I AIq1

4-Experimental set ups

[III. I

-

I _loa

Alf-

1l

r

ijik

t

temperatures at 20 to 25°C and 15 ± 1 hour photoperiod (De Loach and

Cordo, 1976).

5.1.2. Morphology

Adult water hyacinth weevils were procurred from the Project

Directorate for Biological Control (PDBC), Bangalore, India. They were

allowed to acclimatize to the laboratory conditions (31 ± 1°C; 11 ± 0.5 hr

photoperiod; 65 - 70 % r.h). N. bruchi and N. eichhorniae weevils were

maintained separately on fresh water hyacinth plants in plastic troughs

(45 cm diameter, 25 cm height and 20 litres capacity).

Adult weevils (both males and females) were allowed to remain in the

same troughs. Their mating behaviour was studied. The two sexes were

separated and difference in position of rostrum was identified. Male and

female weevils were mounted on a glass slide and rostrum position was

sketched using camera lucida under compound microscope (lOX resolution).

Sketches of the head and leg were also made using camera lucida.

5.1.3. Biology

5.1.3.1. Damaging Potential

The impact of different densities of these weevils on the damaging

potential and consumption rate on water hyacinth leaves were studied at four

density categories such as 2, 4, 6 and 8 animals per container. The different

density categories of both species of weevils were released on two water

hyacinth leaves in separate containers (250 ml capacity). After 24 hours,

total number of scrapings per leaf by each density category was recorded

(De Loach and Cordo, 1976).

In the other experiment males and femalesojN. bruchi and

N. eichhorniae were released and the scrapings were recorded. Control

troughs were maintained for all the experimental set ups and the

experiments were conducted in triplicates to avoid statistical errors.

5.1.3.2. Biocontrol Potential of Adult Weevils

Adult weevils were selected. Two, four, six, eight and twelve pairs of

each species of the weevils were released separately on three water

hyacinth plants of known weight kept in plastic troughs (45 cm diameter, 25

cm height and 20 litres capacity). The plant biomass and number of insects

accommodated per plant was recorded (Bashir et al., 1984). The experiment

was carried out in triplicates. Control troughs containing only water hyacinth

plants were also maintained so as to compare the growth of water hyacinth

with experimental troughs. One set of this experiment was conducted during

summer and the other during winter, so as to compare the seasonal variation

in the biocontrol potential of the weevils.

Initial weight of the plant-Final weight of the plant

Percent Plant biomass

reduction Initial weight of the plant x 100

5.2. RESULTS

5.2.1. Life Cycle

Eggs

Eggs of Neochetina bruchi and Neochetina eichhorniae are whitish in

colour and size ranging between 0.5 and 1 mm in length (Plate 2A).

Temperature was a major factor influencing the fecundity of adult weevils.

Females laid only fewer eggs at lower temperature and at temperature

beyond 30°C also fecundity was gradually reduced. Maximum egg laying

occurred at 30°C for both N. bruchi and N. eichhorniae. However, when egg

development was taken into consideration it varied to a great extent between

both species. It was obvious that eggs of N. bruchi developed better at lower

temperature regimes and the development time was also less at lower

temperatures. However, it was vice-versa in the case of N. eichhorniae.

Survival percentage (% eggs hatched) of N. bruchi was 89.96% at 20°C,

whereas of N. eichhorniae it was 84.54 at 25 °C (Figure 5.01).

Larvae

The eggs hatch within 6 to 15 days at 20 - 25°C. After eclosion, the

newly hatched larvae tunneled towards the base of petiole (Plate 213).

Larvae usually occurred singly. The larvae moved inside the petioles by

forming tunnels and fed on the inner tissues of petioles. They occasionally

burrow up the stem to enter the base of younger petioles and sometimes

reach the stem apex and destroy the apical bud. The larval period probably

requires 30 to 80 days with N. bruchi developing somewhat faster than

N. eichhorniae (Table 5.01). Survival percentage of the larvae of both weevil

species was about 70%.

Pupae

The fully developed larvae burrow out of the stem and move to the

upper root zone just under the surface of the water. They cut off the small

lateral rootlets and form a spherical parchment-like cocoon around

themselves (Plate 20). This cocoon is attached to one of the roots.

Curiously, at the point of attachment, the larvae chew a notch into the root.

This notch possibly functions in gas exchange between the hollow inside of

the cocoon and vascular tissue of the plant. After the cocoon is formed the

larva molts and becomes a pupa. This is an inactive stage during which the

transition from larvae to adult occurs. Pupal period lasts for about 15 - 20

days for both species.

Adult

As the adults emerge they split the cocoon and pull themselves out

through the split (Plate 2D). Once they are out they climb up onto the

emergent leaves of the plant to feed and mate. Adult N. bruchi lives for about

180 days and N. eichhorniae for about 200 days.

5.2.2. Morphology

Weevils are short to long (from 3 mm to 4 mm) and are characterized

by having mouthparts to facilitate scrapping of plant parts, compound eyes

(Eyes not divided), lacking cerci, and possessing thickened forewings or

elytra, which meet at the midline when folded. The membranous hindwings

are the flight organs (although weevils do not use them much) and fold up

under the elytra when not in use. Head usually prolonged into snout,

antennae elbowed. Antennae positions are highly variable. Legs are short

and modified for swimming. Tarsi are primitively five segmented (Tarsal

formula 5-5-5). Bodies are normally stout and sclerotization is extensive.

Metamorphosis is complete. Larvae have poor scierotization and pupae are

exarate (Plates 2E, 2F, 21 and 2J).

Sexes of both weevil species can be separated by the attachment of

antennae to the rostrum. In case of males, antennae are inserted at the

distance from the apex of the rostrum, equal to or less than width of the

rostrum at the point of insertion. In females, antennae are inserted at a

distance from the apex to the rostrum, more than the greatest width of the

rostrum (Plates 2G and 2H).

5.2.3. Biology

The damaging potential of water hyacinth weevils, N. eichhorniae,

N. bruchi and both species together is given in table 5.02. In the four

treatments 2, 4, 6 and 8 weevils per two water hyacinth leaves,

N. eichhorniae scrapped more than N. bruchi and combination treatments as

well. N. eichhorniae made 17.66, 19.31, 18.87 and 18.64

scrappings/animal/day in treatments 2, 4, 6 and 8 numbers of weevils

respectively. Similarly, N. bruchi made 4.75, 11.07, 11.65 and 13.63

scrappings/animal/day in the same treatments. When both species were put

together, number of scrapings/animal/day was 8.76, 8.94, 10.57 and 17.31

in treatments 2, 4, 6 and 8 numbers of weevils respectively (Figure 5.02).

Statistical analysis reveals significant difference between treatments (P<0.05

Table 5.01. Life cycle of the water hyacinth weevils N. eichhorniae andN. bruchi

Developmental stage Approximate duration (days)

N. bruchi N.eichhorniae

Total fecundity 300-900 850-1200

Eggs 6-11 7-15

Larva 30-80 75-95

Pupa 15 -20 15-20

Adult longevity 50-180 30 200

lable 5.02. Number of scrapings made by water hyacinth weevils(animal I day) on water hyacinth leaves (Mean ± S.E)(n=3)

Number Number of Scrapings per leaf!

of animal! day

weevils N. e N. b N. e + N. b

2 17.66 4.75 8.76± ± ±

4.36 1.75 2.31

4 19.31 11.07 8.94± ± ±

3.29 3.53 i c

6

18.87

11.65

10.57

+ + +

2.87

2.64

2.65

8

18.64

13.63

17.31

+ + +

3.64

3.47

3.73

ANOVA

t-Test

Between species

Between species

Calculated F = 13.171

N. e - N. b = S

Table F 5.143

N. b - N. e + N. b = NS

Significant

N. e - N. e + N. b = S

Between numbers

Calculated F = 3.051

Table F = 4.757

Not significant

100

90

80 -U)

70-

w 60-C)

U)

.2 40-

30-

20

10

0-i

10 15 20 25 30 35 40

Temperature

-- N. bruchi- IV eichwrni?a

Figure 5.01. Survival percentage of water hyacinth weevil eggs at differenttemperatures

25 -i

>.

E20-J

EL 10

0-:

2 4 6 8

number of weevils

jNb DN.e+N.b

Figure 5.02. Number of scrapings made by water hyacinth weevils on water

hyacinth leaves

by ANOVA) and among the treatments there is significant difference

between N. eichhorniae and N. bruchi treatments and between

N. eichhorniae and N. eichhorniae plus N. bruchi treatments.

Number of scrapings made by male and female weevils in 24 hours is

given in table 5.03. Results reveal that females have scrapped more when

compared to males. A single N. bruchi male formed only 4.00 scars on the

leaves in a day, whereas the females formed 33.00 scars. Similarly,

N. bruchi males formed 28.50 scars and females formed 32.50 scars. In both

species females scrapped more than males. It has also been observed that

scars were more on the lower surface of leaves than on upper surface.

N. bruchi males formed 1.00 and 3.00 scars on the upper and lower surface

of leaves respectively (Figure 5.03). N. bruchi females formed 7.00 and

23.00 scars on the upper and lower leaf surfaces. N. eichhorniae males

formed 8.00 and 20.5 and females formed 16.50 and 16.00 scars on the

upper and lower surfaces of leaves respectively.

Table 5.04 shows the efficacy of N. eichhorniae in reducing water

biomass during summer and winter. Variation in biomass reduction has been

observed in the different density treatments. In the control treatment water

hyacinth biomass increased from 146.83g to 170.83 and 195.33g on the 7th

and 14th day respectively. In the treatment with 4 weevils/3 plants water

hyacinth biomass decreased to 132.50 and 128.83g from an initial weight of

133.00g. With 8 weevils/3 plants, from an initial weight of 117.00g the plant

weight decreased to 115.17 and 110.00g on the 7th and 14th day

respectively. 12 weevils I trough reduced the initial plant biomass of 140.83g

to 105.17 and 86.83g respectively on the 7th and 14th day. Similarly 18

weevils/trough reduced plants weighing 132.17g to 116.83 and 100.50g on

Number of Change in biomass (g)son

weevils Day 7day 14 day

Control 146.83± 5.60 170.83±5.63 195.33±5.83

nmer

4 133.00±7.32 132.5±7.29 128.83±6.66

8 117.00±8.67 115.17±8.51 110.00±11.59

12 140.83±9.13 105.17±10.93 86.83±9.77

18 132.17±4.91 116.83±4.33 100.5±4.33

24 147.33±11.35 135.50±11.50 116.33±13.27

128.17±2.95

140.33±2.33

141.67±2.33

127.8±4.00

121.0±4.86

127.5±3.06

137.5±2.68

116.33±2.20

109.5±3.75

109.5±4.58

126.5±2.78

134.5±2.36

98.5±2.65

98.83±4.15

101.83±4.53

ter

4

8

12

18

24

Table 5.03. Number of scrapings made by male and female weevils on

water hyacinth leaves in 24 hours (Mean ± S.E) (n = 60)

Species Sex

N. b Male

Female

N. e Male

Female

Number of Scrapingsper leaf on the upper

surface

1.00 ± 0.00

7.00 ± 1.00

8.00 ± 5.00

16.50 ± 2.50

Parameters

Number of Scrapingsper leaf on the lower

surface

3.00 ± 2.00

23.00 ± 5.00

20.5±1.50

16.00 ± 5.00

Total numberof Scrapings I

leaf

4.00 ± 2.00

30.00 ± 6.00

28.50 ± 3.50

32.50 ± 7.50

Fable 504. Biocontrol Potential of different densities of adult N.eichhorniae on water hyacinth plants during summer andwinter (N3) (Mean ± S.E)

ANOVAt-Test

Between days Between

Calculated days

F=4.514 1-7=NS

Table F=3.885 7-14=S

Significant 1 - 14 = S

Between days

Calculated F =2.838

Table F = 3.885

Not significant

the 7th and 14th days respectively. In the treatment with 24 weevils/3 plants,

initial plant biomass dwindled to 135.50 and 116.33g on 7th and 14th day

respectively. Significant difference in plant biomass between treatment is

evident (P<0.05 by ANOVA) and among the days there is significant

difference between days 1 and 14 and between days 7 and 14. Initial plant

biomass was 128.17, 140.33, 141.67, 127.80 and 121.00g in treatments with

4, 8, 12, 18 and 24 N. eichhorniae per 3 plants. In 7 days, in the same

treatments plant biomass leveled off to 127.50, 137.50, 116.33, 109.50 and

109.50g respectively. Similarly on the 14th day plant biomass was 126.50,

134.50, 98.50, 98.83 and 101.83g respectively. No significant difference in

plant biomass change was observed (P>0.05 by ANOVA).

Effect of N. bruchi on water hyacinth biomass during summer and

winter is given in table 5.05. In treatments with 4, 8, 12, 18 and 24 weevils

per 3 plants, initial plant biomass was 107.00, 138.33, 131.83, 110.50 and

153.00g respectively. In the same treatments, plant biomass lessened to

106.33, 135.17, 105.33, 100.83 and 141.00g respectively on day 7, and on

day 14 the same curtailed to 104.83, 132.83, 89.50, 89.17 and 124.83g

respectively. Statistical analysis reveals significant difference in plant

biomass between days (P<0.05 by ANOVA). Effect of N. bruchi on water

hyacinth biomass during winter shows that initial plant biomass was 133.67,

128.33, 145.57, 127.67 and 127.17g in treatments with 4, 8, 12, 18 and 24

N. bruchil 3 water hyacinth plants. The same declined to 133.50, 121.67,

108.50, 113.33 and 116.17g on the 7th day and further more reduced to

130.67, 120.17, 89.33, 98.33 and 106.00g on the 14th day. In the control

trough plant biomass increased to 191.17g on the 14th day from an initial

weight of 143.67g. Statistical analysis shows significant difference in plant

biomass change between days (P<0.05 by ANOVA) and between days there

is significant difference between days 1 and 14 and between days 7 and 14.

Table 5.06 discloses the effect of both species together on water

hyacinth biomass during summer. It is apparent that in treatments 4, 8, 12,

18 and 24 weevils/3 plants, initial biomass of plants was 128.67, 128.83,

113.50, 127.17 and 114.17g respectively. On day 7, the same were reduced

to 128.50, 127.00, 100.00, 109.00 and 104.67g respectively and on day

14 further reductions to 125.33, 124.83, 77.00, 97.83 and 93.17g was

evident. No significant difference in plant biomass between days was evident

(P>0.05 by ANOVA). In the control trough plant weight shot up from 146.83

to 170.83 and 195.33g on day 7 and 14 respectively. Table 5.06 also

discloses the effect of N. eichhorniae and N. bruchi together on water

hyacinth biomass during winter. From an initial biomass of 112.17, 126.33,

123.67, 114.67 and 160.00g in treatments with 4, 8, 12, 18 and 24 weevils

per trough, plant biomass declined to 110.33, 124.00, 107.17, 104.50 and

148.00g on day 7 and furthermore to 110.67, 120.83, 80.67, 93.00 and

131.83g on day 14. There is no significant difference in plant biomass

between days (P>0.05 by AN OVA).

Percent biomass change in plant damaged by weevils during summer

is given in table 5.07. It is obvious that N. eichhorniae reduced water

hyacinth biomass very efficiently. In treatments with 4, 8, 12, 18 and 24

N. eichhorniae, percent biomass reduction was 3.14, 5.98, 38.70, 23.96 and

21.04 respectively. Treatments with 4, 8, 12, 18 and 24 N. bruchi shows a

percent biomass reduction of 2.03, 3.98, 32.11, 19.30 and 18.41

respectively. When both species were put in combination in treatments 4, 8,

12, 18 and 24, percent biomass reduction was 2.60, 3.10, 32.15, 23.07 and

Table 5.05. Biocontrol Potential of different densities of adult N.bruchi on water hyacinth plants during summer and winter(N=3) (Mean ± S.E)

Number Change in biomass (g)ANOVA

son of t-Testweevils Day 1 7 day 14 day

Control 146.83± 5.60 170.83±5.63 195.33±5.83

nmer 4 107.00±6.83 106.33±7.12 104.83±7.34 Between days

8 13833±3.48 135.17±3.63 132.83±3.77 Calculated F = 1.316

12 131.83±6.21 105.33±6.59 89.50±5.63 Table F = 3.885

18 110.50±7.42 100.83±5.73 89.17±6.19Not significant

24 153.00±8.32 141.00±8.22 124.83±8.17

ter 4 133.67±6.69 133.5±6.75 130.67±6.67 Between days Between

8 128.33±2.17 121.67±3.06 120.17±2.68 Calculated F = 4.919 days

12 145.5±9.46 108.5±10.73 89.33±10.08 Table F=3.885 1 —7=NS

Significant 7-14= S18 127.67±2.60 113.33±2.05 98.33±2.80

1 - 14 = S24 127.17±5.93 116.17±6.06 106.0±6.17

able 5.06. Biocontrol Potential of different densities of adultN. eichhorniae and N. bruchi on water hyacinth plantsduring summer and winter (N=3) (Mean ± S.E)

Season Number Change in biomass (g)of weevils

Day 7day 14 day

Control 146.83± 5.60 170.83±5.63 195.33±5.83

Summer 4 128.67±4.18 128.50±4.31 125.33±4.42

8 128.83±5.63 127.00±5.80 124.83±5.83

12 113.50±7.32 100.00±11.85 77.00±16.16

18 127.17±3.77 109.00±3.77 97.83±4.15

24 114.17±4.29 104.67±6.98 93.17±6.36

Winter 4

8

12

18

24

112.17±5.42

126.33±2.89

123.67±3.38

114.67±6.06

160.00±5.63

110.33±5.26

124.00±2.84

107.17±4.11

104.50±5.96

148.00±5.48

110.67±5.42

120.83±1.45

80.67±2.68

93.00±5.97

131.83±5.50

Between days

Calculated F = 1.968

Table F = 3.885

Not significant

Between days

Calculated F = 1.346

Table F = 3.885

Not significant

18.39 respectively (Figure 5.04). Statistical analysis reveals significant

difference in percent biomass reduction in plants damaged by the two

species and different densities as well (P<0.05 by ANOVA). Between

species there is significant difference between N. eichhorniae and N. bruchi.

Between numbers there is significant difference between treatments 4, 8 and

12 weevils.

Table 5.08 shows the percent plant biomass reduction in plants

damaged by weevils during winter. It is obvious that during winter N. bruchi

effectively reduced water hyacinth biomass. Percent biomass reduction in

treatments with 4, 8, 12, 18 and 24 N. eichhorniae was 1.30, 4.15, 30.47,

22.67 and 15.84 respectively. With N. bruchi it was 2.24, 6.36, 38.60, 22.98

and 16.65 respectively. With both species in combination, percent plant

biomass reduction was 1.34, 4.35, 34.80, 18.9 and 17.60 respectively

(Figure 5.05). Statistical analysis reveals that there is no significant

difference between species in reducing water hyacinth biomass (P>0.05 by

ANOVA), however, significant difference between numbers is observed

(P<0.05 by ANOVA).

Figure 5.06 shows the percent plant biomass in plants damaged by

mature adult weevils during summer. It is obvious that during summer

N. eichhorniae effectively reduced water hyacinth biomass. Percent biomass

reduction in treatments with 4, 8, 12, 18 and 24 N. eichhorniae was 3.14,

5.98, 38.70, 23.96 and 23.96 respectively. With N. bruchi it was 2.30, 3.98,

32.11, 19.30 and 18.41 respectively. With both species in combination,

percent plant biomass reduction was 2.60, 3.10, 32.15, 23.07 and 18.39

respectively. Figure 5.07 shows the percent plant biomass in plants

damaged by mature adult weevils during winter. It is obvious that during

Table 5.07. Change in biomass of water hyacinth plants damaged bydifferent densities of adult water hyacinth weevils duringsummer (n=3) (Mean ± SE).

Percentage biomass change inNumber

of weevils14 days ANOVA t-Test

N.e N.b N.e+N.b

Between species Between speciesControl + 33.03 + 33.03 + 33.03 Calculated F = 6.997 N. e - N. b = S

Table F=4.459 N. b - N. e+N. b=NS4 -3.14 -2.03 -2.60 Significant N.e - N.e +N. b= NS

Between numbersBetween numbers 4 - 8 = NS

8 -5.98 -3.98 -3.10Calculated F = 227.323 4- 12 = STable F=3.838 4-18=S

12 -38.70 -32.11 -32.15 Significant 4 -24 = S8- 12 = S

18 -23.96 -19.30 -23.078-18 = S8-24=S12-18 = S

24 -21.04 -18.41 -18.39 12-24=S18-24 = NS

- = increase in biomass; - = decrease in biomass; NS = Not significant; S =significant

Table 5.08. Change in biomass of water hyacinth plants damaged bydifferent densities of adult water hyacinth weevils duringwinter (n=3) (Mean ± SE).

Number Percentage biomass change in ANOVA t-Testof weevils 14 days

N.e N.b N.e+N.b

Control +33.06 +33.06 +33.06

4 -1.30 -2.24 -1.34

8 -4.15 -6.36 -4.35

12 -30.47 -38.60 -34.80

18 -22.67 -22.98 -18.90

Between species Between numbers

Calculated F = 2.185 4-8 = S

Table F = 4.459 4 - 12 = S

Not significant 4 - 18 S

4 -24 = SBetween numbers 8-12 = S

Calculated F= 8-18= S135.064 8-24=STable F=3.837 12-18=SSignificant 12 -24 = S

18 - 24 = NS24 I -15.84 I -16.65 I -17.60

= increase in biomass; - = decrease in biomass: NS = Not significant; S =ignificant

25U)

•20CL

U) 15'4-0I-

20

10

U)(I)U)

E 00.0

-100C)CU)

- -200

-30

upper lower upper lower

leaf surface

jmaIe jfemale

Figure 5.03. Number of scrapings made by male and female weevils onwater hyacinth leaves in 24 hours

30

-40

number of weevils

DN.b EIN.e DN.e+N.b

Figure 5.04. Change in biomass of water hyacinth plants damaged bydifferent densities of adult water hyacinth weevils duringsummer

30

20

10U)

1°. -10a)a)

(0

C.)

-30

-40 -

-50

40

30

20

U)cn 10(0

E0

a)-10

Ce

C.)

-20

-30 -

-40 -

number of weevils

El N. EIN.e ON. e+N.b

Figure 5.05. Change in biomass of water hyacinth plants damaged bydifferent densities of adult water hyacinth weevils during winter

40 -

number of weevils

El Ne E1N.b DN.e+Nb

Figure 5.06. Change in biomass of water hyacinth plants damaged bydifferent densities of mature adult weevils during summer

40

30

20

10Cs

E 0

C -10

1)C)C -Cs.0U -30

qu

-50

number of weevils

EJN.e ED N. ON. e+N.b

Figure 5.07. Change in biomass of water hyacinth plants damaged bydifferent densities of mature adult weevils during winter

winter N. bruchi effectively reduced water hyacinth biomass. Percent

biomass reduction in treatments with 4, 8, 12, 18 and 24 N. eichhorniae was

1.30, 4.15, 30.47, 22.67 and 15.84 respectively. With N. bruchi it was 2.24,

6.36, 38.60, 22.98 and 16.65 respectively. With both species in combination,

percent plant biomass reduction was 1.34, 4.35, 34.80, 18.9 and 17.60

respectively.

5.3. DISCUSSION

In the present investigation life cycle of weevil. is completed in about

51 to 130 days (Table 5.01), which is about 4 months. This is similar to the

observations of Harley (1984), who reported that eichhorniae takes about

4 months to complete its life cycle. Three generations a year have been

observed in the native range in Argentina (De Loach and Cordo, 1976). In

the present study, eggs were deposited in leaves as well as petioles, but in

some cases eggs are deposited in petioles of a mature leaf. Mature larvae

are capable of moving to young leaves. When leaf production slowed, weevil

larvae seemed to injure younger leaves (Center, 1985). Larvae formed

tunnels in the petioles and fed upon the inner tissues of the petioles. Then

they moved to the roots to form cocoon, out of root hairs and pupated with

them. Larvae were found to form root balls in only roots of water hyacinth.

Del Fosse (1978) reported that fully grown larvae were sometimes able to

form root balls on other aquatic plants, but the complete development of

pupae on these plants was not demonstrated. He also reported that pupae

separated from living roots at an early stage in their development died.

Larvae live in tunnels and pupa within the root balls, so the fully-grown

larvae and pupae probably obtained oxygen from the root lesion in a manner

similar to that reported for the aquatic beetle Donacia simplex by Houlihan

(1970).

Development stages may be affected by various environmental

factors. Jamil and Hussain (1993) stated that the uptake of heavy metals by

water hyacinth might reduce fecundity of the weevils that feed on these

plants. High temperature and low humidity may decrease egg production

and reduce adult survival, while low temperature probably below about 150C

arrests development, prevents population increase and decreases survival

(Julien et al., 1999).

Weevils voraciously scrap leaves and petioles. Number of scrapings

per animal per day ranged between 4.75 and 19.31 (Table 5.02). Earlier

studies report that one adult weevil produces an average of about only 20

feeding spots per day, and damage by five adults can kill a medium size

water hyacinth plant in the laboratory in about 10 days. However, feeding at

the junction of the petiole and the leaf blade often kills the blade by cuffing

off the tissues of translocation, so the leaf is effectively eliminated though not

consumed (Perkins, 1974). It is also observed that females scrap more when

compare to males and both males and females prefer to scrap more on the

lower surface of leaf lamina (Table 5.03). Weevils are noctural and hence to

avoid light they tend to scrap the'lower surfaces. During the day adults

usually hide among the ligules or inside young, rolled leaves in buds near

the base of the plant (De Loach and Cordo, 1976). Neochetina species

adults preferred young leaves. The lamina of the youngest leaf was often fed

upon while only partially opened. Adults also fed on older leaves but not to

the same extent (Center, 1985).

In the present investigation, N. eichhorniae caused more biomass

reduction during summer than during winter (Table 5.04). Similarly, N. bruchi

caused more damage during winter, but in summer damage caused was

comparatively lower (Table 5.05). This might be because; the adults of

N. eichhorniae were tolerant to warmer temperatures and fed more than

N. bruchi and N. bruchi was more tolerant to cooler temperatures (Perkins

1974). Environmental factors may alter the feeding rate of an insect as

documented by Englemann (1970). When the two species were put in

combination biomass reduction was quite moderate. The present study also

reveals that when tested in different numbers, the optimum number for

causing effective damage was 4 weevils per plant. This is evidenced from

Tables 5.07. and 5.08. During summer 12 N. eichhorniae13 water hyacinth

plants have brought about a reduction of 38.70%, which is the maximum

among all treatments tested for. At densities above and below this number

reduction percentage is lower. The reason might be that with lower number

of weevils sufficient damage could not be affected, and at higher numbers

there might be competence with each other, which might affect feeding rate.

Forno (1981) and Goyer and Stark (1984) also report that damage by

N. eichhorniae at low densities does not kill the plant, but reduces the growth

rate and fecundity. N. eichhorniae adults prefer feeding on young leaves, but

it appears that eggs are not laid in, and young larvae do not usually feed on

young leaves (Center, 1987). Although adult N. eichhorniae feed on young

leaves, the larvae take the initiative of destroying the mature leaves and

petioles. According to Bashir et al. (1984) N. bruchi population of one

generation reduced the growth of 41 plants by 25.4% in 61 days,

N. eichhorniae by 12.7% and the mixed culture of both species by 22.5%. Of

all, one significant difference between N. bruchi and N. eichhorniae is that

N. bruchi populations develop better under eutrophic conditions (Heard and

Winterton, 2000) and in polluted waterways, may complement the damage

by N. eichhorniae. So even though in combination both species cause

comparatively less damage to water hyacinth plants, both together in the

biological control programmes in fields can work alternatively in the different

seasons to cause effective damage to water hyacinth plants, so as to bring

and keep water hyacinth proliferation below the mark.