Exploiting the Attributes of Biocontrol Agent (Neochetina bruchi) as a Potential Ecosystem Engineer’s

Abstract

The biodiversity of lakes is continuously declining and diverse communities are being substituted by monoculture of invasive Eichhornia crassipes, resulting in a slew of environmental cascade effects. The ability of the Neochetina bruchi to self-perpetuate is a desirable aspect of biological control since it decreases the population to a reasonable level, making the approach more sustainable. N. bruchi is often referred to as “ecological engineers” because of the number of services it provides to the environment and enables herbicide application to be substantially reduced. Despite the presence of highly effective weevils against this weed, its effect on water hyacinth in association with the nutrients present in sites, is likely to vary with levels of disturbance caused by natural and anthropogenic factors. Understanding the aspects that determine the performance of these eco-engineers as valuable management tools will help to guide future endeavors. Our objective is to better comprehend their utility and limitations, along with critical knowledge gaps, to further enhance future applications.

1. Introduction

Wetlands have characteristic aquatic plants called macrophytes that can survive in waterlogged soils. Macrophytes are hydrophytes of freshwater that can be easily seen with the naked eye and are normally found growing in or on the surface of water [1]. As they are primary producers, they form the base of food chains providing food for fingerlings, tadpoles, and other aquatic organisms [2]. The vegetation provides a habitat for invertebrates, protection against predators, and reproduction refuges to young fishes [2]. A total of 106 species of macrophytes were reported in Bhoj Wetland belonging to 87 genera and 46 families together with 14 rare species [3]. Macrophytes are also called the “kidneys of landscape” as they filter sediments and excess nutrients from water [4]. They act as nutrient sinks (uptake nutrients) as well as nutrient pumps (moving compounds from sediment to water column) thus influencing water chemistry [5]. Therefore, nutrient concentrations vary in different limnetic layers and the exchange of these nutrients depends upon the temperature, concentration of dissolved oxygen, and bacterial action [6].

Unethical human activities near wetlands have changed the nutrient dynamics, favoring the growth of overpopulated invasive species of macrophytes at the cost of native, species, thus losing biodiversity [7]. Influx of sewage causes overgrowth, aging, and subsequently decay of macrophytes, devouring the system from life-giving oxygen to anaerobic conditions. The biodiversity of wetlands is continuously declining and diverse communities are being substituted by monoculture of invasive species (e.g., Eicchornia crassipes, Wolfia globosa, and Lemna minor) [8]. After the death and decay of these plants, the scenario does not come to an end, nutrients are further released into the water that again supports the next crop of aquatic weeds. Thus, it is a continuous endless process that impacts the food web and thus the ecological integrity of wetlands [8]. The combined sum of these conditions causes inexplicably severe consequences when taken as a whole. Thus, there is a need to take some serious measures to get rid of these invasive species and conserve the integrity of our wetlands.

2. Chapter review

The Water Hyacinth is a South American perennial free-floating species [3]. It belongs to the family Pontederiaceae, a family of heterostylous flowering plants, which form monospecific, dense mats in lakes and wetlands [9]. The weed is present all over India and causes significant evaporation losses (1.26–9.84%). Eicchornia can give rise to 3000 new offspring in 50 days [10]. It is capable of doubling its area every 12–14 days during the growing season. It reduces light and dissolves oxygen, thus hampering the aquatic life and destroying the food web [11]. The consequences are devastating for those communities reliant on water bodies for water, food, sanitation, and transport [12].

The manual operation to eliminate macrophytes from the lakes requires a large, number of manpower. Though this method is widely used but it is time taking, laborious, uneconomical, and cannot be used for large size water bodies. There are also various types of mechanical devices used to remove aquatic weeds from water bodies but these devices have some limitations. Removal of weeds from the deeper zones of the lakes, is a major constraint as the aquatic weed re-grows up from their rootstocks in this method [13]. Mechanical devices are site-specific and nonselective, that is, they also cut native species with invasive species [13]. The disposal of enormous quantities of removed plant material is another problem in manual and mechanical control.

A wide range of chemical herbicides are now available for controlling the growth of aquatic weeds. Their lethal action is either by direct contact (contact herbicides) or by absorption (systemic herbicides) of the chemical from the treated part of the plant, affecting the biochemical pathways [10]. The chemical methods may be used in a case where the water quality maintenance is not the main issue because, after the treatment of chemicals, the water of the reservoir becomes unsuitable from a drinking point of view [14]. The use of herbicides to eradicate aquatic weeds is a short-term gain only. As long as the chemical effect remains, weed mortality can be observed but after a certain time, frequent use of chemicals is required to control weed growth. These chemicals are costly and after death aquatic weeds started to settle down at the bottom and reduce the depth of aquatic resources as well as release nutrients after decomposition, which supports the next weed [15, 16]. These chemicals also impact the food chain and other micro or macro-organisms of the aquatic system. 2,4-dichlorophenoxyacetic acid, Endothall, Copper sulfate, Diquat, and Glyphosate are some of the commonly used herbicides for the eradication of weeds [17].

As a result, the current scenario necessitates weed control strategies other than chemicals, and in this context, biological control is gaining prominence around the world. Considering the above-mentioned limitations of typically obsolete technologies compels the adoption of novel ways based on biological agents that are environmentally safer, friendlier, economical, and viable. The first serious measure was taken in the US in 1970ʼs for the biological control of E. crassipes. This method controls the excessive growth of a pest by another organism which is naturally a predator.

This goal can be accomplished by natural bio-delegate, such as N. bruchi, which are chevroned Coleopterans belonging to class Insecta. Adult weevil is 4–5 mm long, having a brownish to grey tint. Abdomen is covered with fused brown tan “V”-shaped elytral marking [11]. Male is 3.5 mm in length with a thick and slightly curved snout. Female is 4.5 mm in length with a shiny tip, slender, and more curved snout [11]. Larvae damage the plant by forming tunnels through the petiole [12]. Adult weevils mainly feed preferentially on the slender upper branch of the petiole and the epidermis of the leaves, producing squarish feeding scars. The self-perpetuating existence of the control agents is a desirable feature of biological management, which decreases the population to a reasonable level, making the approach more sustainable [18]. The natural enemies, or control agents, have no undesirable side effects and insignificant impacts on nontarget animals or plants [15]. This method is cost-effective, environmentally acceptable, does not pollute the environment, and enables herbicide application to be substantially reduced [19]. Some of the noteworthy contribution in this field were carried out by Center [20], Center [21], Firehun [22], Heard and Winterton [23], Julien [12], Kumar [24].

The control of invasive species depends on a combination of various factors, such as temperature, nutrient level of the weed, climate and hydrology of the catchment, and number of healthy insects released [4]. The control of E. crassipes through biological control agents will be easier under lower nutrient conditions because plant biomass accumulation will be lower [23]. Understanding the combination of multiple drivers of plant growth, under changing conditions is essential for controlling the growth of water hyacinth [25]. Ray [18] studied the minimum required inoculation load of weevils for the control of three growth stages of water hyacinth based on fresh biomass, plant height and number of leaves and concluded that the smaller growth stages can be controlled earlier than the larger ones.

Based on the findings of previous studies, we should concentrate our efforts on ensuring that the control meets certain criteria, such as being effective against the target organism in a variety of nutrient conditions present in sites, which are likely to vary with levels of disturbance caused by natural and anthropogenic factors and achieving adequate control levels in the field with varying stocking densities. If such faults are not addressed, they create favorable conditions for Eichhornia to reproduce quicker than the weevil's growth rate. Hence, when deliberate introductions of bioagents are to be made, it is essential to release weevils first in the laboratory and assess their effects before releasing them in open field trials. In this chapter, we presented the results of a laboratory investigation to manage E. crassipes with N. bruchi (as the potential eco-engineer) in diverse nutritional situations. Thus, the study will endorse our perception of the degree to which weeds are eradicated in diverse nutritional situations, which will indirectly help to improve water quality and conserve aquatic life. Moreover, these efforts will aid in endeavoring the future application of bio-agents with the explicit goal of restoring biodiversity. The chapter comprehends the utilities and limitations, along with reviews that maintain and enhance the action of natural enemies to boost future applications (Figure 1).

Figure 1.

Flow chart on problems associated with excess growth of Invasive species.

3. Material and methods

This study was conducted in the Animal House Facility of the Department of Bioscience, Barkatullah University, Bhopal, to support studies on weed control in laboratory conditions. Stock cultures of E. crassipes and N. bruchi were obtained from Behata village behind the Sadhu Vaswani College, Bairagarh, Madhya Pradesh.

1. Collection of weevils

   To collect large number of adult weevils without damaging the plants, the plants were sink under the surface of the water, by putting a rigid grid over the top of the plants. Weighing the grid down with bricks and leaving it like this for 2 hours helped in collecting the weevils from the surface of the tank with a small sieve. Authentic collection of adult pairs was a very important issue for this experiment but adults laid eggs, which was very fruitful for the experiment.

2. Storage of weevils

   Weevils were stored in a plastic bucket with freshwater hyacinth plants and covered with mosquito net cloth. This process provides proper light, oxygen while ensuring feeding and restricting the flight of N. bruchi. The proper feed material was also provided from time to time, which supported the ovipositioning of N. bruchi.

3. Cleaning of Water Hyacinth

   The weeds were washed to remove the mud from the roots and leaves, after which they were allowed to dry on paddy straw for 10 min and then weighed to know the wet weight.

3.1 Experimental design

The experiment was conducted in the laboratory conditions in plastic basins or tubs of size 41 cm in diameter and height 13 cm. The monitoring period was 75 days and water hyacinths were kept in 12 different plastic tubs with 10 adult pairs of N. bruchi in each tub. Biological agents were introduced in all the groups except their control. In these groups, different weighable 7-cm long, four water hyacinths were introduced at the primary stage. The weight of the plant was measured by the Analytical and Precision Weighing balance (Endel-JA203P). The bioagents were introduced after 7 days of system establishment.

1. Group T1 = Control I + Natural conditions were maintained + N. bruchi

2. Group T2 = Control II + Hoagland and Arnon solution + N. bruchi

3. Group T3 = Control III + N and P deficient Hoagland and Arnon solution,+ N. bruchi

Generally, natural condition was maintained in T1 for up-gradation of the weed in this condition. Water was added to the T1 set to provide nutrition to the hyacinths and prevent them from drying. The fresh nutrient solution was prepared and added weekly in T2 and T3 groups. Basically, fortnight data were collected for general observation purposes. In the three groups (T1, T2, and T3) different conditions were maintained that provide different sets of data. In all the treatments, all other insects were removed immediately to maintain the original herbivore densities, and any dead adult weevils were replaced with other weevils.

3.2 Statistical analysis

1. The data collected were subjected to statistical analysis (one-way ANOVA) by using Excel–Mac operating system software. All the groups, with overall significance, were further compared for intergroup variation by Tukey’s honestly significant differences (Tukey HSD) test.

3.3 Life cycle of N. bruchi

1. Eggs. Eggs are ovoid about ¾ mm in length and change their color to pale orange as the time of hatching approaches.

2. Larval formation. The larval stage of the N. bruchi starts after 1 week of laying of eggs. Larvae are white or cream-colored and the larval stage continues up to 27 days.

3. Pupa formation. From larva to pupa formation N. bruchi took 29 days. The pupa of N. bruchi is normally found beneath the root of the water hyacinth plant. They cut off the small lateral rootlets and form a spherical parchment-like cocoon around themselves. The cocoon is formed among the lateral rootlets and attached to the main root axis below the water surface.

4. Adult formation. N. bruchi is confirmed as an adult weevil after 29 days of pupa stage. They lived in the adult stage for up to 120 days.

Assessment of the effectiveness of the eco-engineer as management tools

4. Results

In the present study, the insect treatment significantly reduced the plant weight in T1 and T3 (N and P were absent) groups. However, the decrease in weed weight in the T2 (weevil exposed condition with Hoagland’s solution) group was significantly lower compared to the (T1) and (T3) groups after 75 days of the growth period, as shown in Table 1. All herbivory treatments showed lower values of plant weight than their Controls. The change in fresh weight in all herbivory treatments and the control was different, varying from 3.3% to 40.21% during the week 1 to 10th, as shown in Table 2. There were no significant differences between the treatments of plant height parameters, as shown in Table 3.

Macrophytes weight (g)	Tub	March		April		May
		15th day	30th day	45th day	60th day	75th day
T1; Natural condition	A	277	335	298	265	180
	B	272	321	284	234	173
	C	281	339	268	280	168
	Control	244	260	280	330	339
T2; Hoagland and Arnon Sol	D	391	423	478	443	378
	E	388	440	467	430	369
	F	396	435	471	430	385
	Control	390	400	430	450	505
T3; N and P	G	298	350	388	322	273
	H	287	322	340	288	254
	I	292	334	330	298	268
	Control	360	355	330	323	305

Table 1.

Weight of E. crassipes reduced during the experimental period of 75 days.

Control (%)	March (%)	April (%)	May (%)
Control I 28.023	A 35.018	B 36.397	C 40.213
Control II 22.772	D 3.324	E 4.896	F 2.77
Control III 15.277	G 8.389	H 11.498	I 8.219

Table 2.

Percentage of E. crassipes reduced by the N. bruchi during the three different nutrient conditions.

Macrophytes	Aqua rium	March		April		May
		15th day	30th day	45th day	60th day	75th day
T1; Natural condition	A	7	7.4	7.4	7.4	7.4
	B	7	7.3	7.3	7.3	7.3
	C	7	7.4	7.4	7.4	7.4
	Control	7	7.3	7.4	7.4	7.5
T2; Hoagland and Arnon Sol	D	7.8	8.3	8.5	8.6	8.6
	E	7.8	8.4	8.6	8.7	8.7
	F	7.8	8.3	8.5	8.6	8.6
	Control	7.8	8.0	8.4	8.5	8.6
T3; N and P	G	7.2	7.4	7.5	7.5	7.5
	H	7.2	7.3	7.4	7.4	7.4
	I	7.2	7.3	7.4	7.4	7.4
	Control	7.2	7.3	7.4	7.4	7.5

Table 3.

Height of E. crassipes reduced during the experimental period of 75 days.

The results of this study showed that insect herbivory retarded the biomass and growth of E. crassipes and these agents were more effective in the T1 and T3 groups. The natural growth of water hyacinth was higher in March in group T1 because the bioagent (N. bruchi) was residing in this system in an immobile condition. In April and May, the bioagents were found in an active condition, so the wet weight of the plant was reduced in comparison to March. The weight of water hyacinth in tub C was higher than the other two groups in the second half of April month. This incident occurred because at that time population of N. bruchi entered the nether point, that is, they were found dwelling beneath the surface of the plant. This is why the biomass of the water hyacinth increased from 268 gm to 280 g in the second half of April month.

We are speculating that the reason for the success of this experiment in the T1 and T3 groups is that adult weevils scar the leaves of the weed, which lowers the photosynthetic rate and surface area while enabling the access and transfer of saprophytes into the leaves. The introduction of microbes via feeding scars elicits symptoms, such as increased respiration rates, poor chlorophyll and yellowing of leaves decreased buoyancy, and water and nutrient deficiencies [26, 27, 28]. All of these symptoms contribute to decreased growth and cause necrosis of plant tissue thus disrupting the plant leaf dynamics [20, 29]. Naturally, nitrogen and phosphorous are the primary ingredients for the natural growth of plants, so in the absence of these nutrients in the T3 group, slow growth and lower weight of water hyacinth were achieved.

The stress caused by constant herbivory allows energy to be diverted toward the development of daughter plants and new tissues, thereby reducing the overall growth rate of the plants and their sexual reproduction. Byrne et al. [30] stated that water hyacinth density and its spreading capacity were mainly related to asexual reproduction, so a decrease in reproductive capacities would reduce the expansion of mats and invasive potential of the water hyacinth.

In this study, it was observed that young hyacinths were controlled more rapidly than older plants that were confirmed by the studies of Ray et al. [31]. They reported that adult weevils are attracted to young plants because of the presence of some volatile substance that encourages them to feed especially at the previous site of injury but as the age increases it decreases, taking a longer time to be controlled by the weevils. So, managing larger plants is a tedious job in natural conditions that can only be made successful by releasing high inoculation loads of weevils, which increases production of smaller leaves and helps to destroy more leaves.

The change in fresh weights from 3.3% to 40.21 % in our study was very similar to the losses (−5 to −50%) reported by Del fosse and Cullen [32]. Tipping et al. [6] reported that weevil herbivory leads to a 50% reduction in biomass and inflorescence, but had less effect on the coverage area. Firehun [22] reported that three pairs of N. bruchi reduced 30% production of ramets, new leaves, and biomass thus reducing the productive capacity and vigor of the water hyacinth.

In the T2 group, due to high nutrient content (1.6 mg l⁻¹ N and 1.0 mg l⁻¹ P), the mass production of water hyacinths was achieved. In the control group, the wet weight obtained during the 10 weeks of the trial was almost twice as high as treatment (T2), but insect feeding still slightly affected (2.7–4.8%) the plant growth. Heard and Winterton [23] achieved greater damage at higher nutrient concentrations due to the greater production of offspring, that is a high reproductive rate (93 times in a generation). Due to the high reproductive rate, damage by weevils magnify over generations. However, the present findings contrasted with the study of Heard and Winterton, as the study was restricted to only one generation, hence less damage was achieved.

The collapse of the water hyacinth population by weevils in natural water bodies takes a time from 14 months to 24–36 months or 6 years [7, 24, 33]. The reason for this might be that water hyacinth growth and reproduction occur at a more rapid rate than the weevil’s growth rate, so the weevils take a longer time to bring down the population of weeds.

5. Conclusion

Thus, the assessment of optimum densities of the weevil in different nutrients decreases the growth of its host plant up to 3.3–40.21% in 75 days except for extremely eutrophic conditions. The impact of these insects is supposed, to be evident as sudden, widespread eradication of the macrophyte, but rather it occurs gradually in our studies through slight changes in the phenology, morphometry, and productivity in the Eichhornia’s population. We emphasize the need to treat the weed infestations as soon as the growth begins, usually before the plants start flowering, to minimize seed production. Thus, the outbreak of this aquatic weed could be sustainably managed by the judicious use of this potential Ecosystem Engineer. The biological control may act as flawless standalone technique for the control of water hyacinth. However, it should always be noticed that this technique is not always targeted at eradicating but, rather, it aims at managing populations to a level of permanent stress, thus bringing an effective control in the long run. In any case, this study clearly proves the words of T.D. Center that “any number of weevils is better than none”. So, emancipating Neochetina bruchi from our natural water bodies to control this specific weed is a wise approach (Figures 2–4).

Figure 2.

Weight of E. crassipes was reduced during the Natural Condition of Experiment (T1) group after 75 days of inoculation (n = 10). The bars in the data represent the means, and the error bars are the standard error. The (*) over the month of April and May showed significant differences (p < 0.05) when compared to the control group, according to the Tukey HSD test.

Figure 3.

Weight of E. crassipes was reduced in the presence of Hoagland and Arnon’s solution (T2) group after 75 days of inoculation (n = 10). The bars in the data represent the means, and the error bars are the standard error. The (*) over the month of May showed non-significant differences (p > 0.05) when compared to the control group, according to the Tukey HSD test.

Figure 4.

Weight of E. crassipes was reduced in the absence of Nitrates and Phosphate content from Hoagland and Arnon’s solution (T3) group after 75 days of inoculation (n = 10). The bars in the data represent the means, and the error bars are the standard error. The (*) over the month of May showed significant differences (p < 0.05) when compared to the control group, according to the Tukey HSD test.