A Research Work Carried Out to Determine the Acute Toxicity Effects of Imidacloprid on Stingless Bees, Meliponula Bocandei (Spiona, 1853)
Chapter One
Objectives of the Study
- To determine the oral acute toxicity of imidacloprid to the stingless bee(bocandei).
- Also to determine the effects of imidacloprid at different concentrations on the sucrose consumption of bocandei.
CHAPTER TWO
LITERATURE REVIEWS
Biology of Stingless Bees
Stingless bees are a group of small to medium sized bees with vestigial (non-functional) stings. They belong to the genus Meliponula. Stingless bees are highly social bees which live in colonies of thousands of individuals (Michener, 1990). Approximate numbers of species so far identified are 50 in Africa, 300 species in the Americas, 60 in Asia, 10 in Australia and 4 in Madagascar (FAO, 2009). The tribe Meliponini represents the largest biomass of bees within tropical rain forests (Roubik, 1993).
The life cycle and caste systemof stingless bees is different from that of the honeybees. In stingless bees, there can be two or more queens (Plate 1)laying eggs in the same nest and new queens are produced regularly, but most of them are killed and never allowed to produce eggs. Stingless bee queens can provide 10-100 cells with eggs a day, depending on the species. And the fertile eggs from the queens develop into worker bees (Plate 2) and queens. The worker bee task is age dependent and from emergence to 5 days the worker bees brood and nurse the eggs. From 5 days old to 10 days old, the worker bees clean and guard the hive and from 10 days upwards, they turn to foragers. Drones come from unfertilized eggs from the queen, or from egg laying workers. The drones mate with the queen and have no significant role in maintaining the functional system of the colony. After 10 to 15 days, the drones leave their parent colony forever (FAO, 2009).
Stingless bees often need large pre-existing cavities to refuge their heavilypopulated colonies. Many species depend on already existing tree holes;on the other hand, some occupy other substrata such as deserted ant and termitenests, underground cavities, rock cavities and cracks in walls (Roubik, 1989).
Stingless Bees and the environment
To ensure successful pollination and maximum crop production, diverse populations of wild bee species, the most important group of pollinators, are essential (Kremen et al., 2004).Globally, bees are the most important and effective pollinators and are often considered to play a key role within ecosystems (Kearns et al., 1998). Bees are the main pollinators of angiosperms (Bawa et al., 1990).Recent reports have pointed out that most pollinator populations have declined to levels that cannot sustain pollination services in agro-ecosystems due to increased disruption of habitats (Kearns et al., 1998).
Stingless bees can be found in the tropical and subtropical parts of the world where they occur sympatrically with the honeybees (Kajobe, 2007). They are said to have developed before the continents drifted apart from each other and thus the explanation for their presence in all tropical parts of the world (FAO, 2009).Some previous research has shown that stingless bee richness and abundance may be affected by a variety of local forest structures and conditions and overall habitat diversity, or local forest disturbance history (Eltz et al., 2002; Eltz, 2004; Samejima et al., 2004).
CHAPTER THREE
MATERIALS AND METHODS
The research project was carried out in the Laboratory for Systematics and Ecology of Arthropods, Department of Zoology, Obafemi Awolowo University, Ile-Ife.
Collection of Stingless Bee Workers
Adult worker stingless bees (Meliponulabocandei) were obtained from adequately fed, healthy, disease-free colony at the biological garden, Obafemi Awolowo University Campus, Ile-Ife, Osun State, Nigeria (Plate 3). Wooden cages of 42cm by 30cm by 30cm in size (Plate 4) and, hoarding cages each of size 11.5cm by 14.5cm by 7cmwere used for the collection of the bees on the field and for experiment in the laboratory. The size of test cages is such that is appropriate for the number of beesper experiment replicate. The bees collected from the field were sorted into the plastic hoarding cages in the laboratory and acclimatized for 24 hours.
Toxicity Test
Range Finding Test
This is a preliminary test that is usually conducted with a dose range of factor 10 in order to determine the appropriate doses for the formal test. It is usually conducted to identify the concentrations causing mortality (Garcia, 2004). For the test, the following concentrations 0.1, 1.0, 10.0, 100.0, 1000.0 mg of imidacloprid per litre of the solution were used. The bees collected were starved for two hours before sorting them into hoarding cages so that they will all feed on the treated sucrose solution. Ten foraging stingless bees were placed in each hoarding cage in three replicates. Each cage was labelled for easy identification. Following this, they were exposed to the range of doses of the test substance dispersed in sucrose solution in water with a final concentration of 500 g/l (50 % w/v).
CHAPTER FOUR
RESULTS
Mortality
The highest mortality recorded was observed at the highest concentration (4.0 mg/L) with mean percentage value of 83.3% mortality at the end of 48 hours while the lowest mortality observed was recorded at control with the mean percentage value of 3.33%. Mortality of stingless bee (Meliponula bocandei) varies significantly across the different concentrations (F=12.366, df=27, P < 0.05, Fig. 2). Mortality increased over the course of time but was not significantly different between the durations of exposure (i.e., 6, 12, 24 and 48 hours) (F=1.109, df=23, P>0.05). The LC50 recorded at 24 and 48 hours with their confidence intervals are reported in Table 3.
CHAPTER FIVE
DISCUSSION
Toxicity Effect of Imidacloprid on Stingless Bee (Meliponula bocandei)
In this study, stingless bee, M. bocandei showed obvious symptoms of poisoning, such as shaking and tremors, uncoordinated and uncontrolled movements, staggering, inability to take up a correct position of the bodyand prolonged frenetic movement of the legs. These symptoms were observed by Laurino et al. (2011) in honeybees exposed to various concentrations of acetamiprid, clothianidin, thiametoxam and thiacloprid in field concentration, also vomiting was observed in honeybees exposed to these class of neonicotinoids.After imidacloprid application, the German cockroach,Blattella germanica, apparently exhibited comparable behavior, that is, early symptoms of poisoning, including hyperresponsiveness, hyperactivity, and tremors of the body and legs followed by ataxia (Wen and Scot, 1997).
Imidacloprid is a class of neonicotinoids that is systemic on plants and are mostly employed to prevent pest that primarily suck and feed on plant parts. Incidence of neonicotinoid insecticides in causing bee mortalities has been reported in many countries (Greatti et al., 2003 and 2006 Colin et al., 2004). Neonicotinoids permanently bind to nicotinic receptors of acetylcholine, blocking them and consequently the passage of nerve impulses (Suchail et al., 2001; Tomizawa and Casida, 2005). This was observed as mortality increased as the concentration of imidacloprid used for the test was increased.
Mortality observed for the stingless bees exposed to imidacloprid was relatively fast. At 6hours of exposure, mortality was already > 20% in the bees exposed to the pesticides and toxic effects progressed over the course of time. According to Suchail et al. (2001), bee mortality induced by acute exposure to imidacloprid begins to appear generally 4 hours after intoxication and increases progressively during 100 hours; however the symptoms of intoxication appear more rapidly. Although, the observation made by Suchail et al. (2001) was on honeybees, the mode of action of imidacloprid allows control of the insects that attack the roots and the neck as well as feeding on the aerial part of the plant. Acting on contact, neonicotinoids are particularly suited for controlling many insects with biting and sucking mouth parts especially if swallowed. In this study, the imidacloprid LC50 value obtained in M. bocandei after oral application is 1.50 mg/L at 24 hours and at 48 hours it is 0.896 mg/L. These showed that LC50 of imidacloprid is duration dependent and the toxic effect of imidacloprid may increase with time.
Imidacloprid, thiamethoxan, thiacloprid increased mortality after contact exposure in Nannotrigona perilampoides a species of stingless bee (Valdovinos-Núñez et al.,2009). Mortality increased by ingestion and survival was reduced after ingestion exposure. Also,Melipona quadrifasciata was exposed to Imidacloprid and spinosad, both pesticides increased mortality but imidacloprid impaired respiration rate and flight, group activity was increased after imidacloprid exposure while spinosad harmed worker flight (Tome et al., 2005).
Sublethal Effect of Imidacloprid on Stingless Bee
Sucrose Consumption
Imidacloprid induced no sublethal effect on stingless bee, M. bocandei sucrose consumption, at 24 hours and 48 hours after exposure to various concentrations of imidacloprid and the result obtained showed that amount of sucrose consumed by M. bocandei was not affected by either low or high concentrations. This result is in accordance with Decoutye et al. (2004) where during the treatment period (i.e., 11 days) for the nine tested pesticides, the volumes of syrup consumed for control (from 22.0 to 45.2 ll/bee/day) and pesticide-treated groups (from 23.6 to 44.7 ll/bee/day) were not significantly different.
Conclusion
This study demonstrated the toxic effect of imidacloprid to M. bocandei. At a temperature of 22oC, Imidacloprid caused high mortality through ingestion on M. bocandei at 4.0mg/L, although at the lowest concentration used for the experiment, mortality recorded was low but repeated exposure may lead to high mortalities over the course of time. Generally neonicotinoids were used to replace other insecticides that are not selective and highly toxic to bee, this result has shown that imidacloprid may have minimal sublethal impacts on sucrose consumption in the stingless bees,M. bocandei.
Recommendation
Futher studies should be conducted to determine the direct and indirect effects of imidacloprid on stingless bee M. bocandei and also, the sublethal effects on constant exposure to this insectictide should be investigated.
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