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The Asian Citrus Psyllid is referred botanically called Diaphorina citri Kuwayama (Hemiptera: Psyllidae) is a pest of citrus and close relatives of citrus. Together with the African Citrus Psyllid Trioza erytreae (del Guercio) are the two most dangerous citrus pests in the World (Bronson & Gaskalla par.1).
Diaphorina citri Kuwayama causes extreme damage to the crop by spreading pathogens that cause greening or the yellow dragon disease to the citrus plants. The disease can also be referred as the yellow shoot disease because the shoots of the citrus plants become yellow in color. Furthermore the disease also causes chlorosis as if the plant is lacking the zinc nutrient, the twigs are affected and there is reduced fruit size and the productivity declines. The fruit color is not well developed and this to the greening effect hence the name greening disease (Bronson & Gaskalla par.2).
Buitendag & Broembsen argue that the interaction between the vector and the pathogen is not well known and range from 30 minutes for the Asian Citrus Psyllid and 24 hours for the African Psyllid hence the Asian pest is more lethal. The Asian Citrus Psyllid have been a menace to farmers in South Texas (p.269). The pathogens are believed to multiply in the vectors hence increasing in numbers and causing havoc to the citrus plants (Aubert 150). The adult Asian Citrus Psyllid (ACP) are small ranging between 3 to 4 millimeters and have brown wings. These insects are very active and move very fast. The eggs are bright yellow in color and are laid on the feathery flush. Nymphs are either green or dark orange and feed on the leaves and also the stem and can be hard to find in a field unless one is keen (Bronson & Gaskalla par.4).
Citrus is a universal terminology employed to refer to a number of cultivated plant genus belonging to the Rutaceae folks such as Grapefruit, emerald, oranges and lemon. The US produces about forty percent of the collective grapefruit produced globally; this includes both processed as well as fresh fruit bazaar. The states in which this crop is grown include Florida, Arizona and South Texas. This research will be based on South Texas whereby the pest is a menace and has caused poor production in citrus products (Evans et al 126).
Total acreage of commercial Citrus in Texas is at twenty seven thousand acres quantified at a fiscal value worth one hundred and sixty million dollars (Rosson 1). The initiation of citrus production in Texas stretch back in the 1849, when the earliest citrus trees were planted near Brazoria (Anciso 162).The concentration of commercial citrus in southern Texas is due to the regions fertile alkaline soils besides to subtropical environment that are moderately appropriate for citrus construction (Sauls 254).
Nevertheless, owing to the lack of adequate rainfall, almost all of citrus orchards in the LRGV are irrigated, with most of the water acquired from the Rio Grande River (Uckoo et al 25). Roughly 35 natural species are witnessed feeding as well as causing some echelon of spoil in Texas citrus. In the midst of these pests, the ACP is regarded the most detrimental one. In a recent grower survey, ACP has been ranked as the precedence nuisance by growers affecting the production of citrus world wide. The natural enemies of ACP are useful mainly in reducing the population of the pest. These natural enemies are important in the control of the Asian Citrus Psyllids because they are better than controlling then using chemical which in pollute the environment and are known to cause side effects to the people who consume the citrus fruits (Guun & Stevens 190).
2.0. Justification of The Study
2.1. Economic Importance of Citrus
United States is one of the major producers of citrus fruits in the world. It has almost six hundred and twenty one acres of citrus groves which are consumed locally as well as globally. For the total citrus production in the USA, Florida produces 60 percent followed by California 28 percent, then Arizona and Texas four percent. For instance, in 2005 approximately there were forty two thousand commercial fruit farm providing citrus fruits to all states within USA and roughly one hundred and forty countries worldwide. The leading constituent was oranges at 800 million US dollar grapefruits second with an overall value of 174 million US dollar and lastly temple oranges and tangerines. The economic benefits of citrus production in the United States go further than the value of the fruit. The by products like citrus pulp, Molasses and D-Limonene are vital and are used in other ways hence their economic importance can not be underestimated. D-Limonene is significant oil extorted from the peels of citrus fruits and seeds and it is utilized in the cleaning of flavorings and colognes, (Florida Department of Citrus Economic and Market Research Department 86).
2.2. Source of Raw materials
The production of citrus fruits in the United States has a significant impact on other firms including utility industries, transport and warehouse firms, finance and insurance in addition to tax revenues (sales and property taxes). The United State’s citrus industries have a great value on the overall economic growth annually of eight seven billion dollars and offers job opportunities to 769,224 people in the country. In addition, citrus industries has improved the livelihood of people especially the residents of Florida, supports in the conservation of the environment through purification of air, food security.
2.3. Ecological and Aesthetic Benefits
Moreover, citrus orchards have proved to offer sites for rainfall to ooze into the ground and restock the aquifers. Contemporary citrus groves are designed in a manner that is important in the conservation of innate wetlands and generate huge on-site water preservation areas to incarcerate storm-water runoff. Additionally, these sites provide outstanding habitat for wildlife that contribute to overall growth of the economy in form of tourists’ attraction both locally and internationally. Undoubtedly, citrus production is a vital facet in the USA economy.
Lamentably, many factors have contributed to loss of production. Among them includes greening disease which was introduced through infected plants with the pest commonly called ACP. The disease has greatly reduced production levels for instance in 1997 there were eight hundred and fifteen citrus plants generating more than 13 million tons of citrus fruits but this greatly reduced and by almost half in 2006. The south Texas’s farms were highly affected and the blame was pointed on the ACP increase in population. To control this pest is a clear indication to solve the problem of poor production in the farms in the South Texas. Thus this research is aimed at promoting biological pest control instead of relying on the pesticides which are not environmentally friendly and also expensive to undertake.
3.0. Literature Review
3.1. History of Asian Citrus Psyllids
Initially, the Asian Cytrus Psyllid was known to be in the tropical and the sub-tropical area like Asia, Saudi Arabia, Mauritius, Afghanistan, Mexico, Reunion, Central America, the Caribbean, and South America.
However, this pest was found in the United State’s Florida state in June, 1998 in the Palm Beach County (Halbert et al 330). In the state of Texas, its introduction was an accident. It got to the Rio Grand Valley on potted nursery stocks of the plant ‘orange jasmine’, which had been imported from the state of Florida (French et al 15). Due to this, the current fear is that it might end up invading other states especially the state of California, especially from Florida, Mexico or very likely from Asia being as a result of the recurrent interruption of the transportation of plant materials from Asia- especially the citrus plants and so on.
3.2. The Destructive Nature of the Pest
Due to the destructive nature of the Asian Citrus Psyllid, many agricultural organizations have come up with ways of the management of the pest. South Texas, whose citrus orchards make a considerable contribution to the economy of Texas -$ 200 million. The state also has to be keen on the management of the pests since it has already invaded the farms. The concentration of the orchards is in the Lower Rio Grande Valley (Sauls254), where the pest was introduced. The research on the diversity of the natural enemies of the Asian Cytrus Psyllid and their effect will supply good information for the management of the pests, hence boost productivity.
3.3. The Pest Life Cycle of Asian Citrus Psyllid
Its life cycle like that of the grasshopper is divided into three main stages that is egg, nymph and adult. The eggs are laid on growing young shoots and between the leaves. Females normally lay about 300 to 800 eggs during their life time. Nymphs undergo five developmental stages before attaining maturity. The total life cycle requires between duration of half a month to about 47 days depending on the surrounding environmental factors like temperature and seasonal variation. Populations are always low during the winter and and dry seasons and peak during summer. There are 9 to ten generations in a year although more than this also been observed in the field cages (www.hungrypests.com par.7).
3.4. Enemies of the pest at different stages of its life cycle.
There are different enemies to this pest at its different stages. There are those that destroy the egg, those that attack the nymph and those that attack the adult Asian Citrus Psyllid. The enemies range from bacteria and fungus to parasitic insects and beetles. Some of the enemies do affect the plant and cause a lot of harm to it for instance the bacteria. Some completely destroy the pest, but do not cause any harm to the plant, hence are recommended for the control of the pest.
3.4.0 Enemies at the Egg/nymph stage
There are insects that largely feed on the nymphs hence denying them a chance to mature. The most common wasp that are known for attacking the nymphs of Asian Citrus Psyllid pest are referred to as hymenophterous parasites. They are two tiny wasps namely Tamarixia radiata (waterson) whose origin is Taiwan and Vietnam; and diaphorencytrus aligarhensis (shaffee, Alam and Agarwal) whose origin is Taiwan. The adult Tamarixia radiata lays eggs underneath an Asian Cytrus Psyllid nymph and when the egg hatches, it feeds on the Psyllid and kills it. Upon maturing, the adult wasp emerges from underneath the dead nymph or it chews its way out, thus leaving a hole (McFarland & Hoy 227-230). The above is illustrated by the figure below:
There are two nymphal lady beetles that are popular predators of the nymphs of the pest, Asian Citrus Psyllid. They are referred to as Harmonia axyridis (Pallas) and Olla v-nigrum (Mulant). These, while they are at both nymphal stage and adult stage, nourish themselves on the nymphs of the pest thus immensely reducing their population. The Harmonia. axyridis is known for its predatory habits of feeding on aphids of various species.
Normally, it consumes up to 370 aphids through the larval stage, depending on the species of aphids it is consuming (Hukusima & Kamei 55). As it develops, the amount of food consumed increases, translating into reduction of the population of the Asian Cytrus Psyllid (Miua & Nishimura 145). It also has a considerably long life span, and the females are known to feed more than the males.
Citrus greening or Huanglongbing (in Chinese) is a disease that is dreaded by many citrus orchard farmers (Bove 7). It not only affects the leaves, but proceeds also to attack the fruits. The bacterium that causes this disease is referred to as Liberobacter asiaticum and it is parasitic. The bacterium stays in the cells of the plant, mainly the phloem cells (Hoy, Nguyen & Jeyaprakash 2). In Asia, the vector for this parasite is the pest Asian Cytrus psyllid (Halbert and Manjunath 330). It survives in the presence of its host, the pest, Asian Citrus psyllid. However, the details of how the co-existence brings about the deadly greening disease to the plants are still unclear. The only information proven is that the Asian Cytrus Psyllid in contact with a plant that is contaminated, takes approximately 30 minutes to contract the pathogen.
The pathogens survive through transition. They find their ways in the body of the pest and spread to the various developing organs of the pest. The bacterium mostly goes to the salivary glands so as to prepare for transition as the pest feed. The nymphs are the most affected since the bacterium finds an easy time spreading because of the development process. However, if they land on adults, then may not be very lucky to survive because the maturity span is longer and the adult may be already dead. The adults that feed on infected plants are less likely to be affected compared to the you ones that develop feeding on these affected plants because the development of the bacteria spreads more as the pest develops, something unlikely in the old already developed adults (Halbert and Manjunath 332)
Particular fungus is also classified among the pathogens that are enemies of the pest. One of the funguses studied is referred to as Hirsutella citriformis Speare. The study was carried out in Quadeloupe in the year 1998, January. However, the fungi are believed to work under condition that the relative humidity of the area where it is being used is high. Further studies conducted in Florida proved that when the pests were exposed to these fungi, they showed evidence of the presence of diseases in them. These showed that the fungi were affecting the pests negatively and therefore were capable of aiding their elimination. Through this experiment, we learn that the exposure of the pest to a certain species of the fungi - Hirsutella citriformis Speare makes their genetic data distort and increases their mortality rate.
3.4.4. Enemies at the adult stage
They include small insects that predate on them, and man. At an adult stage, there are those enemies that are similar to those that attack the nymphs. A large majority of the pests can be fed on by wide-ranging arthropods like hunting spiders namely Aranae: (oxyopidae, anyphaenidae, salticidae and clubionidae species), whiteflies, minute pirate bugs, lacewings Neuroptera: hemerobiidae, chrysopidae species, scales, syrphids or hover flies, Diptera syrphidae, or mealy bugs like Hemiptera: anthocoridae species (Michaud 1). Others are fed on by the coccinellid beetles that pose as the greatest predators, because they attack the pests at both stages- nymph and adult stage. Wherever they are present, the population of the pests goes considerably low. However, due to excessive use of insect-killing chemicals, the chances are that the pests can easily get a chance to increase rapidly in population because the predators are done away with.
22.214.171.124. Human beings as natural enemies
Known to humans as destructive, the Asian citrus psyllid is seen as something that needs to be eliminated. As such, the elimination of the pest is a survival gesture. The human beings have developed different programmes to manage and control the pests so that they can check destruction and increase production. There are several methods employed by man in the attempt to eliminate these pests.
Chemical compounds have been developed to eliminate these pests, thus posing a great danger to their survival. In addition to the chemical elimination, the trees infested by this pest are usually pruned and destroyed by burning them, so that the chances of reproduction and spread of the pest are eliminated. This considerably affects the population of Asian Citrus Psyllid. In addition to this, human beings can also take to crushing the psyllid physically hence eliminating them.
All the other enemies of the Asian Citrus Psyllid are being utilized by man to ensure elimination, and enhance production of citrus plant products. In fact, parasitic, fungal and predatory enemies are introduced to area infested by these pests to ensure population reduction if not total eradication of the pest. More studies are underway to come up with a more effective biological method of reducing and eventually eliminating the Asian Citrus Psyllid, as it is considered one of the biggest threat to orchard farming.
126.96.36.199. Toxic Plants
No particular plants are known to be poisonous to the Asian Citrus Psyllid. However, in an attempt to ensure the management of these pests, methods have been devised to intoxicate the citrus plants on which these pests feed. There are both foliar and soil treatments that cause the intoxication. When compounds like the neonicotinoid- imidacloprid is applied to the soil, it goes to the sap of the plant and is therefore responsible for the elimination of various plant pests that suck sap from the plant (Boina, Onagbola, Salyani & Stelinski 2). When the concentration is high enough, the Asian Citrus Psyllid dies off immediately from over excitation. When the concentration is not so high to the point of killing the pests, then it affects the fertility rate of the pests, the life span and their productiveness.
The treatment of the soil, though long term, is not as fast as that of the leaves. Insecticides sprayed on the leaves of the plants get more instant results. However, this is discouraged mostly because it affects the parasites that help in the management of the population of the pests.
3.5. Methods and Materials
3.5.0. Sampling Methods
Record Natural Enemies
Select six (sites) citrus groves (80 x 60m) of young grapefruit and orange trees between 2 and 4 years old 3 to 6 feet tall with new flush for visual observations (200 planted trees in plot)
Record average temperature, draw a map of its location on the property, record GPS location grower’s field identification number and dimensions of trees of each study site
Direct sampling observations at each site for 20 minutes on a weekly basis
Using an optic visor and hand lens observe for any visual predators feeding on ACP, including ACP, eggs and nymphs (beetles, spiders, lacewings-if feeding do not record species) log in data into spreadsheet
Transport any predators found in a plastic container at 40°C to the Texas University Inspectory for identification analysis
Determine Impact of Parasitoids (parasites) on ACP
From one random citrus orchard (site) randomly select and label ten fresh flushes infested with nymphs; remove and collect by hand
On selected site (to avoid any wilting) immediately separate and place each flush in a pre prepared aqua tube and place in Petri dish or tube holder
Transport Petri dishes or tube holders in an ice chest at 40°C to the Texas University Inspectory for observation of parasitoids
Record and count number of nymphs per flush microscopically; to evaluate parasitism of ACP nymphs
Place each Petri dish or aqua tube in a ventilated emerge tube
Label emerge tube / Petri dish with parasitized nymphs with a sample number/date/location for emergence; using VWR labeling tape
Store sample emerge tubes in Inspectory at 25°C on a rack; refill tubes with squeeze bottle as needed
After two weeks examine flushes microscopically for parasitized nymphs
Remove emerged parasites from the emerge tubes, record species of parasite live or dead on ACP
Preserve the parasites in ethyl alcohol for identification analysis
Determine the Impact of Natural Enemies to ACP: density (population) Exclusion Intrusion Experiment (how much are they contributing to control the ACP)
From a random citrus orchard (1 site) randomly select twenty fresh flushes from random trees infested with eggs
Label/tag with date and sample number from predetermined numbering system; ten protected with fine cloth sleeve (caged) and ten unprotected; flushes are to be randomly selected from low, middle, and high and from outer and inner parts of the trees
Record an estimate of predators and ACP eggs in the protected and unprotected flushes
After seven days examine the selected flushes for the presence of ACP eggs
Repeat process every seven days
Remove and collect by hand and transport 20 flushes to Texas University Inspectory
At the same citrus orchard (1 site) location select and label another twenty fresh flushes from random (different) trees infested with nymphs
Label/tag with date and sample number from predetermined numbering system; ten protected with fine cloth sleeve (caged) and ten unprotected; flushes are to be randomly selected from low, middle, and high and from outer and inner parts of the trees
Record an estimate of predators and ACP nymphs in the protected and unprotected flushes
After seven days examine the selected flushes for the presence of ACP nymphs
Repeat process every seven days
Remove and collect by hand and transport 20 flushes to Texas University Inspectory
3.5.1. Study Area
Observations will be carried out from early May to early October 2009, in citrus groves at the research field of the Texas. The sampling period will be designed to encompass the period of mortality rate on the Asian Citrus Psyllid (ACP). Twenty flush per grove will be randomly selected and inspected between 8:00 a.m. and 12:00 a.m. every 7 days for a total of 20 weeks. Visual predators feeding on ACP will be used for this experiment.
3.5.2. Experimental Site
The study will be carried out in citrus plantation situated at Rio Red Grape Grapefruit 2006.The site covers an area of 4/8 acreages with a total of 240 citrus tree and 30 tree per row. For regular practices three year grapefruits trees will be selected covering an area of 6/8 acres with 240 trees whereas for Valencia oranges also for regular practice 4 year old trees are the best covering an area of 10/13 acres with 390 trees in total. For conventional practice or research farms four year old citrus trees will be chosen. For Shimotsu farms two years ruby red grapefruits will be selected and finally for Monte Alto Farms two years Valencia Oranges will be ideal.
3.5.3. Field Observation
1. 1.7ml snap cap aqua tubes, (clear/amber and pre-sterilized), Plantariums single vials (7" x 1" diameter), Distilled water, Gampler’s 4-mil Disposable Nitrite Gloves, Para film M IS, Ethyl Alcohol, Zeiss West Germany Microscope, Net sleeves, VWR Scientific Label Tape, Hand Lens, Ice chest, and Ice packs.
3.5.4. Sampling processing
1. 1.7ml snap cap aqua tubes, (clear/amber and pre-sterilized), Plantariums single vials (7" x 1" dia), Distilled water, Gampler’s 4-mil Disposable Nitrite Gloves, Para film M IS, Ethyl Alcohol, Zeiss West Germany Microscope, Net sleeves, VWR Scientific Label Tape, Hand Lens, Ice chest, and Ice packs.
3.6. Exepected Results
Asian citrus psyllid population is expected to decline sharply after the introduction of different kind of natural enemies. This is because the natural predators shall have a deductive effect on the pest population. The various preadots such as the birds, beetles, spiders and other insects shall act synergistically towards the elimination of the pest. In the short run, predator population shall also be expected to increase due to abundance of food (the Asian citrus psyllid). However, this may be determined by the initial population of each kind of predator introduced in the experimental plots and the palatability of the pest to each predator.
However, the predator populations are expected to decline gradually as the pest population gradually declines. This expectation is based on the assumption that the pest forms the main source of food for all the different kinds of species introduced. Again, it shall also be assumed that the experiment shall be conducted in a closed ‘habitat’ system such that due to natural forces of organisms to adjust to ecosystem disequilibrium, non of the predators shall move out of the experimental area to look for food incase the pest population is too low to sustain them.
Worse still, a loss of biodiversity may occur within the experimental site if all the pest population is eliminated. This may lead to new findings of what the predators may turn to do in order to survive (for example, turn to new kinds of food sources, hibernate, migrate or so).
3.6.0. Statistical Analysis
Essentially, two kinds of data are expected to be obtained, qualitative and quantitative data. Qualitative form of data shall be obtained through making observations on both the specimens and experimental plots. Ecological changes, general population trends as well as predator behavior shall be observed and recorded.
Quantitative data shall also be collected in order to have enough information before making major conclusions of the study. Some of the figures that shall be taken include population of each predator at experimental launch and at end, estimated pest population at beginning and end of the experiment, relative consumption of pests for each predator among others. Statistical analysis tools such Excel spreadsheets and SPSS program shall be utilized. Again, various graphs that compare different factors and measurements shall be made. For easy understanding of the results, tables and graphical representations shall also be made.
This study is aimed at making critical conclusion points that shall form a basis for making essential agronomical decisions, especially in the control and management of asian citrus psyllid. The conclusion statement developed here may also form a basis for policy change or a change in agricultural extension approach, depending on their credibility and statistical evidence.
From the gathered literature, a number of recommendations have been considered essential. Natural predators as a mode of pest control is not 100% effective because of the negative ecological imbalance this may bring upon total elimination of the pest. The most important point to consider is to maintain the level of Asian citrus psyllid pest at a level that is not harmful to citrus fruit production. As a result, natural predator method should be intergrated with other modes such as chemical use and physical methods in order to obtain optimum results.
Secondly, there is need to do more research about the extend of the dangers and threats culminating from complete elimination of this pest. Technological advancement should also be incorporated with rgard to genetic engineering. By this, it is meant to conduct more research on possibility of genetically modified citrus fruit trees that are not only resistant to asian citrus psyllid pest, but also to drought and other challenges during growth.
Another study of the pest survival and environmental habitat should be done. This study will lead better understanding of the pest’s life cyle and the best stages of the life cyle to intervene in order to obtain optimum control while ensuring minimal ecological damage and financial losses.
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