Vincentians could experience shortage of citrus fruit

by Rafique Bailey Ph.D. Entomology

Introduction

Almost four years after it was first detected in St Vincent and the Grenadines, the Asian citrus psyllid Diaphorina citri Kuwayama, which transmits the huanglongbing disease or citrus greening, is already having an impact on the citrus farmers here.{{more}}

I, Dr Rafique Bailey, examined citrus trees in different locations throughout St Vincent; I found many trees are showing symptoms of the huanglongbing bacterial disease, with some already dying or underproducing. If nothing is done to remedy this situation, there will be a shortage of some of the main varieties of citrus, including lemons (C. limon), rough lemon (C. jambhuri), sour orange (C. aurantium), grapefruit (C. paradisi) and limes (C. aurantiifolia) in St Vincent and the Grenadines.

The most important damage that Asian citrus psyllid conveys to the plant is vectoring the Huanglongbing bacterium Candidatus Liberibacter asiaticum which causes shoots to yellow and new leaf tips to twist or burn back. It also causes premature blossoming of the tree, abnormal leaf fall, growth of sooty mold on leaves and abnormally shaped fruit with bitter juice. Some of the symptoms of the disease can be found at http://entnemdept.ufl.edu/creatures/citrus/acpsyllid.htm. The disease appears to be widespread throughout the island and is already causing significant impact on the livelihoods of farmers. Field visits to various commercial farms in St Vincent, belonging to Richard Minors (Dauphney), Augustus Tesheira (Yamboo) and Huks Defreitas (Congo Valley) indicate that the insect and bacterium are already impacting heavily on their production.

According to Bob Tesheira, shortly after hurricane Thomas in 2010, he noticed some of his trees turning yellow and starting to look strange; he reported the matter to the Ministry of Agriculture whose officers

visited the farm, but did not do anything to remedy the situation. Mr Tesheira said his production went downhill from a maximum production of 10 thousand fruits per month to zero today.

So, what can we do to help to protect our citrus plant? We can start by examining the young shoots on each plant for presence of the psyllid. Yellow sticky traps can be placed in trees to help monitor, as well as reduce populations of the psyllid. We can try a number of things and see which one works, like regularly spraying the plant with soapy water, spraying with natural insecticides like pepper and garlic sprays, spraying with neem oil or bioneem/neemix, horticultural oils (including coconut oil), or an emulsion of soapy water and oils. We can also try to increase the plants’ resistance by applying compost or biochar, which will help to condition the soil and create an enabling environment for the proliferation of beneficial microorganisms that could suppress the bacteria. Another option that might be helpful is to use microbial fertilisers, such as mycorrhizal fungi and phosphate solubilising bacteria, that can provide nutrients to the plants, as well as help to suppress disease forming microorganisms.

Biological control is the most effective, environmentally friendly way of suppressing populations of the Asian Citrus Psyllid. The parasitoid Tamaxa radiata was effective in suppressing Diaphorina citri in Réunion Island in 1978 and later in Mauritius. It also prevented the transmission of the bacterium Candidatus Liberibacter asiaticum. T. radiata was imported into California in 2010, to counter the Asian Pysillid that was detected there in 2008 and posed a significant threat to their citrus industry. T. radiata was screened for hyperparasitoids and its risk to the environment evaluated. It was subsequently released in 2012 in citrus orchards in California.

Chemical control involves the use of contact as well as systemic insecticides. For a fast knock down, a pyrethroid based insecticide can be used which is broad spectrum and toxic to all stages of the pest, such as Dimethoate or Zeta-Cypermethrin. For a more persistent effect, the soil drench Imidacloprid or Thiametoxam can be used with some caution as these are harmful to bees. In St Vincent, chemical control should no longer be pursued vigorously at this point since the psyllid would have already spread throughout most of the island and biological should be the preferred method.

Conclusions and Recommendations

The Asian citrus psyllid and the bacterium Candidatus Liberibacter asiaticum have joined the band of invaders or Invasive Alien Species (IAS) that has arrived in St Vincent in recent years; some of the others include the Pink Hibiscus Mealy Bug, the West Indian fruit fly, the Mango Seed Weevil, Red Palm Mite, Black Sigatoka, Red Lionfish, Cycad Scale and recently the virus Chikungunya.

In my opinion, the management of these invasive species by the Ministry of Agriculture has been poor, much to the detriment of the farmers. For example, the Citrus Psyllid was allowed to reproduce and increase its population over a number of years, to the point where it has now spread throughout most of the island and is having a significant impact on citrus production.

The Black Sigatoka was virtually left to blow out of control where it decimated an already ailing banana industry, with farmers losing millions of dollars in investments. The same can be said of the pink hibiscus mealy bug, whose population has been allowed to increase unchecked for many years and is now costing the traffickers and farmers thousands of dollars from the rejection of over 2,000 boxes of plantains and bananas shipped to Trinidad during the months of April and May, 2014. As regards the other invasive species such as the West Indian Fruit Fly, Mango Seed Weevil and Red Palm mite, I would like to know the status of these pests.

Clearly, there are many gaps to be addressed, not just about the management of invasive species, but as regards the agricultural sector, on a whole, in St Vincent and the Grenadines and I would be writing about these subsequently.

As regards the management of invasive species, St Vincent does not yet have a National Invasive Species Strategy, despite the increase in the number of foreign species entering the country; the protocols for importing seeds, plants, animals, pets, foods and construction materials, such as lumber and sand, need to be revised. It is not clear if we are signatories to the Ballast Water Convention; this needs to be looked at. There needs to be more effective monitoring at ports by quarantine officers and on sea by the coast- guard. There is an urgent need to restart a Classical Biological Control programme so that the populations of the Citrus Psyllid and Pink Hibiscus Mealy Bug could be brought under control; these natural enemies would have to be brought in, screened for any undue risk to the environment and released.

There is also a lack of public awareness and education as regards to IAS issues; these issues should be mainstreamed across all sectors. Assistance should be sought from organisations that are experienced in managing invasive species, such as CABI, which has just recently executed the project “Mitigating the threats of Invasive alien species in the Caribbean,” from which five Caribbean countries benefitted.

Any comments and questions can be emailed to rskbailey@hotmail.com.