Factors that make it difficult to control the leprosy mite

In recent years, many citrus growers have reported difficulties in managing the citrus leprosy mite in their orchards, observing shorter periods of control of the mite population after the application of acaricides and having to carry out additional applications of acaricides during the year to obtain good results. The objective of this text is to point out and analyze possible factors that are contributing to the increase in the citrus leprosy mite population in orchards and making its control difficult.

Until the early 2000s, control of the leprosy mite was based on the principles of Integrated Pest Management (IPM), in which the control action or application of the acaricide was determined by the population density of mites present in the field and also by the historical the presence of the citrus leprosis virus, evidenced by the presence of lesions of the disease on the plants. The level of control action, or mite infestation for the application of acaricides, adopted by the citrus grower depended greatly on his aversion to risk and varied between 1 and 10% of the organs sampled with the presence of the mite, and in cases of absence of disease lesions in the sampled area in up to 15% of the sampled organs. To estimate the level of leprosy mite infestation in the plot, sample monitoring by zigzag or systematic walking was carried out every 7 or 10 days, on 1 to 2% of the plants in the plot, observing each plant the presence of the mite on 3 to 5 fruits or, in the absence of fruits, on the first 30 cm of branches. Once the action level was reached, the application of acaricides was carried out observing the rotation of products from different chemical groups available and in high volume. Thus, a long period of control of the leprosis mite was achieved, that is, over 300 days between the application of the acaricide and the moment in which the mite infestation reached the established level of action again. With this management, generally with 1 to 2 applications of acaricides per year, no leprosis epidemics were observed in the orchards. So the question arises: What changed from the early 2000s to the present day that has made controlling the leprosis mite more difficult? Is the climate more favorable to the multiplication of dust mites? Has mite monitoring changed? Has the citrus grower's reaction time to spray the infested field changed? Is acaricide application technology more flawed? Have orchards changed and make mite sampling and acaricide applications difficult? Acaricides available on the market are not as efficient as previous acaricides? Is the leprosy mite population resistant to the acaricides used? Does mixing acaricides with other products reduce the effectiveness of stroking? Is managing other pests affecting mite control and behavior?

Climate and leprosis mite

The reproduction of the leprosy mite increases significantly during dry and hot periods of the year, when there are long days, high temperatures, low rainfall and low relative humidity, and, consequently, low water availability in the soil. Work carried out at FCAV/UNESP showed that in plants maintained with soil water availability at 25% of field capacity for 60 days, the leprosy mite population grew twice as much as in plants maintained at 70% of field capacity. Furthermore, high temperature and low relative humidity favor the evaporation of droplets, especially fine and very fine drops, during acaricide sprays, reducing the coverage and deposition of the acaricide mixture applied to the plants.

These weather conditions were most recently observed in 2013 and 2014, and not coincidentally were the years with many reports of leprosy mite control problems.

Leprosis Mite Monitoring and Sampling

As mentioned, monitoring the leprosy mite population is an essential tool to support taking action to control it. Sampling, as previously recommended, incurs estimation errors above 60%, and it is common for different inspectors, sampling the same field on the same day, to rarely reach similar estimates. Furthermore, this sampling does not consider the number of fruits on the plants, that is, the same number of fruits and/or branches are sampled per plant regardless of the quantity of fruits on the plant. To reduce these sampling errors, both the percentage of plants sampled per plot and the number of fruits and/or branches sampled per plant should be increased, taking into account the load of fruits present on the plant. The interval between samplings should also be reduced or the number of inspectors in each sampling should be increased, to take an average between them.

However, with the increase in labor costs for pest inspection in the field, observed in recent years, the inspection protocol is moving contrary to what is recommended and is becoming less and less intense. Today, it is common to observe intervals between inspections of more than 14 days in orchards, and not uncommonly 30 days. Additionally, a reduction in the sample size per plot is also observed, with less than 1% of the plants sampled and with the observation of fewer fruits and/or branches per plant. All of this leads to greater errors in estimating the mite population density and to detection that is often well above the stipulated level of control action.

Time between mite detection and acaricide application

The MIP provides that once the level of leprosy mite control action is reached in the field, the application of the acaricide will be carried out immediately. However, what is observed in orchards, in an attempt to reduce production costs, is that the number of equipment and sprayers is increasingly leaner and, often, there are no machines available to apply the acaricide when the acaricide is detected. control action level. Therefore, it is common for it to take more than two weeks for the control measure to be adopted after observing the level of control action. This means that at the time of application of the acaricide the mite population is already at a density above the level of control action and, even if the acaricide has the same mite control efficiency (causes around 90% of mite mortality ), the residual mite population that did not die during the application of the acaricide will be larger and, consequently, it will take less time to reach the level of control action again, resulting in a shorter period of mite control by the acaricide.

Miticide application technology and leprosis mite control

For good control of the leprosy mite, the acaricide must be applied in such a way as to have a good deposition and coverage above 50% on the fruits, branches and leaves located internally on the plant and throughout the crown of the plant (skirt, middle and top). Until the early 2000s, it was common to use a spray volume of around 8 to 10 thousand liters per hectare in orchards with adult plants. Aiming to reduce application costs while maintaining the same control efficiency, research was carried out to adapt the sprayers and the volume of acaricide spray applied, increasing the number of nozzles in the spray branch and using nozzles that produce fine drops. between 100 and 200 µm in volumetric median diameter for greater droplet penetration inside the plant crown. It was concluded that using this droplet size, acaricides applied in volumes between 100 and 400 mL/m3 of canopy have the same efficiency in controlling the leprosy mite without the need to correct the acaricide dose. Since then, good control of the leprosy mite has been achieved in adult orchards with volumes between 2 and 3 thousand liters per hectare.

However, it is not uncommon to see citrus growers reducing the volume of acaricide syrup simply by increasing the speed of the turbo sprayer or changing the working pressure without adopting the technology in the correct way, with adequate calibration and adjustment of the sprayer and appropriate choice of the number of nozzles and size. of drops.

Densification of orchards and control of the leprosy mite

In search of greater productivity (tons per hectare), mainly in the initial stage of orchard formation, and thinking about maintaining a plant stand that allows good productivity even with the elimination of plants with greening (HLB), from 2005 , there was a trend towards increasingly denser orchards. Orchards planted in 2005 had an average of 440 plants per hectare, while orchards planted in 2016 had an average of 719 plants per hectare.

The density of orchards in the planting line makes it difficult for pest inspectors to move from one street to another, hindering mite inspection, and increases contact between plants, which facilitates the movement of the mite from one plant to another. The density between planting lines makes it difficult to achieve good coverage of the acaricide applied, as the plant canopy is very close to the spray nozzles, often even touching the nozzles and causing uneven coverage of the applied spray, which leaves areas of the canopy without the acaricide applied. Ideally, the spray nozzles should be at least 40 cm from the edge of the plant canopy to ensure good spray fan formation.

Mite resistance to acaricides

Successive applications of the same acaricide molecule accelerate the process of selecting resistant mite populations, being one of the factors linked to failures in mite control in orchards. There are reports of leprosy mite resistance to the acaricides hexythiazox, dicofol, propargite and flufenoxuron. Therefore, rotation of acaricides from different chemical groups and mode of action is recommended for leprosy mite management.

However, today there are basically two groups of acaricides on the PIC List with good efficiency in controlling the leprosy mite, spirodiclofen and cyflumetofen, which makes an acaricide rotation program very difficult.

Although there are still no reports of leprosy mite resistance to these two acaricides, mite mortality above 90% in different mite populations in the state of São Paulo in laboratory tests, the selection of resistant populations with continued use of the same acaricide may occur in the future and the need for new acaricides is urgent.

Mixing acaricides with other products in the spray tank

As a way of reducing production costs, product mixtures in the spray tank are common in orchards and can also affect the effectiveness of acaricides, mainly due to changes caused in the pH of the spray solution and electrical conductivity, as well as possible incompatibility between the compounds.

Furthermore, after the detection of HLB in Brazil, it became common practice to add insecticides to control the psyllid. diaphorina citri in all spraying operations (application of foliar fertilizers, fungicides and acaricides). Research carried out at FCAV/UNESP found that the foliar fertilizers potassium phosphite, magnesium sulfate and the mixture of zinc and manganese chlorides with magnesium sulfate resulted in a decrease in the effectiveness of the acaricides propargite and acrinathrin on the leprosy mite. They also observed, under laboratory conditions, that the mixture of the insecticides phosmet and imidacloprid with the acaricide spirodiclofen, although not showing physical or chemical incompatibility, reduced leprosy mite mortality seven days after application by between 28 and 44%. A lower efficiency of the acaricide in leprosy mite mortality will result in a shorter period of control of the applied acaricide.

New research with other combinations of mixtures of acaricides and insecticides is being carried out in the laboratory and in the field by FCAV/UNESP and Fundecitrus and, soon, it is expected to have a clear idea of ​​which mixtures with insecticides can affect acaricides in controlling the mite of leprosis. Therefore, the recommendation is that other products should not be mixed with the acaricide solution to control the leprosis mite.

Management of other pests and diseases in leprosy mite control

The growing incidence of important diseases, such as early blight and citrus canker in the São Paulo citrus park, has considerably increased the use of fungicides or copper-based products to control them. Likewise, the need to control the HLB psyllid from 2004 onwards significantly increased the use and frequency of application of insecticides in orchards. This also directly affects the management of the leprosy mite, considering that many insecticides and fungicides have an effect on natural enemies (entomopathogenic fungi and insects) that help control the leprosy mite and other pests that were previously secondary, such as mites. defoliating tetranychids and flour scale cochineal.

In addition to the effect of insecticides on natural enemies, the hormesis effect caused by the application of subdoses of insecticides, mainly pyrethroids and neonicotinoids, is widely reported in the literature, stimulating reproduction (increased egg laying and viability) or reducing mite mortality in diverse cultures. Although not yet proven for the leprosy mite (studies are ongoing), it is believed that this hormesis effect may be occurring due to successive applications of insecticides to control the citrus psyllid. The doses of insecticides used to control the psyllid do not cause mortality in the leprosy mite, but could stimulate its oviposition, increase the viability of the eggs or their longevity, which would result in an increase in the rate of reinfestation of the orchard after the application of the acaricide. If this hypothesis is confirmed, the problem will be very difficult to overcome because we still cannot give up intensive control of the psyllid in the management of HLB.

In conclusion, there is no single factor responsible for the increase in the difficulty of controlling the leprosis mite, but rather a combination of factors that must be taken into account and corrected to allow for an improvement in the management of the leprosis mite and the disease it transmits.