Introduction

The purpose of this publication is to provide an in-depth profile of the lychee erinose mite. It is intended for the use of interested laypersons with some knowledge of biology as well as academic audiences.

The lychee erinose mite (LEM), Aceria litchii (Keifer) (Acari: Eriophyidae), is one of the most important pests of lychee (Litchi chinensis Sonn., Sapindaceae). This eriophyid mite pest is native to Asiaand has been reported in India (Sharma 1985), Pakistan (Alam and Wadud 1963), Bangladesh (Haque et al. 1998), Thailand (Keifer and Knorr 1978), China and Taiwan (Huang 1967), Hawaii (Keifer 1943), and Australia (Pinese 1981) (Figure 1). More recently, LEM was found in Brazil (Raga et al. 2010; Fornazier et al. 2014), where it has spread to all major lychee-producing areas and has caused an estimated 70–80% yield reduction and a 20% increase in production costs (Navia et al. 2013). Prasad and Singh (1981) also reported an 80% yield reduction in India caused by LEM. In February 2018, LEM was found in Lee County, Florida, and since then it has spread to several counties in central and south Florida (Carrillo et al. 2020). As a result of this incursion, the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (FDACS-DPI),established an eradication program and a quarantine in Lee county.

Synonymy

Eriophyes litchii Keifer (1943)

Identification, Damage Symptoms and Host Range

As a typical eriophyid mite, LEM is vermiform (tubular-shaped) and has two pairs of legs. It is approximately 150 µm in length and cannot be seen with the naked eye (Figure 2).

LEM uses a series of stylets (Figure 3) to pierce and feed on leaf epidermal cells. Punctured cells often die but surrounding epidermal cells undergo morphological alterations (structural changes), resulting in the enlargement (hyperplasia) of leaf hairs (trichomes), referred to as “erinea” (Karioti et al. 2011). The enlargement and excessive branching of leaf hairs provide mites with a favorable habitat, protecting them from natural enemies and environmental adversities.

Erinea develop initially on the lower side of the leaves, presenting a white/transparent coloration, and causing leaves to become distorted or curled (Figure 4a). However, at this stage of the infestation, a color change of the leaf can also be visible on the upper side (Figure 4b). As LEM populations grow, the erinea change color, thickness, and density. Dense white erinea (Figure 4c) have fewer mites present in comparison to amber colored erinea (Figure 4d). Erinea of dark brown or black color (Figure 4e) have little to no mites present. At this stage, the mites have overexploited the leaf and have dispersed in search of a new flush within the same plant. Erinea may also develop on petioles, stems, panicles, flower buds, and fruit. They may vary in size, shape, and color (Figure 5). Heavy infestations have typically multiple, much larger erinea that may vary in maturation due to the time it takes for the mites to disperse within the plant (Nishida and Holdaway 1955; Sabelis and Bruin 1996). The exact mechanism lying under the erinea formation remains unknown. Research currently conducted at UF/IFAS Tropical Research and Education Center (TREC) aims at understanding the mite-plant interactions and investigating how the erinea are formed.

LEM is highly host-specific and is well documented as a pest of lychee (Oldfield 1996). There is a single report of LEM attacking longan (Dimocarpus longan) in Taiwan (Huang 2008); however, this most likely is an anomaly. The author either confused LEM with a closely related mite, Aceria dimocarpi (Kuang), that attacks longan, or the mite was intercepted on longan but without causing any damage. In Brazil and Florida, longan planted alongside lychee infested with LEM never developed symptoms. Young lychee trees are more susceptible to LEM infestations, and some lychee varieties may be more susceptible than others (Arantes et al. 2017). However, all lychee varieties grown in Florida have shown susceptibility to LEM.

Life Cycle and Dispersal

LEM eggs are only laid after the erinea formation and are located at the base of erinea (Figure 6). They are approximately 32 μm in length and hatch into larvae within three to four days. Larvae are on average 49 μm in length, and they molt to nymphs after two to three days. LEM nymphs are on average 80 μm long and molt to adults within five to seven days (Alam and Wadud 1963). The development from egg to adult takes approximately 14 days depending on environmental conditions (Jeppson et al. 1975). Multiple, overlapping generations can occur over the course of one year. Population growth is favored by new growth on trees during moderately hot and dry periods. High temperature, high relative humidity, and heavy rainfall were unfavorable for LEM development in Pakistan (Alam and Wadud 1963), but in Brazil no correlation was found between LEM population densities and these environmental factors (Azevedo et al. 2014).

The mites prefer to feed on young new flush, which they infest by walking from leaf to leaf among the new flush (Alam and Wadud 1963; Azevedo et al. 2013) (Figure 4). For long-range dispersal, LEM disperses by phoresy, “hitching a ride” by attaching to the bodies of honeybees during the blooming season (Waite and McAlpine 1992; Waite 1999). Mites can also disperse using wind currents or by the plant propagation through air-layers. Air-layering is a method of propagating new trees from stems still attached to the parent tree. Air-layers produced from LEM-infested parent trees can facilitate movement of the mite to new locations where the air-lyaers are taken.

Management

Frequent and regular monitoring of trees should be conducted for early detection of LEM infestations. Any shoots with emerging stems and leaves and/or panicles are especially susceptible to LEM attack. Monitoring for the presence of LEM requires regular and careful inspections of the foliage to detect symptoms, especially around the time when trees are expected to flush or are actively flushing. The mites cannot be seen with a hand lens; high magnification (stereomicroscope) is required.

Chemical control in combination with pruning is the main approach that is followed worldwide for managing LEM infestations. Several active ingredients have shown some efficacy (Schulte et al. 2007; Azevedo et al. 2013), but satisfactory control can only be achieved through carefully timed treatments to protect the new flush (Waite 2005; Picoli et al. 2010). In other regions with LEM, foliar applications are made of various miticides (Castro et al. 2018; Nadhida and Holdway 1955; Schulte et al. 2007; Waite 2005). However, most of the products are not approved for use in the United States. All these miticides are prophylactic (applied before an LEM infestation) and do not kill LEM once the erineum is established.

Until recently, only two acaricides, bifenazate and abamectin, were registered for use in lychee in Florida. Bifenazate is an acaricide used against many species of spider mites, but it is not known to be active on gall/rust, broad, or flat mites (Cloyd 2004). Abamectin is used for a broader spectrum of mite species, including multiple gall/rust species such as the tomato russet mite (Aculops lycopersici) and the citrus bud mite (Aceria sheldoni). This acaricide, however, can only be applied twice per year on lychee and is not compatible with sulfur. In early 2021, EPA made available a Special Local Needs label (EPA Registration: 70506-187), which was approved by the FDACS-DPI, for the use of sulfur (Microthiol Disperss^®, UPL, King of Prussia, PA, USA) for control of LEM on lychee.

The main cultural control strategy against LEM consists of removing and burning infested branches (Waite 2005; Castro et al. 2018). Pruning must be followed by sulfur applications to protect the new flush (see below). Pruning without supplementary sulfur applications may aggravate LEM's spread. Cultural practices are combined with repeated sulfur applications to prevent colonization of the new shoots and leaves by LEM. Once the trees are pruned, a sulfur application of Microthiol Disperss® is made. Sulfur is applied to run-off to all parts of the tree, including the trunk. Subsequent sulfur applications start with the emergence of the new flush and are repeated every 15 days until the leaves have hardened and the tree has stopped producing new flush. Phytotoxicity trials conducted at TREC showed little to no phytotoxicity caused by sulfur application. However, during periods of high temperatures sulfur may burn foliage and fruit. Avoid sulfur applications at temperatures over 90°F for three consecutive days. Additionally, sulfur products are not compatible with oil sprays.

Havesting lychee entails pruning off the fruit-ladened panicals. After harvest, pruning the trees is recommended. The purpose of the postharvest pruning is to control the tree size, maintain canopy light exposure and fruit production along the sides of the tree, synchronize the flush and development of stems, make cultural practices such as foliar pest control and nutrient applications more efficaceous, and reduce the potential for mechanical damage as a result of tropical storms and hurricanes. Note that postharvest pruning without supplementary sulfur applications may aggravate LEM's spread.

Due to current regulations, lychee producers at LEM-positive locations may ship lychee fruit to other non-lychee-producing states but are not allowed to sell the fruit in the state of Florida. In response to these limitations, Revynthi et al. (2020) developed a postharvest treatment using paraffinic oil dips that can be used to disinfest lychee fruit of LEM. This postharvest treatment did not result in fruit quality reduction. FDACS has approved this postharvest treatment, which allows growers in Lee County and other quarantine areas to move lychee fruit within the state of Florida.

Several natural enemies have been reported in association with LEM in India (Lall and Rahman, 1975; Thakur and Sharma 1990), Australia (Schicha 1987; Waite and Gerson 1994), Brazil (Picoli et al. 2010; Azevedo et al. 2014) and China (Waite and Hwang 2002). However, predation on LEM was only confirmed for a few species of predatory mites, including Amblyseius largoensis (Muma) (Acari: Phytoseiidae) in China (Cheng et al 2015), and Phytoseius intermedius Evans & MacFarlane (Acari: Phytoseiidae) in Brazil (Evans and Macfarlane 1961; Azevedo et al. 2014). Phytoseius intermedius was the predator most frequently found associated with LEM, and detailed studies determined that LEM is a suitable prey for this predator (Azevedo et al 2014). However, despite the frequent occurrence of P. intermedius, this predator was unable to prevent visible damage to the trees (Azevedo et al. 2014). Picoli and Vieira (2013), who reported the mite fungal pathogen Hirsutella thompsonii (Fischer) naturally infecting LEM in Brazil, suggested that the erinea may facilitate the development of the fungus and its persistence on the plants. Three phytoseiid predators have been found associated with LEM in Florida, Phytoseius woodburyi De Leon, A. largoensis and Euseius mesembrinus (Dean) (Acari: Phytoseiidae). However, their potential as biological control agents of LEM has not been assessed.

Please contact FDACS-DPI (tel: 1-888-397-1517) if you suspect a LEM infestation and contact your local UF/IFAS Extension agent for more information. Also visit the UF/IFAS Tropical Research and Education Center Lychee erinose mite website for current information (https://trec.ifas.ufl.edu/Lychee-Erinose-Mite/).

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