A comprehensive review of the coffee leaf miner Leucoptera coffeella (Lepidoptera: Lyonetiidae), with special regard to neotropical impacts, pest management and control

The coffee leaf miner (CLM) Leucoptera coffeella moth is a major threat to coffee production. Insect damage is related to the feeding behavior of the larvae on the leaf. During the immature life stages, the insect feeds in the mesophyll, triggering necrosis and causing loss of photosynthetic capacity, defoliation, and significant yield loss to coffee crops. Chemical control is mandatory to sustain the coffee production chain, though market requirements move towards conscious consumption, claiming for more sustainable methods. In this overview, we discuss aspects about the CLM concerning biology, history, geographical distribution, economic impacts, and the most relevant control strategies in progress. Insights to develop an integrate approach for a safer and ecofriendly control of the CLM are discussed, including bio-extracts, nanotechnology, pheromones, and tolerant cultivars.


Life cycle
L. leucoptera is an holometabolous insect [20], (Figure 2). Considering a 25°C temperature, the egg stage usually lasts about five days, twelve days for the larval development and more five days as pupae, totaling about 22 days until reaching adulthood [21]. Total life cycle varies according to temperature, relative humidity, and rainfall. In the dry season, the attack of the pest is usually stronger than in wet periods [22,23]. The egg is about 0.3 mm, made by a translucent structure, with an oval, concave shape, with expanded sides [9,22]. After hatching, the larvae leave the underside of the eggs, which are in contact with the upper leaf epidermis, and gets into the leaves [24] (Figure 3). The L. coffeella larval phase has four instars [25]. Newly hatched larvae have a translucent whitish color, but throughout their development they take on a greenish yellow tone. The last larval instar is 5 of 25 about 4-5 mm, flattened, segmented with 11 segments and yellowish in color [9,11] (Figure 4a). Fourth instar larvae have a flat head and mouthpiece of the chewing type (Figure 4b, c), prolegs and brackets [9,26] (Figure 4d). After accomplishing the larval stages, the larvae leave the mines and weave a silk X-shaped cocoon, usually in the axial region of the leaf, forming the pupae [9,22] (Figure 4e). Pupae have an approximate length of 2 mm, milky color, small black eyes, antennas, and legs ventrally fused, and wrinkled wings [9] (Figure 4f). Usually, more pupae are found in the 'skirt' region of coffee plants, which is the underside of the plant where dead leaves accumulate. [24].
From pupae, adults emerge with an average body length of 2 mm and a wingspan of 6.5 mm (Figure 5a). They have a head with 'white hair scales', long antennae that reach the end of the abdomen, silver white chest, legs covered with white bristles, wing with three rows of yellow bristles at the apex with a black circle, yellowish abdomen and covered with white scales and genital organs covered by a tuft of white scales [9,11] (Figure 5 b, c).

3.2.Larval feeding behavior
CLM feeds only in coffee leaves [27] and the larvae are the causal agent of the crop damage. When feeding on the mesophyll of the coffee tree leaves, the insect creates mines that justify the common name of the pest -coffee leaf miner ( Figure 6a). The mines cause necrosis (Figure 6b), decreasing the photosynthetic leaf surface (Figure 6c, d), leading to a lower photosynthetic rate of the plants and consequent depletion of the plant and productivity diminish [20]. The damage caused by this insect includes defoliation [22] (Figure 6e). Eventually, without adequate cultural treatments, the infestation can lead to the death of the plant.
A relationship between the feeding damage of CLM and the application of synthetic fertilizers has been described in the literature [28,29]. The amount of free amino acids and reducing sugars in the metabolic system of coffee plants is related to nutritional imbalance and susceptibility to pests. Plants fertilized with organic material showed a decrease of up to 50% of leaf mines [28].

3.3.Adult behavior
L. coffeella is depicted as the quintessence of sensitivity level [23]. In adulthood, the insect has a nocturnal habit and during the day, it shelters beneath the coffee leaves [22]. Mating and laying occur preferably at night [25]; [24,30]. The sexual behavior of adults is very peculiar and can present the following stages, (1) females in a resting position with the abdomen curved downwards, exposing the pheromone gland in continuous movements from the inside out, to attract males; (2) when perceiving the pheromone, males remain in the same place, moving their antennae and flapping their wings, and then walk towards the female; (3) male touches the female with his antennae, female retracts the pheromone gland and places his abdomen towards the male; (4) the male places his abdomen towards the female abdomen, releases the edeago and fits the female, initiating copulation

4.Damage and Losses Caused by the CLM
Brazil, Vietnam and Colombia are responsible for about 50% of the global production of coffee. The Brazilian coffee commodity comply more than one third of world coffee production and exports, figuring in the 5th place in Brazilian exportation agricultural trades [32,33] performing US$ 1,3 billion in 2020 [34].
Until 1970 CLM outbreaks in coffee plantations in Brazil were sporadic because of the effective action of natural enemies on CLM population. Moreover, coffee crops used to be organized with narrow spacing, which is an adverse condition for this pest. In the 1970s, the mechanized model replaced the former one, requiring large areas of extensive agriculture with greater spacing between trees. The new plantation areas expanded onto drier and warmer regions, such as Brazilian Cerrado biome [20] During their lifecycle CLM females are able to oviposit around seven eggs per night and more than 50 eggs during their lifetime [22]. Increased temperature in coffee fields often allows two CLM rounds in producer regions [21]. In a few days, the injuries area evolves from some millimeters to several centimeters ( Figure 5) ending up to the falling of the leaves and lowering the productivity. The damage caused by the insect is not restricted to coffee, since current CLM control practices require pesticides that contaminate workers, consumers, and the environment.

Control and Management of the CLM
The hot climate shortens the plague cycle, resulting in remarkably high populations of adults, caterpillars and chrysalis, in addition to a large number of eggs in leaves [35]. On average, 8 generations can occur per year in Brazil, even reaching 12 during the crop year [36]. Climate change scenarios predict an increased infestation of CLM due to a greater number of generations per month [17,37]. The impact of climate change on C. arabica production was recently revised [38] Accurate models to estimate CLM levels of infestation are proposed [39].
Ideally, the first generation of the CLM must be controlled efficiently to prevent a growing population throughout the year. Chemical control is needed because the CLM reduces drastically the productivity of the C. arabica and C. canephora cultivars, otherwise the farmers would have to abandon their coffee growing activity. However, the chemical control used alone presents disadvantages for the environment, especially in relation to the survival of natural enemies, which are mostly affected by the products used for pest management. In addition, chemical control does not have complete efficiency, and several sprays are required, increasing the production costs. Finally, chemicals lose their effectiveness because they allow the selection of naturally resistant individuals, and consequent increase of resistant populations in field.

Chemical control 9 of 25
Nowadays the chemical control of CLM is mandatory for the maintenance of coffee crops. There are some active ingredients available and the most efficient are thiamethoxan [2], chlorantraniliprole [40], cartap hydrochloride [41]. Studies on the stability of those products detected their presence in plants up to eight and six months after application, respectively. The best performance in CLM control was observed with soil application for both systemic action and selectivity results [2,42] verified thiamethoxam protection above 180 days whereas control plants without insecticide showed a drastic decrease of more than 50% in production.
Nowadays the chemical control of CLM is mandatory for the maintenance of coffee crops. There are some active ingredients available and the most efficient are thiamethoxan [2], chlorantraniliprole Thus, chemical control has been required, especially in areas with high CLM incidence, such as the Brazilian Cerrado biome, which is characterized by high temperatures and long drought periods. The intensive use of chemical insecticides, however, can lead to pest resistance, especially when the same active ingredient is continuously used. In addition, chemical pesticides pose risks to workers and the environment.
[43] recently reported that 94% of the populations were resistant to the active ingredient chlorantraniliprole in some producing areas of the Bahia state.

Genetic resistance to CLM
Coffee farming began in Brazil in 1727, with the Típica cultivar introduction. This genetic material was nearly the only one exploited in a commercial basis until the middle of the 19th century [44] Since then, several traits of interest, such as high yield and improved tolerance to diseases/pests and abiotic factors, have been introgressed using genetic breeding and generating several new cultivars. Those genetic breeding programs allowed the expansion of coffee growing in several biomes in the country. However, traditional cultivars such as Catuaí and Mundo Novo, and even other cultivars with higher levels of tolerance to other pests, suffer heavy CLM infestation.

Breeding
With regard to susceptibility to BMC, [45] classified the species of Coffea as: highly resistant -C. stenophyila, C. brevipes, C. liberica and C. salvatrix;moderately resistant -C. racemosa, C. kapakata, C. dewevrei and C. eugenioides, susceptible-C. congenesis, C. canephora and C. arabica. However, coffee breeding programs deal with serious limitations to perform interspecific hybridization of C. Arabica associated with the low genetic variability and the allotetraploid features of this commercial species [44].
The main source of resistance to BMC are plants derived from a natural cross between C. arabica and C. racemosa carried out in the 1950s (Medina, 1963 percentage of leaves injured by the CLM, these cultivars were able to keep the leaves longer than Ouro Verde Amarelo IAC 4397, which can mitigate the pest damage. In addition, it was noticed that the destruction of the leaves is milder in C. canephora than in C. arabica. Resistance to BMC may be of a biochemical nature [16] and larvae probably have a protective mechanism against a possible toxic effect of caffeine [48]. Data on the influence of chemical composition and leaf age (Guerreiro Filho, 2006) suggest that new leaves are more tolerant, with significant reduction in egg laying and increased larvae mortality, probably due to the higher concentration of secondary metabolites, such as phenols.

Genetic modification
The biotechnology achievements on genetic improvement of coffee plants were revised by [49], and more recently by [50]. Genetic engineering techniques were developed to C. arabica [51] and C. canephora [52] to express genes aiming to control coffee insect-pests.
Transgenic plants carrying the cry1Ac gene from Bacillus thuringiensis showed good resistance to L. coffeella in greenhouse conditions and initial field experiments [53]. Nonetheless, the constitutive promoter EF1α-A1 provided too low Cry1Ac protein levels in the transgenic leaves to confer efficient and sustainable protection against L. coffeella in the field [54].

Biological control of CLM
Population dynamics of CLM can be strongly affected by host-plant attributes and environmental conditions, but also by the abundance of natural enemies [15], [55,56]. Parasitoids and predators (wasps and ants) have been extensively reported in coffee plantations in several Latin American and African countries since 1970s as natural mortality factors [4,[57][58][59][60][61][62]. Despite the great number of hymenopteran species parasitizing Leucoptera sp. larvae in coffee growing areas worldwide (Supplementary Table 1) and their contribution on population dynamics of the pest, no significant attempt was made to use these natural enemies as biological control. As reviewed by [63], few cases of introduction of new parasitoids species or augmentation of indigenous species have been made for the suppression of CLM populations. Although unsuccessful in most of the cases, the author stressed the great potential of periodic releases of natural enemies of L. coffeella under certain conditions. Moreover, some invertebrate-pathogenic fungi were already tested against different developmental stages of species in Leucoptera genus. Eggs and larvae of L. coffeella were susceptible to infection by the fungus Metarhizium anisopliae [64]. Also, the species Beauveria bassiana was described as pathogenic to L. malifoliella, infecting last instar larvae when leaving the leaf mines for pupation after exposure to this pathogen [65].
Crop management practices and landscape structure can affect insect communities and the ecosystem services provided by natural enemies, enhancing their diversity and abundance. Ecologically complex coffee systems are associated with higher biodiversity of parasitoid wasps, ants, and other predators [66]. For instance, richness and abundance of social wasps is positively correlated with forest cover in coffee-producing regions, increasing L. coffeella predation [67].

Semiochemicals
Pheromones may be used to manipulate or disrupt the natural behaviors of insects to reduce population levels to diminish crop damage. Mass trapping offers a good alternative over huge plantations, disruption of mating processes to regional application, and "attract and kill" to smaller fields. 5,9-dimethylpentadecan and 5,9-dimethylhexadecane are the main and secondary components of L. coffeella sexual pheromones [68,69]. [70] synthesized the four possible stereoisomers of 5,9dimethylpentadecane. Subsequently, [71]demonstrated the synthesis in 3 stages of a mixture of 5,9dimethylpentadecane stereoisomers that showed high biological activity in the field. The racemic mixture of pheromone components was synthesized from citronellol [72].
The female production pattern of 5,9-dimethylpentadecane is related to the period of the day and the time after adult emerging from pupa [73]. The results in virgin females showed that the higher amounts of pheromone were produced in the period comprised between 4h before and 2h after dawn and in 1day-aged females.
Host plants release volatiles which influence the mating and oviposition of lepidopterans and may increase biosynthesis of sexual pheromone. In the case of L. coffeella, the volatiles compounds liberated by the C. arabica plant increase the mating ratio by about 90%. Moreover, they accelerate the onset of copulation and soar mating duration. The CLM oviposition observed in C. arabica was higher than in non-host plants [74].

Monitoring
The 5,9-dimethylpentadecane is indicated for the integrated management of the CLM. Delta trap monitoring [75] can be done by installing one trap for every 4ha.
[31] used the traps 0.5 m above the ground containing synthetic sex hormone and observed that the peak of male capture occurs around noon, coinciding with the mating peak time.

Mating disruption and mass trapping
Mating disruption (MD) techniques provide prevention of mate location and mating, and factors that interfere with or delay the normal insect mating processes [77]. Successful cases have long been reported to codling moth in pome fruit, oriental fruit moth in peaches and nectarines, tomato pinworm in vegetables, pink bollworm in cotton and omnivorous leafroller in vineyards [77]. The management of caterpillars of the pine processionary moths Thaumetopoea was achieved in restricted areas using pheromones as a practical alternative to insecticide sprays [78]. Some mating disruption treatments are still to be improved [79][80][81][82][83], being CLM one of them.
The viability of MD to reduce coffee leaf miner populations by application of synthetic 5,9dimethylpentadecane was evaluated by synthetic-baited pheromone traps or the level of damage that the insect caused to the leaves. The results showed failure of the MD that may be attributed to a combination of several technical factors [84]. Dispenser types, like aerosol [85] or aerial applications [86], release points in the field and treatment period as yet to be tested to CLM control by confusion.
Recent advances are reported for lepidopteran pests in agriculture using "attract and kill" approaches [87]. Effectiveness of mass trapping (MT) by sticky traps was observed to the leopard moth in the olive orchards [88], the tea geometrid [89], chickpea pod borer [90].

Botanical pesticides
Biopesticides based on plant sources present remarkable advantages over conventional synthetic chemical pesticides, including: lower persistence in the environment, lesser phytotoxicity, more effectiveness, higher specificity towards target organisms, reduced pest management costs, and a low toxicological and ecotoxicological risk for field workers, consumers and the environment [91].
Studies on the efficacy and the use of botanical pesticides has been largely reported in the literature, and even more, recommended by international organizations as a more sustainable manner to control pests [92][93][94][95][96]. Raw vegetal materials for biopesticide development are obtained from barks, leaves, roots, flowers, fruits, seeds, cloves, rhizomes, and stems of plants belonging to several botanical families. The derived substances from the materials processing are generally plant extracts, essential oils or both [97]. Commercialized pesticides from plants, such as pyrethrum, neem and sabadilla, are some examples of biopesticides of the least toxicity to non-targets organisms, such as pollinators and fish [98].
Combining biopesticides and nano-based delivery methods at the nanoscale is now being explored to increase efficacy while limiting the negative impacts traditionally seen through the use of pest control means [102]. Nanotechnology offers the advantages of using nanomaterials presenting novel and enhanced features compared to bulk materials. The remarkable physicochemical properties of these materials generate applications in agriculture as pesticides and platforms for gene delivery [103,104].

Nanobiopesticides
Nanopesticides constitute nanoencapsulated (or nanoentrapped) pesticides, which can be bioactive compounds (biopesticides) and/or agrochemicals (e.g. insecticides), capable of controlling and inhibiting the growth of plant insect pests. Thus, nanobiopesticides comprise the encapsulation and/or entrapment of biopesticides, which are obtained from bacteria, fungi, plants, and animals (e.g. plant extracts and essential oils, fungal and bacterial biomolecules) [105]. The encapsulation not only optimizes stability, solubility, permeability, and specificity of pesticides, but also promotes a sustained release of them [106].
The agricultural nanoformulations are commonly based on metallic nanoparticles, polymeric nanoparticles, nanoemulsions, lipid nanoparticles, or carbon-based nanostructures. Silver nanoparticles (AgNPs) synthesized using the leaf extract of Annona reticulata, and the AgNPs showed insecticidal activity against Sitophilus oryzae, an insect that damages rice grains [107]. Nanemulsions produced with Pimpinella anisum essential oil presented activity against the red flour beetle (Tribolium castaneum), a stored grain pest [108]. Likewise, solid lipid nanoparticles produced with geranium essential oil (Pelargonium graveolens) were reported as a control agent of black cutworm Agrotis ipsilo [109]. Similarly, graphene oxide nanocomposites loaded with pesticides (pyridaben, chlorpyrifos, and beta-cyfluthrin) enhanced acaricidal activity against spider mite [110].

Conclusions
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 October 2020 doi:10.20944/preprints202010.0629.v1 There is great demand for control products of this pest that are less toxic, highly specific, with less impact on the population of natural enemies, and that result in lower production costs (GABRIEL CASTILLO, 2016). Some biotechnological alternatives can generate products to meet the demand for sustainable, durable, and safe solutions for the specific control of this pest.
Development of coffee leaf miner resistant/tolerant cultivars remains a strong tendency. The current breeding programs began with C. arabica and C. racemosa crossings. Individuals belonging to the offspring of the second round of natural backcrosses (RC2) were hybridized with C. arabica commercial cultivars and generated new registered cultivars [111], [112] However, further investigation concerning the molecular basis of the resistance introgressed in C. arabica cultivars is required to keep the high-performance traits of grain yield and quality of the new genotypes.
CRISPR/Cas9 technology could circumvent some traditional breeding limitations to develop cultivars resistant to the CLM. Gene editing could provide both precise genome modifications and attenuation of regulatory restrictions on genetically engineered crops [113]. Despite the controversy surrounding GMOs in the agricultural sector, coffee is one of the very few woody species that has a validated protocol to mutate C. canephora with CRISPR/Cas9 A [114].
Biorational pesticides strategies to control L. coffeella must consider the important role of parasitoids, predators, and insect pathogens on enhancing the natural mortality of the CLM in field. Albeit biological control alone presents limitations in the efficiency and durability of treatments, it is positive in integrated control systems, in special to organic farming.
Promising results using plant extracts against CLM encourage the research of improved biopesticides to be integrated in more robust and sustainable pest management systems. Despite the lack of scientific papers describing the use of agricultural nanoformulations to control major coffee pests like the CLM, these recent advances to control pathogens and pests in other crops strongly suggest this possibility in the forthcoming years.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.