**5. Current strategies to combat citrus HLB**

#### **5.1 Vector control**

#### *5.1.1 Biocontrol of vector*

Biocontrol uses natural enemies by import, augmentation, and conservation to control the population density of disease pathogens or pests in agriculture [87]. Asian citrus psyllid (*Diaphorinacitri*) was controlled using a practical method of import (from Asia) and free of *Tamarixia radiata* in Florida citrus groves [88]. Natural enemies, *Diaphorencyrtus aligarhensis* and *Tamarixia radiata*, were imported from Taiwan and Vietnam to Florida citrus orchard and released as a biocontrol agent for *D. citri* [89]. *T. radiata* became more widely established parasitoid wasps than its counterpart *D. aligarhensis*. In urban and suburban regions, the release of *T. radiata* could considerably benefit commercial citrus growers by reducing latent psyllid populations and preventing the further spread of HLB disease [87]. *T. radiata* has been firmly established in commercial citrus-producing countries, including Réunion Island [90], the Philippines [91], Indonesia [92], Guadeloupe [93], and the USA [94], where it is spread throughout the state [88]. It appeared inadvertently in Brazil, Venezuela, Mexico [95], Puerto Rico [83], and Texas [31].

In addition to wasps, many insects native to Florida are recognized as *D. citri* predators, including several ladybeetle and spiders [81]. *Cyclonedas anguinea*, *Harmonia axyridis* Pallas (Coccinellid beetles), and *Olla v-nigrum* Mulsant were the leading killer of nymphal psyllids. In addition to *Ceraeochrysa*, *Hibanavelox* (becker), spider, and *Chrysoperla rufilabris* Burmeister (lacewings), *Tamarixia radiata* (parasitoid) also contributed to the additional mortality in Florida citrus groves. Coccinellid beetles are considered one of the most important natural enemies of *D. citri* populations in central Florida. Besides this, intraguild predation (IGP) causes more than 95% of immature *T. radiate* mortality [96]. Van den Berg et al. [97] noted that the spiders are the critical predators of *T. erytreae*, followed by chrysopids, coccinellids, syrphids, hemerobiids, Hemiptera, and predatory mites in citrus groves under the control of an integrated management program. Adults and larvae of *O. v-nigrum* (Mulsant) preying on premature Asian citrus psyllid were noticed in citrus groves throughout Florida [98].

A range of fungi species was reported to infect Asian citrus Psyllid, particularly under humid conditions [81], including entomopathogenic fungi, *Isaria fumosorosea*, *Lecanicillium lecanii*, *Beauveria bassiana*, *Capnodium citri*, *Cladosporium sp. nr. Oxysporum*, *Metarhizium anisopliae*, and *Hirsutella citriformis* that were used against HLB vectors as biopesticides [99–101]. *Isaria fumosorosea* is reported to have great

potential to control different insect pests [102]. *H. citriformis* conidia with *synnemata* produced *in vitro* and *in vivo* were subjected to adult *D. citri* exhibits an increased mortality rate [100, 103]. In a 2-year field investigation in citrus groves in Florida, adult *D. citri* (ACP) was killed by *H. citriformis* following the rainy season [104]. In laboratory conditions, the fungal strains of *Isaria fumosorosea* (ESALQ-1296) and *Beauveria bassiana* (ESALQ-PL63) accounted for 77.8 and 78.4% of adult *D. citri* mortality, respectively, while in semifield conditions, adult *D. citri* mortality rate was as high as 83.5% with ESALQ-PL63 and 80.6% with ESALQ-1296. During 1 year, the monthly use of these two fungal strains in commercial citrus groves exhibited adult *D. citri* mortality ranging from 96.1% in December 2011 to 57.8% in October 2012. In addition, this study found that the mortality rate increased under high humidity conditions [99]. *Isariajavanica* and *Acrostalagmus aphidum* were also identified as biopesticides against *D. citri* in China [105]. The use of fungal species, such as *Metarhizium anisopliae*, *Cordyceps bassiana*, and *Isaria fumosorosea*, was shown to decrease larger populations of nymphs than adults of *D. citri* in the Persian lime groves [106].
