Survey of phytoseiid mites (Acari: Mesostigmata, Phytoseiidae) in citrus orchards and a key for Amblyseiinae in Vietnam

Citrus is the second largest fruit crop in Vietnam. Phytoseiid mites are important natural enemies of mite pests and small insects in citrus orchards. By examining the specimens collected from three citrus orchards in the Mekong Delta in the south of Vietnam, a new record of phytoseiid mite Amblyseius obtuserellus Wainstein and Begljarov, 1971 was found and two other species Amblyseius largoensis (Muma, 1955) and Euseius ovalis (Evans, 1953) were also obtained. Herein, they are re-described and illustrated based on the current specimens. A key for Amblyseiinae in Vietnam is also provided


Introduction
Vietnam is long and narrow, located in the east of Indochina Peninsula. Vietnam shares its land borders with China to the north, Laos and Cambodia to the west, and surrounded by the south China sea to the east and south. Its climate is characterized by the interplay of the East Asian and South Asian Monsoons, cold surges in the winter months and a long rainy season which extend beyond summer (Nguyen et al. 2014). Latitude of the south (8°10′ N-10°48′ N) is more southern than Hainan Island (18°10′ N-20°10′ N) in China, and its temperatures vary less in the southern plains around the Mekong Delta. Vietnam is one of the 25 countries considered to possess a high level of biodiversity (Vietnam Environment Protection Agency, 2005, electronic source) (Sohn and Park, 2009). Until now, only 15 species of phytoseiid mites (Acari: Phytoseiidae) have been recorded in Vietnam (Nguyen et al. 2016;Nguyen and De Clercq, 2018;Thuan et al. 2019;Nguyen, 2019aNguyen, , 2019bNguyen and Dao 2019;Nguyen et al. 2019aNguyen et al. , 2019b. Citrus is the second largest fruit crop in Vietnam, with Mekong Delta the main citrus production area. The sampling sites in this survey are all located in Mekong Delta. Phytoseiid mites are important natural enemies of pest mites and small insects in citrus orchards (Wu et al. 2009;Huang 2011). Phytoseiid mites have been mass-reared and widely used for biological control of pest mites and insects in citrus orchards in China (Zhang et al. 2002;Ouyang et al. 2007;Fang et al. 2013Fang et al. , 2018. It is important to survey the native species of phytoseiid mites in citrus orchard, which is supposed to be the most adapted bio-control agents can be used in local citrus orchards. This study proposes a new record A. obtuserellus Wainstein and Begljarov, 1971 and two other species, Amblyseius largoensis (Muma, 1955) and Euseius ovalis (Evans, 1953), from citrus orchards in Mekong Delta. (6-9), Z4 82 (75-88), Z5 161 (155-167), s4 68 (63-72), S2 8 (7-9), S4 7 (6-9), S5 7 (5-8), r3 12 (11-14), R1 8 (7-9).
Amblyseius obtuserellus has 11 well-developed teeth on fixed digit and four teeth on movable digit of the chelicerae and this species may play an important role in bio-control. Its biology and bio-control application need to be further studied.
Remarks -The calyx of the examined specimens is tubular, similar to illustration in Wu et al. (2009), but not distal flaring as illustration in Gupta (1986). Fixed digit of the examined specimens with three teeth is similar to illustration in Liao et al. (2017), but not only with one tooth as the original description (Evans, 1953).
Euseius ovalis can be mass-reared on pollen and it is the dominant natural enemy in controlling Aceria litchi (Keifer) (Wu et al. 1991). This species also had potentiality in controlling Tetranychus kanzawai Kishida (Shih et al. 1993) and Oligonychus mangiferus (Rahman and Sapra) (Nguyen and Shih, 2010). Liao et al. (2017) reported E. ovalis rarely located on the leaves with Tetranychus colonies and only located close to Oligonychus and Panonychus colonies.
Key to the Vietnamese Amblyseiinae species