First report of Cheiloneurus exitiosus (Perkins, 1906) and Helegonatopus dimorphus (Hoffer, 1954) (Hymenoptera: Encyrtidae) from Japan, with remarks on their abundance in rice paddies

Abstract Background Encyrtid secondary parasitoids of Delphacidae have not been recorded in Japan. However, they may play an important role in the rice ecosystem because they can reduce the number of Dryinidae, the natural enemies of rice planthoppers. New information We found two encyrtid species, Cheiloneurus exitiosus (Perkins, 1906) and Helegonatopus dimorphus (Hoffer, 1954), from rice paddies and the surrounding environment. Haplogonatopus oratorius (Westwood, 1833) and Anteon sp. were newly recognized as hosts of He. dimorphus. Parasitism of C. exitiosus was rare, but He. dimorphus was common in Kumamoto Prefecture. The sex ratio (male proportion) and clutch size of He. dimorphus was estimated as 0.19 and 4.95, respectively.


Introduction
Some encyrtid wasps are known as parasitoids of Dryinidae (Guerrieri and Viggiani 2005, Olmi and Xu 2015). There are two genera of encyrtids reported as parasitoids of Dryinidae, namely, Cheiloneurus Westwood and Helegonatopus Perkins. Hitherto, Helegonatopus is considered the specialist of Dryinidae. Among these, H. dimorphus (Hoffer, 1954) has been widely recorded from Europe to Sakhalin (Noyes 2016). This species exhibits a peculiar sexual dimorphism. The head of the male is compressed laterally, whereas that of the female is unmodified. As for Cheiloneurus, C. exitiosus (Perkins, 1906) is recorded widely from South Asia to Oceania . As is often the case with small parasitoid wasps, a substantial number of Encyrtidae show a cosmopolitan distribution. Furthermore, some of their primary hosts such as rice planthoppers are known as long-distance migrants (Otuka 2013), and dryinid larvae can also be transported (Mita et al. 2013). It is possible that those primary hosts carry not only dryinid larvae, but also their hyperparasitoids. In East Asian countries, the secondary parasitoid of rice planthoppers has seldom been investigated. However, a consideration of their distribution range and the host's dispersal ability suggests they should be found widely in East Asian countries including Japan, and they may play an indispensable role in the rice ecosystem. We herein report the occurrence of these encyrtid wasps in Japan, and present results of a preliminary survey of their field abundance.

Materials and methods
Materials used in the study are preserved in 99.5% ethanol. They were mounted on pieces of cardboard or glass slides when necessary. Original pictures were taken by a digital camera (Olympus E-5) attached to an Olympus SZX10 stereomicroscope. Photo images were processed using image-stacking software (Combine ZP). Materials are deposited in the Entomological Laboratory, Faculty of Agriculture, Kyushu University (ELKU). Distribution and host records were obtained from Guerrieri and Viggiani (2005),  and Olmi and Xu (2015).
Some encyrtid wasps collected before 2014 could not be separated into each host individual. To confirm the number of encyrtid wasps per dryinid larva, the parasitism ratio, and the sex ratio of encyrtid hyperparasitoids, specimens of Laodelphax striatellus (Fallén, 1826) parasitized by Dryinidae and dryinid cocoons on leaves were collected from rice paddies in Kumamoto and Kagoshima prefectures on Kyushu Island during September 17-28, 2014 (Table 1). Two other species of rice planthoppers, Nilaparvata lugens (Stål, 1854) and Sogatella furcifera (Horváth, 1899), were seldom collected during the survey. Cocoons were additionally collected at Suya from September to October 2014. Parasitized planthoppers were reared separately in small glass tubes plugged by cotton with a stem of rice. Field cocoons were similarly treated, but a stem was not inserted. They were kept in an incubator (25°C, 16L/8D) or the laboratory at room temperature. When no parasitoid emerged, the cocoon was dissected and any dead wasps were identified.
Field cocoons and emerged parasitoids collected by field survey in 2014.

Clutch size and sex ratio of Helegonatopus dimorphus
Olmi and Xu (2015) reported that 10-12 individuals of He. dimorphus emerged from a cocoon of G. rosellae (Currado & Olmi, 1974), while 7 individuals were recorded as having emerged from G. solidus (Haupt, 1938) in Italy. According to our field survey, 3-8 (mean 4.95±1.40SD) individuals emerged from Ha. oratorius. The average male proportion is 0.19 (Table 4), and represents 0.95 per clutch. The smaller number relative to the other two dryinid species may be due to the smaller body size (2.0-3.1 mm for the females of Ha. oratorius, compared to 3.0-3.4 mm in G. solidus and 3.5-3.8 mm in G. rosellae). This number is similar to that of He. pseudophanes (Perkins, 1906)

Host stage
As for C. exitiosus, they attack the "later instar" larva of G. nudus (Perkins, 1912) (as Pseudogonatopus nudus) (Manickavasagam et al. 2009). According to the ovservations of De Santis and Virla (1991), both genera may have the ability to begin attacking from the larval-sac stage on the primary host to the pupa in the cocoon. In the present study, we confirmed that He. dimorphus can attack the larval sac, but the proportion of this parasitism is much smaller than that of the cocoon. The higher proportion of parasitism can be at least partially explained by the difference in developmental period. The developmental period of the larval sac of Ha. oratorius (as Ha. atratus, 4.5 days at 25°C) is shorter than the period from removal from the host planthopper to adult emergence (11.2 days) (Kitamura 1983).
Our results indicate that L. striatellus possibly transports its primary and secondary parasitoid together, but the amount of secondary parasitoid should be very limited not only because of their scarcity. Host stage preference should be investigated to understand the effect on host-parasitoid dispersal dynamics. Table 4.
Two encyrtid species collected by field survey in 2014.