common name: an Asian citrus psyllid parasitoid
scientific name: Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal) (Insecta: Hymenoptera: Encyrtidae)
Introduction - Synonymy - Distribution - Description - Life Cycle - Hosts - Biological Control - Selected References
Diaphorencyrtus aligarhensis (Shafee, Alam and Argarwal) (Hymenoptera: Encyrtidae) is a host-specific, solitary endoparasitoid of Diaphorina citri Kuwayama (Hemiptera: Psyllidae), the Asian citrus psyllid (ACP). Diaphorina citri is a serious pest throughout citrus growing regions of the world (da Graca 1991, Halbert and Manjunath 2004). Psyllid feeding activities cause leaf chlorosis and promote the growth of sooty mold, but more significantly vector the bacterium Candidatus Liberibacter asiaticus, a causal agent of citrus greening disease or huanglongbing (HLB). Infected trees begin to decline, producing yellow shoots, leaf mottling, low quality fruit and may experience mortality (Aubert et al. 1996, Bové 2006). Diaphorina citri was discovered in south Florida in 1998 (Halbert 1998), while citrus greening first appeared in 2005 (Halbert 2005).
In addition to utilizing chemical and mechanical methods to control both the Asian citrus psyllid and huanglongbing, the biological control agents Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) and Diaphorencyrtus aligarhensis were imported into Florida and released (Hoy and Nguyen 2000). Both parasitoid species have been credited with reducing psyllid populations in Réunion (Etienne and Aubert 1980) and Taiwan (Chien and Chu 1996).
Figure 1. Adult female Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), a parasitoid of the Asian citrus psyllid, Diaphorina citri (Kuwayama). Photograph by Eric Rohrig, University of Florida.
Ruggiero et al. (2011) reviewed past taxonomic names published for D. aligarhensis including:
- Aphidencyrtus aligarhensis
- Aphidencyrtus diaphorinae
- Aphidencyrtus sacchari
- Diaphorencyrtus diaphorinae
- Psyllaephagus diaphorinae
- Syrphophagus aligarhensis
Diaphorencyrtus aligarhensis was originally recorded from India (Shafee et al. 1975) and later from the Philippines, Vietnam (Aubert 1987), and China (Yang et al. 2006). Specimens were imported and released for classical biological control programs in Réunion, Taiwan and the United States (Etienne and Aubert 1980, Chien, 1995, Hoy and Nguyen 2000).
Adults: Adult wasps are small (~ 1-1.5 mm) and possess yellow legs and antennae and a black head and thorax. Sexual dimorphism is exhibited in slight differences in the antennae and abdomen. Female antennae are smooth and clubbed while male antennae are slightly longer, lack clubs and are covered with short hairs. Females possess a large, rounded yellow abdomen with a black posterior while male abdomens are smaller, solid black, and more cylindrical.
Eggs: Eggs are oval, with an average length of 130 ± 10 µm and a diameter of 100 ± 1 µm. Newly laid eggs are milky white but begin to clear within 12–24 hours. After clearing, the developing embryo can be seen through the clear smooth chorion.
Larvae: All larval stages are hymenopteriform with a weakly developed head capsule. Larvae lack hairs or spiracles and possess short, small, reddish-orange mandibles that are approximately 20% of the total body width. First instar larvae average 410 ± 7 µm long by 100 ± 5 µm diameter and increase in size to 1190 ± 10 µm long by 400 ± 20 µm diameter by the fourth larval instar. Larvae entering prepupation shorten in length by constriction.
Figure 2. Immature life stages of Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), a parasitoid of the Asian citrus psyllid, Diaphorina citri (Kuwayama). Eggs shown 12 hours (A) and 24 hours (B) after oviposition. First (C), second (D), third (E) instar larvae, and a fourth instar larva (F) shown shortening by constriction upon entering the prepupal stage. Photograph by Eric Rohrig, University of Florida.
Pupae: Pupating larvae begin to develop adult head and abdomen definition first, followed by legs, mouthparts, and antennae. Next, the thorax, eyes, and ocelli begin melanization, followed by the abdomen, legs, and lastly the antennae.
Figure 3. Pupating Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), a parasitoid of the Asian citrus psyllid, Diaphorina citri (Kuwayama). The pupa is beginning melanization. Photograph by Eric Rohrig, University of Florida.
Diaphorencyrtus aligarhensis generally reproduces by thelytoky, which results in all female offspring (Chien 1995, Hoy 2003). Imported wasps tested positive for infection with the bacterial endosymbiont Wolbachia through PCR analysis (Jeyaprakash and Hoy 2000, Rohrig et al. 2011a). Wolbachia infection can induce parthenogenesis as well as other reproductive anomalies in insects (Werren 1997). Shafee et al. (1975) reported small numbers of males in Asian populations surveyed.
Female D. aligarhensis oviposit a single egg into the host, usually through its abdomen. If two wasps lay an egg in the same host nymph (superparasitism), only one will complete development to adulthood (Rohrig et al. 2011a). Diaphorencyrtus aligarhensis are considered koinobiont parasitoids because host D. citri nymphs continue to feed and develop following a parasitism event until they are eventually killed before reaching adulthood.
Females parasitize 2nd through 4th instar nymphs and host feed on 1st through 4th instars (Chien and Chu 1996, Skelley and Hoy 2004). Adult females are capable of killing up to 280 D. citri nymphs through host feeding and parasitism combined (Chien 1995). Skelley and Hoy (2004) reported that there was no significant difference in the mean number of progeny or adult wasp size when reared from 2nd, 3rd or 4th instar nymphs. Females produced an average of 6.6 progeny per day during laboratory rearing and only 10% of adult D. aligarhensis could survive for 50 days at 25°C (Skelley and Hoy 2004). Chein (1995) determined mean longevity at 20 days.
Figure 4. Adult female Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), a parasitoid of the Asian citrus psyllid, Diaphorina citri (Kuwayama), parasitizing a second instar D. citri nymph. Photograph by Eric Rohrig, University of Florida.
Figure 5. Adult female Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), a parasitoid of the Asian citrus psyllid, Diaphorina citri (Kuwayama), host feeding on the internal fluids of a second instar D. citri nymph. Photograph by Eric Rohrig, University of Florida.
Life cycle from egg to adult requires 16 to 18 days at 25°C (Rohrig et al. 2011a, Chien 1995). Diaphorencyrtus aligarhensis development includes an embryonic stage (~2 day duration), four larval instars (~6 days), a prepupal (~1 day), and a pupal stage (~7 days) that occur within D. citri nymphs. Parasitized nymphs die and begin to harden into a brown mummy as the developing parasitoid nears the end of the larval stage of development (~8 days post parasitism).
During pupation, D. aligarhensis develops with its ventral surface toward the nymph's ventral surface and its anterior end toward the nymph's posterior end. Upon reaching adulthood, the wasp rolls 180° so that it is now facing the host's dorsal surface. The adult wasp chews a round exit hole through the nymph's abdomen, crawls out, grooms, moves its wings and begins walking. This exit hole can be used to distinguish between a mummy parasitized by D. aligarhensis from T. radiata which exits dorsally through the thorax (Hoy 2005) rather than the abdomen.
Figure 6. A mummy of the Asian citrus psyllid, Diaphorina citri Kuwayama, with an abdominal exit hole characteristic of the emergence of its endoparasitoid Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal). Photograph by Eric Rohrig, University of Florida.
There is no evidence that D. aligarhensis is capable of developing on any host other than D. citri (Aubert and Quilici 1984).
After a successful classical biological control program targeting D. citri was conducted in Réunion, both D. aligarhensis and T. radiata were evaluated for release in the United States.
Diaphorencyrtus aligarhensis was originally imported into Florida from Tawain in 1998 and field releases began in 2000 (Hoy and Nguyen 2000). Another population of D. aligarhensis from Guangdong China was imported in 2006 and released a year later (Rohrig et al. 2011b). Tamarixia radiata from Tawain and Vietnam were released into Florida beginning in 1999. Currently, T. radiata has established in most citrus growing regions of Florida, providing varying levels of control (Michaud 2002, Quereshi et al. 2009), while D. aligarhensis appears not to have established despite several recoveries.
It is believed that a combination of the superiority of T. radiata as a biological control agent of D. citri, increased/more efficient use of pesticides to control Asian citrus psyllids, inconsistent populations of immature psyllids, and predation of parasitized hosts by generalist predators likely impeded the establishment of D. aligarhensis (Rohrig et al. 2011b).
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