Since 2008, a wasp species has been observed to predate nymphs of Diaphorina citri Kuwayama (Hemiptera: Psyllidae) (Fig. 1), the Asian citrus psyllid, one of the vectors of the bacterial pathogen Candidatus Liberibacter asiaticus, causal agent of the deadly citrus greening disease (Xu et al. 1998; Garnier & Bové 2000). The wasp species was identified as the Mexican honey wasp, Brachygastra mellifica (Say 1837) (Vespidae: Polistinae) by M. Buck. This wasp was first observed attacking D. citri nymphs in 2008 in sweet orange, Citrus × sinensis (L.) Osbeck (Rutaceae), and lemon, Citrus × limon (L.) Burm. f. (Rutaceae), groves, planted 3 to 5 yr ago in Río Bravo, Tamaulipas, Mexico (25°56′39.12″N, 98°07′3.9″W, 28. 2 elev.). We have been observing B. mellifica since 2008 at Rio Bravo, and have found that its predatory activity is greatest during Jul through Sep when a high population of D. citri nymphs is present on new flush.
B. mellifica are small ocial wasps (body length 7–9 mm), and are one of the few insect species, other than honey bees, that produce and store honey (Sugden & McAllen 1994). The species constructs paper nests of 40–50 cm in diam when mature. The 10 or more horizontal cell combs are surrounded by an outer envelope. Nests are placed in the mid-canopy of a tree or shrub 1 to 9 m above ground level (Sugden & McAllen 1994).The nests of B. mellifica are populous and can house from 3,500 to 18,700 wasps (Hastings et al. 1998).
The genus Brachygastra occurs from the southern U.S. (Texas, possibly Arizona) to northern Argentina and includes a total of 16 species (Richards 1978). Three species are known from Mexico: B. mellifica, B. azteca (Saussure 1857) and B. smithii (Saussure 1853). B. mellifica ranges from Texas into Nicaragua. It appears to be the only species of the genus in northeastern Mexico, whereas B. azteca has a more southern and western distribution, and B. smithii is known from Chiapas only (Richards, 1978). Brachygastra is a fairly distinctive genus of social wasps that is unlikely to be confused with other wasps except for certain species of the solitary genus Pachodynerus (Vespidae: Eumeninae), which possess a much larger first metasomal segment.
In the nearby Lower Rio Grande Valley of Texas, wasps gather nectar from wild or exotic flowers including sunflower (Helianthus annuus L., Asteraceae) and mesquite (Prosopis glandulosa Torr., Fabaceae) (Sugden & McAllen 1994). The wasps have also proven to be efficient pollinators of avocado (Persea americana Mill., Lauraceae) (Castañeda-Vildozola et al. 1999; Ish-Am et al. 1999). Besides nectar, they gather honey dew excreted by aphids (Aphididae) and psyllids (Psyllidae) (Sugden & McAllen 1994).
Reports on predacious activity of Brachygastra are scarce and refer mostly to boring or plant tissue mining of prey species. In the U.S., B. mellifica was reported to prey on the flower-mining larvae of the weevil Anthonomus aeneolus Dietz (Curculionidae), and on a many-plumed moth (Alucitidae) (Sugden & McAllen 1994). In Brazil, the closely related species B. lecheguana was observed hunting cotton boll weevil larvae, Anthonomus grandis Boheman (Soares & Lara 1994) and white coffee leaf miner larvae, Perileucoptera coffeella Guérin-Meneville (Lyonetiidae) (Parra et al. 1977; Gravena 1983).
We observed that B. mellifica generally attacks D. citri nymphs on the new flushes of citrus (Fig. 1). They appear to be efficient predators because they explore the new flushes rapidly and move much more quickly from one twig to another than other predators of D. citri nymphs, such as the Asian lady beetle, Harmonia axyridis (Pallas), and the lady beetle, Cyclonedasanguinea (L.) (Coleoptera: Coccinellidae).
In 2008 the distance between the wasp nest and the foraging place was approximately 30 m. On the other hand, in 2009 we observed that the wasps had built a nest inside a grove in an orange tree; it lasted for about 2 yr, but was abandoned early in 2011. During 2008 and 2011, we have observed pysllid nymphs being predated by B. mellifica in absence of any nests in the vicinity. During the summer time, when the population of D. citri nymphs increases, the predation by B. mellifica was observed all day, but predation was most intense from 800–1200 h and from 1600–1900 h.
To determine the amount of nymphs consumed by B. mellifica, a video was filmed by a Canon® Power Shot S3IS de 6.0 Megapixeles. The numbers of nymphs consumed were registered by analyzing the video in a personal computer. This was accomplished by selecting flushes from 5 to 10 cm long infested by 4th to 5th instar D. citri nymphs. These leaves were removed, and those with nymphs were then exposed to B. mellifica at its foraging time.
The period of time that B. mellifica was foraging on new flushes infested by D. citri nymphs varied from less than 1 min up to 16 min approximately, depending on the size of the nymph population.The average consumption of B. mellifica was one 4th or 5th instar nymph every 30 seconds (n = 50). The maximum number of nymphs consumed at one time was 31 nymphs in 16 min. Evidence of the such predatory activity of B. mellifica was documented on the video: http://www.inifapcirne.gob.mx/Videos.htm.
Based on our field observations in Río Bravo, Mexico, we believe that B. mellifica has the potential to be a suitable biological control agent against D. citri. Further studies should focus on the efficiency of B. mellifica as a predator, development of a protocol for the relocation of wild colonies to citrus groves, and possibly mass rearing this species.
Since 2008, in México, a new natural enemy of Diaphorinacitri has been observed predating on nymphs of this psyllid vector of citrus greening disease. This wasp was identified as Brachygastra mellifica (Say 1837), the Mexican honey wasp. This wasp was observed attacking all D. citri nymph instars, and it appears to have the potential to serve as a tool in integrated biological control programs.
Present research was supported by FONSEC SAGARPA-CONACYT, through the project: Manejo de la enfermedad Huanglongbing mediante el control de poblaciones del vector Diaphorina citri (Hemiptera: Psyllidae), el psílido Asiático de los cítricos (2009-108591).