Asphondylia websteri Felt (Diptera: Cecidomyiidae) attacking Hass  Avocados

Asphondylia websteri Felt 1917 (Diptera: Cecidomyiidae), the alfalfa gall midge, is a polyphagous galling midge that attacks immature fruiting bodies of native (Gagné & Woods 1988) and exotic crop plants in the southwestern USA (Barnes 1946, Gagné & Wuensche 1986). This midge emerged as a pest of cultivated legumes in the southwestern USA in the early 1900's where larvae caused damaged to pods by feeding in the ovary and immature fruiting bodies. Galling of very young fruit can affect seed set and honey production. Damage to alfalfa at this time appeared to range from inconsequential to 20-30% damage being observed in some areas (Barnes 1946).

The exact home range of A. websteri is uncertain, but the southwestern USA and Mexico are thought to be areas constituting the natural range in North America as this midge was first recorded from this area in 1909 (Barnes 1946, Gagné & Orphanides 1992, Gagné & Woods 1988, Rogers 1972).

Prior to 1917, economic entomologists in the USA considered this midge to be A. miki Wachtl, a pest of legumes in central Europe, France,Italy, Hungary, Russia, and possibly England (Barnes 1946). Felt (1917) considered this highly isolated population of an invasive "European" pest in the southwestern USA "remarkable" but the identity of the midge could not be reassessed until a sufficient series of material was available for examination and comparison to European material. Reared material was made available in 1917 and sufficient morphological differences  between the A. miki from Europe and the southwestern USA Asphondylia were determined to exist (e.g., smaller size, darker color, and less setae for the US species) that the US Asphondylia was named A. websteri after Professor Webster who supplied the type series of reared material for examination (Felt 1917).

There are over 250 described species of Asphondylia, with many additional known species that remain undescribed (Gagné & Waring 1990). Asphondylia spp. are cosmopolitan in distribution and the majority appear to be mono- or stenophagous (Barnes 1939). Some Asphondylia spp. exhibit sufficiently high host fidelity they have been considered as classical biological control agents for invasive weed species (Cruttwell-McFadyen & Bennett 1995). Other species of Asphondylia, like A. websteri and A. gennadii, are polyphagous, and in some instances, can be important pests on non-native crops (Gagné & Orphanides 1992, Gagné & Wuensche 1986, Rogers 1972).

Surveys for fruit feeding pests of Hass and non-Hass avocados over November-December 2007 in Guatemala detected large numbers of deformed avocado fruit < 2cm in length in a commerical Hass avocado orchard in Sumpango, Sacatepéquez. Visual examination of fruit indicated that up to 85% of "off bloom" fruit set from flowering that occurred over the period August-September was attacked despite prophylactic applications of broad spectrum insecticides. Deformed fruit did not set on trees and was observed to drop to the ground.  Dissection of deformed fruit in the field revealed the existance of internally feeding larvae. Subsequent rearing of field collected material revealed that A. websteri was the agent responsible for fruit damage that was promoting premature abscission. The rearing and identification of A. websteri from Hass avocados in Guatemala is the first official record for this midge from Central America and the first time this insect has been recorded attacking immature fruit of avocados.

Adult Asphondylia websteri: Adult males and females of this midge are small and typically around 2.25 mm in length. Antenna are long often greater than 3/4 of the body length. Eyes are large and black, the thorax is a dark reddish brown with occassionally yellowish brown markings. Legs are brown in color, halteres are pale yellow, and the wings are hylaine almost without setae and characterized by a light brown costal vein.

Asphondylia websteri escape tunnel entrance shown in a deformed fruit (bottom fruit in left photo). Protruding pupal case from which the adult midge has emerged (center). A. websteri pupae that emerged fully from fruit prior to the emergence of adult midges (right).

  Hass fruit undergoing normal development are shown on the right in the photo above. Elongate fruit on the right have Asphondylia websteri larvae developing inside.

Torymus spp. (Hymenoptera: Torymidae) (photo above) were the most common parasitoids reared from Asphondylia websteri larvae infesting Hass avocados in Guatemala.

Epipona guerini opening an immature Hass avocado fruit infested with a larva of Asphondylia websteri (left photo). Epipona guerini extracted Asphondylia websteri larvae from fruit and ate them (middle photo). Fruit from which Asphondylia websteri have been removed by Epipona guerini were characterized by large circular holes in the side of the fruit
(right photo).

CRUTTWELL-MCFADYEN, R. E., AND F. D. BENNETT. 1995. Potential biocontrol agents of Portulaca oleraceae L. from the Neotropics. Biol. Cont. 5: 189-195.

FELT, E. P. 1917. Asphondylia websteri n. sp. J. Econ. Entomol. 10: 562.

GAGNÉ, R. J., AND G. M. ORPHANIDES. 1992. The pupa and larva of Asphondylia gennadii (Diptera: Cecidomyiidae) and taxonomic implications. Bull. Entomol. Res. 82: 313-316.