Disholcaspis quercusvirens (agamic)

Disholcaspis quercusvirens (sexgen)

Disholcaspis quercusvirens

Bullet galls were initiated by the sexual generation of D. quercusvirens during early shoot expansion in the spring. Slight stem swellings were visible by June, and small red-colored bullet galls with soft and spongy tissue (Fig. 1A) emerged through the bark by August. A sweet and sticky exudate occurred on the bullet galls from August to mid-October, when only larvae were present inside the galls. This exudate attracted red imported fire ants (Solenopsis invicta Buren), velvet ants (Hymenoptera: Mutillidae) (Fig. 1B), and paper wasps (Hymenoptera: Vespidae), and supported sooty mold growth (Fig. 1C). D. quercusvirens bullet galls were mature and woody (Fig. 1D) in the fall, reaching their maximum diameter by November (Fig. 2).

Mean maximum bullet gall diameter was 7.7 ± 0.1 mm in 2007 and 4.8 ± 0.1 mm in 2008 on the trees at the nursery (Fig. 2). Compound galls were 30.4 ± 1.6 mm in length with 9.3 ± 0.6 bullet galls per compound gall (range: 1–44 bullet galls) in 2007 and 25.5 ± 1.4 mm in length with 11.0 ± 0.9 bullet galls per compound gall (range: 1–52 bullet galls) in 2008.

Asexual females (Fig. 3A) emerged during cold weather periods from late-November through January and oviposited into dormant buds (1.3 ± 0.07 mm in length, 1.2 ± 0.09 mm in width at base). Females (n = 20) carried 64.0 ± 3.0 eggs (range: 43–88 eggs). All eggs within female abdomens appeared to be of similar size, but dimensions were not measured.

In total, 1,650 bullet galls were collected from 1 August to 18 December 2007, and 91.3% were parasitized (Table 1). The natural enemy complex included species of Synergus (Cynipidae), Eupelmus (Eupelmidae), Eurytoma and Sycophila (Eurytomidae), Ormyrus (Ormyridae), and Acaenacis (Pteromalidae) (Fig. 4).

Bullet gall height, diameter, and number of bullet galls per compound stem gall were influenced by the gall inhabitant. Bullet galls containing D. quercusvirens and parasitoids in 2007 were significantly wider than those containing inquilines. Bullet galls with live D. quercusvirens were significantly taller than those containing parasitoids or inquilines. Furthermore, compound galls with more than five bullet galls in a cluster produced more D. quercusvirens than compound galls having five or fewer bullet galls.

Bullet galls from which gall makers successfully emerged did not produce any parasitoids or inquilines. However, two or more parasitoids and/or inquilines emerged from many D. quercusvirens bullet galls and presumably killed or out-competed the gall maker. Although inquilines are phytophagous, their development may cause gall maker death because of overcrowding or competition.

Disholcaspis quercusvirens sexual generation

Small bud galls developed beneath bud scales (Fig. 1E) and were located on the branches at the base of a petiole, hidden between young leaves (Fig. 1F). Bud galls were 2.5 ± 0.02 mm in length and 1.1 ± 0.01 mm in width. They likely were initiated by asexual wasp oviposition (i.e., November–January), and remained on trees until May. Bud gall tissue was soft and spongy and continued to grow and harden until the bud scales fell off the galls. The tips of young bud galls, sometimes red in color (Fig. 1E), were visible in early- to mid-March. Emergence of D. quercusvirens ended by mid-April, and parasitoid emergence continued until all bud galls fell off the trees in May. Bud galls were fragile after hardening, and, similar to the D. cinerosa system (Frankie et al. 1992), could be easily dislodged after adult emergence or inclement weather.

Galled (2.8 ± 0.1 mm length, 1.7 ± 0.08 mm width) and nongalled (2.5 ± 0.1 mm length, 1.6 ± 0.05 mm width) buds collected on 9 March 2009 did not significantly differ in length (F = 1.70; df = 1; P = 0.1983) or width (F = 2.44; df = 1; P = 0.1251). Live oak buds collected on 9 March 2009 ranged in development from dormant to green-tipped.

Sexual generation pupae (Fig. 3B) were present in bud galls by 11 March 2009, and 65% of the bud galls collected on 16 March contained white pupae. First adult emergence (Fig. 3C and D) occurred on 21–23 March 2009 from laboratory rearings, and emergence continued through ≈21 April 2009. 

In total, 480 bud galls were collected from 4 April to 14 May 2008, at the end of the sexual D. quercusvirens emergence period. Parasitoids comprised 99.1% and D. quercusvirens comprised 0.9% of the specimens reared (Table 1). Parasitoids reared from these bud galls included a species of Aprostocetus, Baryscapus, and Pediobius (Eulophidae); Brasema auratus (Ashmead) (Eupelmidae); and Acaenacis lausus (Walker) (Pteromalidae). The most abundant species was B. auratus. No inquilines were reared from the bud galls; a feature shared with some other small and structurally simple oak cynipid galls (Stone et al. 1995, Schönrogge et al. 2000).

From 19 March to 14 May 2009, 3,884 bud galls were collected. These bud galls had a more diverse parasitoid complex (Table 1), which included a species of Aprostocetus, Baryscapus, and Pediobius (Eulophidae); B. auratus and Brasema gemmarii (Ashmead) (Eupelmidae); Sycophila and an unknown species (Eurytomidae); Ormyrus hegeli (Girault) (Ormyridae); and A. lausus (Pteromalidae). Only one specimen was reared from each bud gall (e.g., bud galls were monothalamous), as observed in other small sexual-generation oak cynipid galls (e.g., Stone et al. 1995, Schönrogge et al. 2000). The most abundant parasitoid in 2009 was A. lausus, and it was reared also from bullet galls in 2007. Pteromalid wasps in cynipid galls are typically parasitoids, and their larvae feed on the eggs or larvae of their hosts (Krombein et al. 1979, Askew 1984). O. hegeli (Girault) (Ormyridae) was also reared from both bud galls in 2009 and bullet galls in 2007.

The parasitoid species Brasema ficigerae (Ashmead), Eupelmus quercus Ashmead, Eurytoma querciglobuli rubra Bugbee, Eurytoma studiosa Say, Torymus elegantissimus (Ashmead), Torymus fagopirum (Provancher), and Torymus lissus (Walker) were previously reared from the bullet galls of D. quercusvirens (Krombein et al. 1979), but were not found in the current study. Eurytoma and Tetrastichus (Eulophidae) can feed on developing larvae of D. perniciosa in bullet galls (Seibert 1993). It is assumed that the parasitoids reared from the D. quercusvirens bud galls also fed on the gall maker larvae or pupae.

Life History, Natural Enemies, and Management of Disholcaspis quercusvirens (Hymenoptera: Cynipidae) on Live Oak Trees