Gall Volume Research Articles

Influence of tree growth rate, shoot size and foliar chemistry on the abundance and performance of a galling adelgid

1. The abundance and performance of the Eastern Spruce Gall Adelgid, Adelges abietis, was evaluated on young, open‐grown trees of White Spruce, Picea glauca, whose growth rates had been increased through fertilization or decreased through root‐pruning. 2. In general, gall densities were highest on control trees and on mid‐crown branches. Reduced galling success on fertilized trees was largely due to higher overwintering mortality of first‐generation nymphs. Foliar magnesium, total tannin and total phenol contents were positively, and nitrogen and total monoterpene contents negatively, related to gall density and/or galling success. 3. Although short shoots were more abundant, shoot length was parabolically related to gall density. There was a non‐significant parabolic trend between shoot size and galling success and volume. The number and average size of A. abietis females emerging from galls were positively related to gall volume, and realized fecundity was positively correlated to female size. Consequently, fitness was approximately twice as high for nymphs colonizing intermediate‐sized than small or large shoots. 4. These results do not support the plant vigour or plant stress hypotheses. The results do, however, agree with predictions of the modified plant stress hypothesis for sucking insects. It is speculated that Adelges abietis lacks the necessary resources for successful gall formation on small shoots and is unable to produce a stimulus large enough to induce gall formation on large shoots.

Life History and Host Specificity of Rhopalomyia n. sp. (Diptera: Cecidomyiidae), a Potential Biological Control Agent of Scentless Chamomile

The life history and host specificity of Rhopalomyia n. sp., a multivoltine gall former and potential agent for the biological control of scentless chamomile [Tripleurospermum perforatum (Merat) Lainz, Asteraceae], was investigated at field sites in Eastern Austria and under seminatural conditions in Switzerland from 1995 to 1997. Rhopalomyia n.sp. produced 4 generations per year in Switzerland and induced galls in different meristematic tissues of T. perforatum, including the apical meristems of rosettes and bolting plants, leaf axils, buds, and flowers. Galls formed in axillary buds or flowers were usually single-chambered, whereas the majority of galls induced in apical meristems consisted of several chambers (up to 80), each containing 1 larva. Gall volume increased with the number of chambers per gall. The gall midge was attacked by 1 parasitoid species (Aprostocetus n. sp.) in the field. The probability of parasitoid attack was higher in multichambered galls than in single-chambered galls, whereas percentage parasitism per gall was either unaffected by gall size or declined with an increasing number of chambers per gall. Gall midge females contained on average 92.5 eggs, of which 20-96% were laid, depending on temperature. Females in culture produced an average of 61 offspring, and the gall midge population increased up to 150-fold during 2 generations. A threshold temperature of 7.02 degrees C was calculated, and 478.5 DD were necessary to complete development from egg to adult. A total of 51 plant species and varieties in 6 tribes in the family Asteraceae were offered in no-choice and multiple-choice oviposition and development tests under seminatural conditions. Development to adult was restricted to the genus Tripleurospermum, and T. perforatum was the preferred host plant in all tests. Rhopalomyia n. sp. is a multivoltine species with the potential for rapid population increase. Its ability to induce galls on all plant stages, especially to attack the rosettes of scentless chamomile in late summer and early spring, should supplement the effect of 2 univoltine weevil species already released. Permission for field release of Rhopalomyia n. sp. in North America was granted in October 1998.

Growth of coyote willow and the attack and survival of a mid-rib galling sawfly, Euura sp.

We studied the relationship between variation in age and shoot characteristics of the host plant Salix exigua Nuttall (coyote or sandbar willow) and the attack and survival of Euura sp. (an unnamed leaf-midrib galling sawfly). Variation in shoot characteristics resulted from reduced growth as willow ramets aged. Mean shoot length per ramet and mean longest leaf length per shoot decreased by 95% and 50% respectively between 1- and 9-year-old willow ramets. All measured shoot characteristics-shoot length, longest leaf length, number of leaves per shoot, and mean internode length-were significantly negatively correlated with ramet age (r 2 ranged from -0.23 to -0.41). Correlations between shoot characteristics were highly positive, indicating that plants also grew in a strongly integrated fashion (r 2 ranged from 0.54 to 0.85). Four hypotheses were examined to explain sawfly attack patterns. The host-plant hypothesis was supported in explaining enhanced larval sawfly survival through reduced plant resistance. As willow ramets aged, the probability of Euura sp. attack decreased over 10-fold, from 0.315 on 1-year-old ramets to 0.024 on 2- to 9-year-old ramets. As shoot length increased, the probability of sawfly attack increased over 100-fold, from 0.007 on shoots <100 mm, to 0.800 on shoots in the 1001-1100 mm shoot length class. These attack patterns occurred even though 1-year-old ramets and shoots >500 mm each represented less than 2% of the total shoots available for oviposition. Host plant induced mortality of the egg/early instar stage decreased by 50% on longer leaves and was the most important factor determining survival differences between vigorous and non-vigorous hosts. Sawfly attack was not determined by the resource distribution hypothesis. Although shoots <200 mm contained 82% of the total leaves available, they contained only 43% of the galls initiated. The attack pattern also was not explained by the gall volume hypothesis. Although gall volume increased on longer shoots, there was no significant variation in mid or late instar mortality over shoot length, as would be expected if food resources within smaller galls were limited. The natural enemy attack hypothesis could not explain the pattern of oviposition since predation was greater on longer shoots and leaves. In addition, larval survival was related to oviposition behavior. Due to a 69% reduction in late instar death and an 83% reduction in parasitism, survival of progeny in galls initiated close to the petiole base was 2.8 times greater than in galls initiated near the leaf tip. A 75% reduction in gall volume over this range of gall positions may account for the observed increases in late instar mortality and parasitism.

Utilization ofApionsp. (Coleoptera Apionidae) galls by an ant community in southeastern Brazil

The ant community associated with Apion sp. (Coleoptera Apionidae) galls on Diospyrus hispida DC (Ebenaceae) was studied in a cerrado (savanna) area in southeastern Brazil. We asked the following questions: (a) what ant species occupy the galls, (b) are there any relationships among gall occupation, plant height, and gall distance from the ground, and (c) do ants differentially select galls? Seven ant species were found: Camponotus sp. 1 and Camponotus sp. 2, Crematogaster brevispinosa (Mayr 1870), Zacryptocerus pusillus (Klug 1824), Pseudomyrmex gracilis (Fabricius 1804), Solenopsis sp., and Leptothorax sp. Plant height, distance of the gall to the ground, gall volume, number of channels inside the galls, the length of ant's body and the width of ant's head were recorded. Ants occupied the largest galls (t = 2.653, n = 144, P < 0.05). The number of channels in occupied galls was significantly different from that in unoccupied galls (t = 5.565, n = 144, P < 0.05). Differential utilization of galls may be ...

Effects of gall volume on survival and fecundity of gall‐making aphids Adelges japonicus (Homoptera: Adelgidae)

SummaryThe number of larvae reaching maturity within the gall of Adelges japonicus was positively related to gall volume, and the relationship between the number of mature larvae and gall volume did not change with different densities of colonized larvae.The population changes in the bud galls of A. japonicus were surveyed by collecting the galls, which did not suffer predation or parasitism within the galls, from young artificial plantations of Picea jezoensis over two years. In the year when the density of colonized larvae was high, they suffered a 42% mortality within the galls, whereas mortality was nearly zero in the low density year. The numbers of larvae per gall were positively correlated with gall volume. The regression lines of the number of colonized larvae on gall volume did not differ significantly in the regression coefficients between the two years, whereas the intercept was significantly higher in the year when the density of colonized larvae was higher. However, different within‐gall mortality nullified this difference, and the regression lines of the number of mature larvae on gall volume had no significant difference both in the regression coefficients and the intercepts. This suggests that the number of mature larvae per gall was limited by available resources within the gall which were positively related to gall volume. In 25% of the galls in which mature larvae inhabited, the space within the galls were completely filled by the larvae, indicating that space was one of the limiting resources. Gall volume also affected the number of adults that emerged from the gall and the potential number of their progeny.