Hawthorn pod galls are fully expanded and very evident on hawthorns (Crataegusspp.) in southwest Ohio. The galls are produced under the direction of the gall-midge fly,Blaesodiplosis venae(family Cecidomyiidae).
The image below provides proof the galls are constructed of leaf tissue. It shows that aGymnosporangiumrust fungus will infect the gall tissue just as it would “normal” leaf tissue.
Like all leaf galls, the gall-maker females take advantage of undifferentiated meristematic leaf bud cells to form both a home and food source for their offspring. The leaf galls cannot be formed from leaf cells once they have differentiated into their final form. That’s why hawthorn pod galls develop in the spring.
Research on other gall-making midge flies has shown the female flies inject phytohormones along with eggs to start hijacking the nascent leaf cells. The process continues with the hormones arising from the eggs and midge larvae (maggots) to turn plant genes on and off at just the right time to direct the growth of the gall-maker species-specific plant structure. It’s a wonderous but still poorly understood process.
The elongated hawthorn pod galls arise from the veins on the underside of the leaves of their hawthorn host. Mature galls become crimson red and this coloration coupled with the gall's sometimes irregular surfaces gives rise to the alternate common name of hawthorn cockscomb galls. Opening the hollow galls will reveal the tiny white, semi-transparent midge fly maggots developing within the plant structure.
As with the vast majority of leaf galls, hawthorn pod galls appear to cause little to no harm to the overall health of their tree host. It’s clear they do not disrupt the vascular flow in the leaf veins, so infested leaves remain functional. In fact, the galls are constructed of functional leaf tissue with their green color indicating they contribute to carbohydrate production through photosynthesis.
Although the galls cause little harm to the health of the tree, heavily galled leaves become deformed and detract from the aesthetic value of infested trees. However, as is true for most leaf galls, there is no efficacy data on using insecticides to prevent the development of hawthorn pod galls.