The "architect" of willow leaves - the willow gall sawfly

On the willow trees beside the road, we sometimes see many small beans densely hanging on the willow leaves and swaying in the wind. Some are light yellow in appearance, while others are light green, and are about the same size as soybeans.

Are these “little beans” the fruits of willow trees?

Galls on willow trees (Photo from the Internet)

If you break open these small beans, you will find "hermits" living inside, which are the larvae of some insects. The lumps are galls, which are also jokingly called "houses" built by parasites on plants. Just like human houses can shelter from wind and rain, parasites wrap themselves or their eggs in galls to form a protective cover and facilitate feeding.

The inside of the gall (Photo from the Internet)

How are galls formed?

Gall-forming insects have a unique way of building their homes. When laying eggs or feeding, they produce a chemical secretion on plant tissues. This secretion can increase the local growth hormone in the plant body, thereby accelerating cell division and eventually growing into deformed tumors. In most cases, it is caused by aphids, scale insects, sawflies, weevils, leaf rollers, and gall midges. However, sometimes nematodes can also form at the roots of plants, which will affect the normal growth of plants.

Different types of galls on willow trees (photo from the Internet)

When the gall begins to develop, even if the gall-forming insect leaves, the formation of the gall is not affected, so the gall is essentially a response of the plant to the secretions of the larvae, that is, the real builder of the gall is actually the plant itself. In layman's terms, the gall-forming insect is the architect and the host plant is the contractor. After the architect hands the house design drawings to the contractor, the remaining work is completed by the contractor, and the subsequent work has nothing to do with the architect. The willow gall sawfly is a common architect on willow trees.

Gall pictures (pictures from the Internet)

What is the willow gall sawfly?

The willow gall sawfly is the thick-walled willow sawfly Pontania bridgmannii Cameron. It belongs to the order Hymenoptera, family Tetraptidae. It is a leaf-feeding pest that harms various willow trees. Once a willow tree is damaged, the back of the branches and leaves of the affected tree crown will have egg-like bumps, and the young branches will droop and turn yellow due to the heavy load. The harmful plant rate in the damaged section is 60%. The insect has one generation per year, with mature larvae cocooning in the soil around the roots of the tree for the winter. The adult insects emerge from late April to early May of the following year, with the emergence peak in mid-May, and lay eggs in the tissue on the edge of the willow leaves. After the larvae hatch, they eat the leaf flesh on the spot, causing the upper and lower epidermis of the leaves to gradually swell, and small reddish-brown galls appear on the edges of the leaves. As they feed and grow, the galls become larger and thicker, bulging downwards, and appearing oval and kidney-shaped. The galls of old trees finally turn purple-brown.

Adult of willow gall sawfly (picture from the Internet)

The larvae infest the galls until the end of September or the beginning of October, when they fall to the ground with the leaves, crawl out of the galls and burrow into the soil, where they form cocoons to hibernate. The adult is 6 mm long with a wingspan of about 16 mm. The body is earthy yellow with brown markings, and the wing veins are mostly black. The eggs are oval and yellowish-white. The mature larvae are about 15 mm long, yellowish-white, slightly curved, with a smooth body surface and dorsal wrinkles, 3 pairs of thoracic legs, and 8 pairs of abdominal legs. The leaves are infested with yellow-green galls, and the larvae live in the galls.

Willow gall sawfly larvae (picture from the Internet)

What are the methods for preventing and controlling the willow thick-walled sawfly?

1. Implement mixed planting of tree species to reduce and avoid single species pure forests as much as possible.

2. Manual control: During the growth period of young trees, organize and mobilize local people to remove leaves with galls from each tree. After autumn, remove and dispose of fallen galls, and burn and bury them.

3. Drug control: (1) When the disease is serious from late April to early May, choose appropriate pesticides to control it while ensuring the safety of people and animals as much as possible: spray the whole tree with 1000-1500 times diluted 40% omethoate EC or 2000 times diluted 40% chrysanthemum mixture. (2) Use systemic pesticides for root irrigation control: that is, bury the pesticides in the roots where the tree has the most fibrous roots. The pesticides can be 3% furadan granules or 15% aldicarb granules, with 1.5-2g per centimeter of trunk diameter; you can also irrigate the ditch with 40% omethoate EC, 1.5-2kg per centimeter of trunk diameter at 1000 times, and cover the soil after infiltration; you can also inject 10 times of 40% omethoate around the base of the trunk.

Author: Chen Xiaotong (graduate student at the School of Life Sciences, Hubei University)

Scientific review: Xu Letian (Associate Professor and Doctoral Supervisor at Hubei University)

Planning: Zhao Qingjian

Editor: Wu Yuetong