Cone insects – new threats to our future forests?

By Paal Krokene and Bjørn Økland, Norwegian Forest and Landscape Institute

Insects exploit almost all possible food sources in the forest – including the trees’ flowers, cones and seeds. Insects that feed on spruce cones and seeds have not traditionally been considered a significant threat to forestry in Norway. However, since an increasing number of our spruce trees now originate from seeds produced in seed orchards, this situation is starting to change. The threats the spruce trees are facing may also be changing, as the abundance of our two most important cone insects seems to be affected by climate change.

Plant breeding has provided spruce seeds that yield up to 20% higher growth than wild seeds from the forest. Consequently, an increasing number of the 25 million spruce trees that are planted in Norway each year now originate from seed orchards. This increased dependency on seed orchards makes us more vulnerable to pests and diseases, as losses in seed orchards can be very high. This is illustrated by the experiences from 2002 – a good cone year with extensive cone production. The Norwegian Forest Seed Center was planning to harvest 170 tons of spruce cones that year, but due to severe damage from insects and diseases the final harvest turned out to be less than 4.5 tons. Two insects were found in about 40% of the harvested spruce cones – the spruce seed moth and the spruce seed gall midge.

The spruce seed moth (Cydia strobilella) is found throughout the range of Norway spruce in Norway, but is most common in high altitude spruce forests in Southern Norway. It is considered to be one of the most damaging cone and seed insects on spruce. The eggs are deposited between the shells of the open spruce flowers, whereas the larvae live inside the closed spruce cones. Heavy attacks by the spruce seed moth can destroy almost all the seeds inside the cone, since there may be as many as 10 larvae in a single cone and each larvae eats 7-10 seeds during their development.

The spruce seed gall midge (Plemeliella abietina) is a tiny fly which develops inside the spruce seeds. It is most common in the lowlands of South Norway. The eggs are deposited in the open spruce flowers at about the same time as the spruce seed moth. Although it reduces the number of viable seeds in the cones the spruce gall midge is less abundant than the seed moth and has therefore been considered to be less of a problem.

In a master’s thesis from the Norwegian University of Life Sciences Sakib Kisja found a decline in the abundance of the spruce seed moth since the late 1960’s, while the spruce seed gall midge became more abundant and extended its range both northwards and into higher altitudes. These changes may be linked to a gradual increase in temperatures and more frequent seed years over the same period. This suggests that climate change may be affecting the distribution and abund-ance of two important insect pests in Norway spruce cones. The comprehensive archives of the Norwegian Forest Seed Center are a good basis for future studies of how the abundance of important cone and seed insects may be influenced by climate change.

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Picture 1: X-ray image of spruce seeds showing many dark, empty seeds, a number of normal, white seeds, and a single seed attacked by the spruce seed gall midge (a white sausage-shaped larvae in the bottom left of the image, about four seeds from the edge). (Photo: The Norwegian Forest Seed Center)

Picture 2: A dissected spruce cone showing two spruce seed moth larvae in the centre of the cone. In the spring, the larvae pupate and the adult butterflies leave the cones to oviposit in the spruce flowers. (Photo: The Norwegian Forest and Landscape Institute)