It is difficult to predict the impacts of climate change on forest insect pests because of the complexity of the interactions between insects and trees. This overall response is dependent on the impacts of climate change on the insect- tree host-natural enemy relationship. However, some generalised predictions can be made, based on current pest distributions and the severity of insect outbreaks in individual regions. New (alien or exotic) insect species that may present a future threat to woodland are also assessed, although their introduction is as likely to be driven by global trade as by climate change.
The analysis is based on expert judgement, and is very much a 'vision of the future' and not 'hard science'. It should be placed in the context of indicating those insect pests for which there is an ongoing and future need for monitoring, and not as a hard-and-fast prediction of future impacts.
- Elatobium abietinum (green spruce aphid).
Populations are largely controlled by low winter temperatures and over-wintering populations are therefore thought likely to increase with climate change. This effect of temperature on population dynamics has been clearly demonstrated and serious outbreaks are predicted to become more commonplace. Research indicates that a serious outbreak reduces increment in the year of damage by 20-30%. In addition, a higher frequency of outbreaks could have more profound effects on site productivity.
- Hylobius abietis.
A serious and widespread problem during restocking of conifer plantations. The weevil has a large climatic range (across most of Europe), although activity increases to some extent with temperature. Climatic warming might thus be expected to increase its impact, although the use of small felling coups which is now common standard practice, particularly in CCF systems, may act to moderate any increase in activity.
- Dendroctonus micans (great spruce bark beetle).
Infestations are sporadic, but excessive bark damage leads to tree death, especially in Sitka spruce. The species is likely to benefit from an increased frequency of summer drought and climatic warming, although its specific predator (Rhizophagus grandis) might benefit to a greater extent, thus potentially reducing the impact of Dendroctonus.
- Ips acuminatus (and others: bark beetle on pine trees).
Reports of serious damage in the UK are not common. It is largely a secondary pest, but primary infestations have been reported, particularly at the young thicket stage on light soils. Primary attacks are linked to drought stress, so again, climate change may worsen the situation in some areas. Population build-up also occurs in areas of wind-blow, which may therefore present a future climate change impact should storm damage become more commonplace.
- Ips cembrae (large larch bark beetle).
Its distribution is currently limited to areas in Scotland and northern England where it is a primary agent of tree death. If populations were established in other areas, Ips cembrae could cause serious damage in stands subjected to the increased frequency of summer drought that is predicted for much of England.
- Pissodes castaneas (small-banded pine weavil).
Infestations have been periodic and heavy, and characterised by top-death of affected trees. Drought stress is seen as a trigger for infestation; climate change is therefore expected to make infestations more frequent and severe.
- Rhyacionia buoliana (pine shoot moth).
Serious infestations occur when trees are under stress. It is thought likely that its impact will increase with climate change.
- Agrilus biguttatus (buprestid beetle).
Associated with oak decline, but it is not certain whether the relationship is causative, and there is also conjecture as to whether it is a primary or secondary pest – it is seen as a primary agent of decline in Germany, but only as a secondary agent in the later stages of decline in the UK . At present, its distribution is restricted to southern England. Climatic warming may extend its range.
- Platypus cylindicus (oak beetle).
A secondary pest affecting the value of felled broadleaf timber, particularly oak. It was a ‘red data-book’ species in the early 1980s, but became a serious pest following the 1987 storms as a result of large quantities of lying timber. Is only likely to be a problem if there are widespread losses for other reasons (drought or storm).
- Longhorn beetles.
Only infest dead trees at present, but may attack living trees under conditions of extreme drought. They are relatively common in old timber.
- Tortrix and winter moth.
At present there is no link between climate and damaging episodes. However, if synchrony were to break down between the egg hatch and canopy development in the spring (which it has not to date), defoliating episodes may become less common.
- Cameraria ohridella (horse chestnut leaf miner).
This insect has spread rapidly across Europe from Macedonia, in northern Greece, and is now established in London and other towns in south east England. It is thought that the speed of spread may be related to recent climate change. A Forestry Commission Exotic Pest Alert on the moth has recently been published. It is likely that the range of the leaf miner will expand. The long-term impact of the moth and methods of control are currently under investigation.
- Sesia apiformis (hornet moth).
This species is a stem borer of poplar, and its prevalence may be linked to climate. It is a secondary pest, and only presents a problem when trees are stressed by summer drought.
New alien/exotic pests include the following, and vigilance should be maintained, particularly regarding the importation of timber and wood products.
- Anoplophora glabripennis (Asian longhorn beetle).
Populations could establish in warmer coastal areas if the beetle was introduced, and they would benefit from climatic warming. Amenity and street trees are particularly susceptible.
- Ips typographus (eight-toothed spruce bark beetle).
Pheromone traps in ports have recently recorded higher numbers than in previous years. If predictions of increased soil moisture deficits are borne out, drought-stressed trees could provide a resource from which wider outbreaks might develop. As with Ips acuminatus, populations build up in areas of significant wind-blow.
In general, it is a reasonable assumption that many insect pests have the potential to become more damaging as a result of climate change. Most of these predictions are driven through expectations that trees across much of Britain will become more drought stressed during the summer months, and that development of pests will accelerate, possibly leading to higher population growth. At the same time, it is likely that the pests’ natural enemies will also benefit, so it is, to some extent, unclear as to what the overall effects will be.