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How to Manage Pine Weevil Control

How to Manage Pine Weevil Control

It is estimated that on average 50% of the seedlings on untreated reforestation sites in Ireland and the UK are killed by large pine weevil (Hylobius abietis), in the early years. John Casey, Teagasc Forestry & Dr. Louise McNamara, Teagasc Crops advise on predicting the risk and management options

While many forest owners are unaware of the threat posed by large pine weevil (Hylobius abietis), it is estimated that on average 50% of the seedlings on untreated reforestation sites in Ireland and the UK are killed by pine weevil during the first few years. In fact, estimates indicate that the pine weevil is the most important pest of reforestation sites in Europe.

Teagasc, in association with Maynooth University and the Department of Agriculture, Food and the Marine (DAFM), are organising a free, one-day conference on 4 May 2022 focusing on Integrated Pest Management (IPM) for pine weevil in Ireland.

At this conference, experts from Teagasc, Maynooth University, Coillte, DAFM, private forestry and the UK will cover topics including the lifecycle and management options of the large pine weevil, pesticide regulations, reforestation and continuous cover forestry. For further information and to register, please go to www.teagasc.ie/crops/forestry/news/2022/pine-weevil-conference.php

An example of the threat

The pine weevil risk was highlighted to farm forestry owner Brendan Keane (pictured) of Dunhill, Co. Waterford when he participated in a forestry Knowledge Transfer Group (KTG) back in 2019. At the time, Brendan was preparing to put in a forest road and to thin his 8 hectare (ha) conifer plantation, made up mainly of Sitka spruce. Unfortunately, Ash Dieback Disease was also discovered in his nearby 7 ha crop of 14 year old ash in 2019.

Following consultation with his Teagasc forestry advisor, Brendan decided to clearfell the infected ash, with assistance from Ash Dieback Reconstitution & Underplanting Scheme (RUSS). This DAFM scheme provides financial support for the site clearance or partial clearance of the ash and the reconstitution or the replacement of ash trees with alternative species following the clearance. Having looked at his options, Brendan decided to replace the ash with a crop of Sitka spruce and alder, which were planted in inverted mounds in March 2022.

Brendan was very aware that felling an adjacent coniferous crop, even in a thinning, can produce a large increase in potential breeding material for the large pine weevil (Hylobius abietis). The combination of the conifer thinning operation, the replacement of the ash with Sitka spruce and the resulting insect pest would challenge the most gifted multitasker. In Brendan’s case, he rose admirably to the challenge, by weighing his options and by seeking advice, before mapping his path forward.

Quantifying the threat & potential cost

The replacement young Sitka trees could potentially be attacked by adult pine weevils feeding on the stem from the root collar upwards. Heavy damage can completely girdle stems and cause plant death. Naturally, Brendan wishes to avoid young tree losses, as it could lead to substantially increased re-establishment costs through the cost of insecticide application, replacing dead plants and additional weeding.

Photo shows weevil scar damage

The adult Hylobius (the large pine weevil) may live for up to 4 years and may attack at any time of year when it is warm enough for insect activity. There is a tendency for two peaks of damage to occur yearly, one in spring before egg-laying and the other in late summer before the adults hibernate underground. It is important to note that the knapsack application of the insecticide Cypermethrin, for example, is only effective for approximately 6 weeks. Predicting the optimum time of application will be a critical task for Brendan, since the young trees will remain vulnerable for the first couple of years.

Photo above: weevil growth stages

Predicting the risk

Stump hacking is a method of predicting a weevil outbreak. It is recommended to clear the soil away from 1 /4 of a the felled Sitka spruce stumps, at least 40cm out and 30cm down from soil level (include at least one major root and two root- stump junctions). The bark can be removed from the cleared area using a spade.  The number of any weevil larvae and pupae should be counted. Weevil larvae are not segmented or ridged and tend to form a C shape.

Photo: Stump larvae of weevil

Brendan was advised to sample at least 5 stumps. If the average stump count on a site felled more than 12 months is 5 or more per stump then spraying will be necessary. If the count is 1 or less then spraying may not be necessary. If average count is between 1- 5 the felled site should be checked again during weevil feeding periods (April & August). This method should be viewed as an indicator and is not 100% accurate.

Controlling pine weevil

No fully-successful means of controlling the population in the stumps is currently available and it is therefore necessary to protect the plants directly through dipping and/ or spraying with the insecticide Cypermethrin. The use of such pesticides is governed by the European Communities Regulation 2012, Sustainable Use of Pesticides. It should be noted that alternative insecticides are emerging in the marketplace, and there is now a strong focus on lower impact Integrated Pest Management (IPM) strategies. A range of chemical, physical & biological controls will be discussed at the upcoming weevil conference on 4 May.

Management options

Reforestation, whether due to disease or at full rotation clearfell, allows forest owners to pause and to consider their options before choosing the next rotation’s path. The owners’ objectives may have changed or increased to reflect the multi- functional nature of forestry as a whole. Research in terms of improved species, species mixes and management techniques is on-going, to encourage natural resilience. Teagasc continues to be available with comprehensive supports to assist forest owners, individually and as groups, to access the latest advice and knowledge. Our joint aims are to maximise our forestry resource, while seeking to manage the potential risks. 

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