Controlling Pear and Cherry Slugs

Last season during food garden visits I saw many fruit trees devastated by Pear and Cherry Slugs. Ideally action against this pest begins in winter and if continued during the season can result in complete removal of the pest, and a return to healthy leaves and good crops.

Pear and Cherry Slugs may look like slugs, but they are not actually members of the slug family. They are the larvae of Caliroa cerasi, also known as Black Sawfly. 

Black Sawfly larvae eat the soft tissue of leaves of pear, cherry, plum, apple and quince trees. A bad infestation weakens a tree considerably. There will be many leaf skeletons, less fruit, smaller fruit, or no fruit.

The damage done my Pear and Cherry Slugs

As with all food garden pests, understanding the life cycle of the Black Sawfly is key to controlling it, or ideally, getting rid of it all together.

A Sawfly larva and a lot of leaf damage

The Life cycle of the Black Sawfly can be summarised as follows

1. Adult Sawflies lay eggs on leaves
2. Larvae emerge from the eggs and eat the soft tissue of leaves
3. They drop to the ground and dig into the soil
4. They form pupae.
5. Adult Sawflies emerge and fly to the leaves (back to Step 1)

An adult Black Sawfly - photo Margaret W.

There are two of these cycles per season.

The first generation of larvae appear on leaves in early summer.

The second generation of larvae appear later in summer. That second generation is often more numerous and more devastating.

When the second generation drops to the ground and pupates, the pupae overwinter underground, and re-emerge as Sawflies in late November to re-start the cycle.

Based on this information, we are going to look at two distinct containment periods.

Containment period 1 – when the tree has no leaves

It is winter. There are no Sawfly larvae visible, but if you had an infestation last season, you can safely assume that pupae are in the soil under your tree, up to the furthest point where they could have dropped off the tree.

If your tree is large I recommend you prune your tree at this time. If your tree is more than 2.5 metres high prune it down to or below 2.5 metres, so it is not going to be too high for an effective treatment in Containment period 2 (see below) later in the season.

Do you have chooks, or geese or other soil foragers? 

They are the easiest and best way to catch all over-wintering Sawfly pupae, in fact all ground-dwelling pests. Put them around your fruit trees, at least during winter, and they will do a wonderful job.

If you don’t have soil foragers, a good strategy, successfully trialed by Peter Cundall, is …..

Trapping and killing young sawflies in the ground in spring:

Over a good sprinkling of pelletised chook manure, I spread thick, overlapping layers of newsprint, extending from the trunk to well beyond the drip line, wetting and weighing the paper down with more fertilisers. This is covered with a thick straw mulch so this dense combination acts as a very tight, impenetrable barrier, preventing the emergence of young sawflies.

(from https://www.weeklytimesnow.com.au/country-living/gardening/peter-cundall-how-to-get-rid-of-leaf-graziers-on-fruit-trees/news-story/cdfb2774af931b58d661bcca8da4b144 )
Thank you, Peter Cundall!


Containment period 2 – when the tree has leaves

As young leaves emerge in spring check your tree regularly. Containment period 2 starts when you find the first Sawfly larvae of the season. It finishes when the tree loses its leaves at the end of autumn.

This is what you are looking for

If there are only a few larvae and your tree is small, you can remove them by hand, but if this is going to be your only containment method, you will have to check the tree two/three times a week.

For a slightly bigger tree you can hose the larvae off the tree with a strong jet of water (away from leaves larvae are without their food source and will die), but again, you will have to do this regularly.

The best containment method is to cover all leaves with ash or powdered lime.

If you have a wood heater, use wood ash (it won’t cost you anything and it is very effective). If you don’t have a wood heater, use builders’ lime. Buy it from a hardware store.  It is a powder. It is cheap.  It is not recommended for use on organic food gardens. Dolomite and other limes commonly used in organic food gardens are often not fine enough for what you are going to do.

Wood ash and lime are highly alkaline.  The treatment described below is quite effective, but you need to make sure that you do not breath in these powdery substances, get them in your eyes, or on your skin.  Please wear eye protection, cover your nose, wear long trousers, and a long sleeved shirt.

Do what is described below as soon as you find the first Sawfly larvae. The sooner you take action, the less damage will be done to the tree.  Now do the following:

• Put lime or ash in a bucket and in that same bucket put a large scoop (half a plastic milk container will do the job). How much lime or ash do you need? We will aim to cover all leaves of the tree with ash or lime, so it depends on the size of the tree. That same quantity you will ideally apply a number of times during the season, so if you use wood heater ash, start saving it up!

• Choose a windy (but not stormy) day, and make very sure you know what direction the wind comes from.

• With a fine spray of water from a hose wet the leaves on the tree. For this, and for what you are going to do next, it is not handy to have a really high tree and this is why I recommended you prune the tree at the start of the season (see pruning in winter above).

• With the bucket in hand, position yourself facing the tree, with the wind blowing away from you towards the tree.

• Throw a scoop-full of ash or lime up in the air, and the wind will take it to the leaves. Any larvae present on leaves will be covered and die within a very short time. The ash or lime will stick to the leaves (because they are wet), so new larvae when they emerge also won’t survive long. Throw up more scoops until all leaves have some ash or lime on them. It does not matter if some leaves are completely covered. The tree won’t mind.

• Repeat this process when there is no longer any ash or lime on the leaves. This might be after the next rain.  If your infestation was bad last season, repeat the process regularly, even if you don’t see any larvae.

Ash and lime are free or cheap and very effective, but there are alternatives.

It is worth considering the organic control Bacillus thuringiensis (traded under various names e.g. Dipel).  Look for a label that specifies that it works against pear and cherry slug.

Limil is lime that has been made soluble in water.  It can be bought at nurseries and hardware stores.  Mix half a cup of limil in half a bucket of water, and spray it onto your tree with a sprayer.  Make sure the limil is mixed in well to prevent your sprayer from clogging up.

Yates Success is a biological control against Sawfly larvae, but it costs a lot more than some of the controls mentioned above.

Please always wash fruit from trees that have been given any pest treatment before eating.

In summary

Ideally action against Sawfly larvae (Pear and Cherry Slug) starts in winter. What you do then, is very different from what you do in summer. At no point do you do anything against adult Sawflies. It is the larvae and pupae that you try to eradicate.

May your tree(s) soon look like this …

Thank you, Max K (fruit tree professional), for your valuable feedback re a draft of this blog post.