Ion Staunton*

…And then there are cherry slugs (sometimes also known as pear slugs). Yuk!

Slimy little mongrels… except they are purebred sawflies… a wasp that doesn’t sting or parasitise. The larvae eat the leaves of cherry, pear, plum, quince, apple and a few other trees. They are a worldwide pest… including in Oz.

Introducing… Caliroa cerasi.

Identification and life cycle

You may never notice the mostly black flying adult, all of 10mm long. It is without the normal skinny waist and stinging equipment of the other families of wasps… see my drawing. The females have the business-like antennae shown and have a saw tooth spike at their egg-laying end which allows her to insert her eggs into leaf tissue alongside a leaf vein. She lays about 50 eggs in her 2-week life as an adult. That’s a short time… they have mandibles, but I don’t think they eat anything.

The eggs hatch in about a week and the larvae emerge from within the leaf and begin eating the soft tissue between the veins (even the tiny veins). They produce a slime coating which darkens to shining black as they get older over eight stages (instars) over almost 3–4 weeks… by which time they can be about 15mm long.

They are a streamlined slug shape (without a slug’s snail-like antennae) but underneath there are three pairs of proper legs and five pairs of prolegs which help them undulate along, especially to get to the edge of the leaf before dropping off (without a parachute) to the soil where they dig in, 30–50mm deep, before going into pupal mode over another couple of weeks. If it’s getting toward winter, they will stay in this dormant pupal case until spring, otherwise, during summer while there are leaves on their choice of deciduous fruit trees, they can make it through 2–3 generations.

Damage

Skeletonised leaves do not work well. If there are a lot of damaged leaves, not enough photosynthesis is going on to help pump up the developing cherries with the sweet sugary taste sensation that makes cherries so wonderful to eat. Or pears, plums, apples… maybe quinces (which are not what I’d call sweet).

Control

Well, we know birds don’t help with biological control of sawflies and, beneficial wasps aren’t ‘traitors’ either. This pretty much leaves it up to you, the grower, to do something.

Theoretically, the systemic insecticides that enter the plant sapstream after the leaves are wetted, will flow through the entire plant, and kill any pest that is eating tissue (leaves, fruit or sap). The label is the law, and you’ll find there are varying withholding periods for different crops from 1-day to maybe two or so weeks… which means by the time that period is up on the named plant, the effectiveness of the toxicity to the feeding sawfly is also past.

Spraying or misting with contact killers is effective against the cherry/pear slug during the day or evening. As always, if you have bees around pollinating flowers at the same time as leaves are being attacked, after-dark application of natural pyrethrum will kill the sawfly larvae while the bees are back in the hive. Next morning, the bees are still repelled by the pyrethrum residues until UV light degrades it, somewhere around morning tea time, so bees are not killed.

How often will you need to apply? Here are some timing factors to consider: If a spray kills all the insects feeding on the leaves, that means any eggs have up to two weeks to hatch and another 3–4 weeks feeding before they drop off to pupate. The pupal stage is another couple of weeks so that is about two months between when those hatching eggs become females ready to lay the next generation of eggs.

l and changing into randy adults? Next day they will begin arriving to lay eggs for a new generation, so, about two weeks after your first spray, slugs will be noticed again. You can wait as long as you decide the leaf damage is not affecting fruit development before a follow-up application.

I checked other websites on control options and found references to dusting lime or wood ash all over the foliage and to bring in poultry to forage through the soil to try to find and eat the 7–8mm pupae 30–50mm below soil level. Best of luck with that. That advice may suit someone with a couple of trees… which is why you read this column I guess.

*Ion Staunton, entomologist, is the manufacturer of Py-Bo Natural Pyrethrum Insecticidal Concentrate. See pestech.com.au or phone to ask him questions on 0407 30 88 67.

Other Sawflies

Heard of spitfires? Lots of larvae with prickly bits that can sting if you’re foolish enough (or careless enough) to touch them. There are a few species; mostly native, attacking the foliage of native trees. During daylight hours, they hang out in full ugly sight in clusters of say 30–50 on top of each other. They’re hoping birds do not see them or that the weight of numbers might intimidate; then again, a bird that suffered a stung throat would be unlikely to want to have second helpings and may even warn their young not to test their luck. As soon as dusk delivers darkness and birds go “home”, these sawfly larvae peel off each other and spread-out seeking leaves to eat.

Sawflies are all in the primitive Family Symphyta of the Order Hymenoptera. Other Hymenoptera Families include bees, ants and other wasps. Of course, bees and ants are highly communal, living in hives or nests… wasps can still be communal but are not subjected to royal rule by queens. Citrus gall wasps may cause the stem to swell in distortion as the plant tries to combat the larva but, there is no party going on.

While you are here reading about packs of insect larvae hurting plants, you may recall another insect group called the bag shelter moths. These are the caterpillar larvae which also go into hiding in a self-woven community silken ‘bag’ during the day, emerging after dark to eat leaves, maybe flowers and developing fruit, while no one is watching. These pests are from the Order Lepidoptera (moths and butterflies).