Common name: pear sawfly; pear slug; cherry slug; pear slugworm; cherry slugworm
Description & Life Cycle: Adult pear sawflies start emerging from the soil over a few weeks starting in mid-spring. They are non-stinging wasps, shiny brownish-black in colour and have two pairs of transparent wings which they hold folded over their backs when at rest. After mating, the females lay their eggs under the epidermis on the upper surface of a leaf using their saw-like ovipositor.
The oval-shaped eggs are a tan-pale orange colour and roughly 0.04 of an inch (1 mm) long. They look like a small blister on the leaf.
The first generation larvae begin to emerge in May. Early instar stages are grey-green in colour and have no distinct legs. They tend to hold their rear end up slightly. Body shape is wide in the front, narrow in the rear giving them a slug-like appearance, especially as they secrete a slime which completely covers the bodies of the early instar stages. As they progress through the 2nd to 4th instar stages, their body colour darkens to a green-black, then to black caused from a coating of their liquid body waste. They feed on the upper leaf surface, eating through the epidermal layer but stopping before they have eaten all the way down to the lower epidermal layer of the leaf. The removal of the top layers of the leaves produces tan skeletal-like patches…a sure sign there are pests at work on your plant. By the last instar stage, the larvae have developed a more caterpillar appearance…green body with an orange head and 10 pairs of distinct legs. At this stage, they are 0.4 of an inch (9.5 mm) long.
When the first generation of larvae are ready to pupate, they crawl or drop to the ground…digging themselves into the soil. Duration is roughly 3 – 4 weeks with the second generation adults emerging in late-July or early August. Females lay their eggs in the same manner directly after mating. Second generation larvae appear in mid- to late August.
These second generation larvae do the most damage, likely because their numbers are higher and they are preparing to over-winter. Severe feasting damage to the leaves can cause them to drop prematurely, thus weakening the plant. Luckily, they are not known to destroy their host plants. When these larvae reach the fifth and last instar stage, they crawl or drop to the ground and dig themselves 2 to 3 inches (2.5 – 7.5 cm) down into the soil where they spin a cocoon and over-winter.
Special Notes: Caliroa cerasi is native to Europe. It is unknown when this pest was first discovered in North America but speculation puts the timing as far back as colonial times. Today, it is found throughout Europe, Asia, Africa, Australia, Tasmania, New Zealand, parts of South America (Argentina, Chile, Uruguay), the United States, and Canada.
“Pear slug” is the officially recognized common name.
Remedial Actions: Usually, the damage has been done before the gardener notices there is a pest at work. Best line of defense against the pear slug is to hit the affected plant with a jet of water or a dousing of insecticidal soap. However, this may prove difficult if the tree is way above your head.
If the larvae are dropping to the ground to pupate, there is nothing available which will destroy the pupae once they are underground.
In our Zone 7a garden: We have been fortunate up until this year (2017) to have dodged this pest…and we have seven out of the ten host plants I noted above! (Pear tree, plum tree, apple, a flowering quince (Chaenomeles), a serviceberry (Amelanchier), a cotoneaster, and a Sorbus (mountain ash).
This year, my Aronia melanocarpa (black chokeberry) has been hit. I only just realized the damage as I was harvesting the berries this week. Too late to do anything this year but I will definitely be watching for the adult sawflies to appear next April and May…although there is not much one can do to deter the adult females from mating and laying eggs. But I have read Neem oil may be effective in retarding the larvae from molting to the next instar stage. Worth a try. If it does not work, I can always turn a jet of water on them.
Posted on September 20, 2017