Unwelcome Guests: Roseslug Sawfllies

If your rose leaves start to look as though they have become dried and scorched, a closer look may reveal the culprit, the larvae of the roseslug sawfly.  The common name is something of a misnomer:  this pest, which resembles a small caterpillar, is neither a slug nor a fly, nor a true caterpillar.

There are three main varieties of roseslug sawflies but if you live where we do in northeastern New England, you are probably dealing with Endelomyia aethiops. The good news is that this particular pest, unlike it's kissing (um, chewing) cousins, has only one life cycle each gardening season.  Once you conquer the current infestation, you're all set for the season.  Your roses stand an excellent chance of re-foliating and recovering from the havoc it causes.  And if you take steps to mitigate re-infestation next year, you will likely be able to avoid most of the problem you're facing now entirely.

Identifying the Pest

Sawfly larva;  it resembles but is not a true caterpillar.

First about the bug:  The pest you're dealing with is actually the larva or maggot form of a small  black wasp that you'd probably mistake for a  fly if you even noticed it at all.

Since the adult wasp neither eats nor bites nor stings,  it probably doesn't leave much of an impression on you.  But it leaves quite an impression on your roses.

The adult female sawfly wasp uses her very sharp, knife-like ovipositor to make a tiny cut into the flesh of the rose leaf and sometimes (although rarely), into the cane or a stem of a rose shrub.  She then inserts her tiny eggs right inside the tissue of the rose leaf.  If you are very observant and have excellent vision, you might see small irregularities along the edges of the leaves where the eggs have been inserted into the leaf but quite honestly, my vision is so poor, when they started attacking our roses, until the larvae were well into their feeding frenzy, I was blissfully unaware that they were even there.

The eggs hatch into tiny, clear maggots.  Once they begin eating the green, fleshy part of the leaves, in addition to the damage on the leaves which quickly becomes devastatingly obvious, you will notice what looks like small green caterpillars with orange heads.  The green color of the larva's body is actually the green leaf material filling their GI tract and visible through their clear body  They max out at 3/4 to an inch at most in length. Thus, once they become really noticeable, they are well on the way to devouring your rose's leaves.  Voracious eaters, they will eat through the fleshy portions of the rose leaf leaving a web of veins behind, hence the papery thin, brown look of the leaves.  Multiply one by tens or hundreds, and they can munch their way through your rose garden in a matter of days to weeks.

A hallmark of the roseslug sawfly is that it is species specific -- it only eats roses, and for the most part, it only affects the leaves.  Generally, the buds and blooms themselves are unaffected, and it doesn't bother neighboring plantings.

Although there is significant damage to the rose leaves, the heuchera and violets are completely unaffected. Buds at the top of the frame and the bloom that is almost out of the frame at the bottom were unaffected.

The larvae (maggots) resemble small caterpillars and are usually 3/4 inch to no more than an inch in length at maturity.

They can affect part or all of a plant, adjacent plants, but the buds and blooms are usually fine.

After the larvae finishes this stage of the life cycle, they drop off the leaf and onto the ground where they burrow into the soil to form a cocoon like case where they pupate. They overwinter underground and hatch in the spring to start the cycle all over again.

Control of the Pest

Although the rose shrub may appear nearly destroyed, chances are you can not only save it but also rid your plant and rose bed of the problem relatively easily.

The first step is to recognize the problem early on and treat it quickly.  IF you catch it early, you can effectively halt the problem with relatively little damage to your roses.

The way to control the problem is to interrupt the life cycle of the wasp at every stage of its development.  Whereas Endelomyia aethiops only has one full life cycle per season, if this is the particular pest you are dealing with, that is relatively easy to accomplish.

If you catch the problem when only one rose or a few leaves on one or two roses are affected, remove the affected leaves and don't compost them -- they should be incinerated, put with the trash, or treated chemically as you don't want the larvae to pupate in your compost pile where they will develop into wasps that will attack your roses next spring.  If you aren't the squeamish type, you may simply need to pick off the offending maggots and squish them so they don't pupate.  (I'm squeamish.)  Cutting off affected leaves that you think still harbor eggs or tiny maggots is also helpful, although you may be well past that stage when you discover the problem.  Remember, proper disposal of infested material is a major part of controlling this pest.

Spraying the rose and the soil with insecticidal soap - even the organic products - is generally effective.  It's essential to treat the soil as well as the shrub so that the pupating wasps are also killed. The granular and liquid Bayer products for rose care with systemic insecticide take care of all of the different life cycles of this pest and I recommend them if you have a heavy infestation and can safely use them in your garden.  For the most part, the common insecticides (including those labeled as organic) that are effective against typical rose pests will be effective against the roseslug; however, do keep in mind that since this is not a true caterpillar, "Bt" (Bacillus thuringiensis), which is effective against most caterpillars that affect roses will not be affective for this pest.  Pyrethrins are also effective but should not be used near fish or if you have cats as pets.

We generally don't use chemicals in our garden as we abut a conservation area and we also have a large koi pond, but our all-natural garden protocol is usually very effective in keeping these pests in check.  Early detection and prompt spraying minimizes the problem.  This spring, our usual early-in-the-eason spraying was delayed due to rain in May and the maggots began feeding during our vacation week away.  We returned to find a full blown infestation and so we had to play catch-up in dealing with it last month.  It meant spraying when we returned and then again after  3-4 days to be sure we caught all of the maggots.  That was followed by staggered pruning to give the shrubs a chance to re-foliate.

Here is the spray that we recommend for a natural approach: 

In a one gallon sprayer, mix the following:

2 ounces Neem Oil Concentrate, or amount recommended on label; 

   check the label as different concentrations are mixed differently (Neem is optional but I recommend it)

4 ounces of canola oil (any brand, canola cooking oil;  other vegetable oils are okay but not as effective)

2 ounces of all natural dish liquid - Seventh Generation is one brand that is available at Lowe’s, 

   Shaw’s, Albertson’s, Wal-Mart;  Palmolive also has a clear, all-natural dish soap; pure castile soap or any all natural 
   dish liquid will do)

2 Tablespoons of Baking Soda 

2 ounces garlic oil (optional, see recipe below, can be substituted for canola above) 

Add water to make one gallon of solution.  When spraying, shake the sprayer frequently to keep mixture homogenous.

Garlic Oil:   To make your own garlic oil, peel most of the outer paper covering from a medium sized garlic bulb and separate the cloves.  It is not necessary to peel the cloves.  In a a small electric food chopper, finely mince the garlic and blend with 4 ounces of canola or soybean oil.  Let sit in refrigerator for at least 48 hours and then strain through cheese cloth or a fine mesh strainer.  This can be made in larger quantities and kept refrigerated for several weeks.

To Spray:   Spray shrubs liberally when there is no rain expected for 24 hours, being sure to spray top and bottom of leaves and also spray soil surrounding plant.  If using Neem, spray at dawn or dusk . If we spray in the morning, we spray at about 5:30 AM.   If we spray in the evening, after the beginning of May, we wait until 7 PM or later. This is necessary because Neem is toxic to honey bees and butterflies.  

Follow-up:  In the fall, be sure to spray with horticultural oil or canola.  In early spring, before roses leaf out, we spray the soil in our beds with a 10% solution of ammonia (effective against many pests that are harbored in the soil) and all plants with the above mixture as soon as the first leaves begin to appear. 

Care of Affected Roses

The damage  wrought by the sawfly is localized and the rose itself is not infected systemically.  Once the rose has been effectively treated, removing the affected leaves and pruning heavily is done as much for cosmetic and aesthetic reasons as for stimulating new growth of healthy leaves and canes.

When pruning, it's important to leave as much intact leaf material on the plant as possible as even damaged leaves produce critical nutrients and energy that the rose needs to continue growing.  We usually prune in 2-3 stages, depending on how badly the rose is affected.  Carefully dispose of leaf litter and prunings as composting affected leaves that still have eggs or larvae will only carry the problem into the next growing season.  Small amounts of cuttings make excellent fuel for your chiminera or decorative fire pit.

Once the affected leaves are pruned away, the shrub should grow and thrive, but TLC in the form of some extra compost, compost tea, and fertilizing at regular intervals will help the shrub to re-foliate quickly and regain its vigor.  In areas where the species of wasp has multiple growth cycles in a single growing season, repeated serious infestations can weaken a rose to the point where it might not survive a difficult winter or other harsh weather conditions, or it may so weaken it that it is susceptible to other pests and fungal infections.  That said, infestation with the rose-slug sawfly in and of itself is generally not lethal to the shrub.

Only a month has passed since we first encountered this problem and the roses have almost completely recovered.  Steve just pruned Macy's Pride, which is shown in several of the above photographs, for the third and final time.  With aggressive spraying of the ground and shrubs in the spring with oil and ammonia, we hope to avoid a repeat of this spring's attack of the rose-slugs.