The large rose sawfly (Arge ochropus) is a member of the family Argidae. Native to Eurasia, this garden pest has been introduced to eastern Canada and the northeastern United States. To the untrained eye, the insect can look like a fly from certain angles, but it is actually a wasp relative. It is not the adult rose sawfly, itself that causes damage to rose bushes but rather its larvae, which are called "rose slugs" (even though these caterpillar-like creatures are not really slugs). In fact, while the larvae eat rose bush leaves, the adults feed on the nectar and pollen of entirely different plants. The larvae can do significant damage to roses; here is how to manage them.
What Do Rose Sawflies and Slugs Look Like?
The adult large rose sawfly is about 1/3 inch long. Its head and thorax are black; its abdomen and wings are orange.
Since it is the presence of the larvae that you will be checking for, it is more important to be able to identify the rose slugs than the adults. They are an olive color except for the head, which is orangey-brown. They can be as much as 3/4 inch long. Their bodies are somewhat slimy.
4 Ways to Get Rid of Rose Sawfly Larvae
Control methods are targeted at the larvae, not the adults. Both manual methods and insecticides can be used to manage these pests, including that living insecticide known as the "predatory insect." Manual methods have the benefits of being free and not only organic, but also safe for beneficial insects. All in all, manual methods are your best option.
Hand-Pick Them Off
Regularly inspect the undersides of the leaves of your rose bushes. If you spot rose slugs on them, pick them off by hand. This is the most straightforward manual means of control.
Knock Them Off With a Hard Spray
A second manual method to use, upon spotting rose slugs, is to direct a blast of water at them from the garden hose. This will knock them to the ground; they are unable to climb back to the leaves of your rose bush, and birds may eat them while they crawl helplessly on the ground. This manual method will be preferred by those who are squeamish about touching caterpillar-like critters.
Insecticides that you can apply to an active infestation of rose slugs include carbaryl (sold under the brand name, Sevin), Neem oil, and pyrethrin. All three kill rose slugs when they come into contact with them. Of the three, Neem oil and pyrethrin are organic, but many gardeners still avoid them because they can harm beneficial insects as well as pests.
Use Beneficial Garden Insects
Rose slugs have natural enemies that will eat them. These include parasitic wasps and predatory beetles. However, you can't just assume that these predators will be around to do the job. Such beneficial insects can be bought online. You can also foster an environment in the garden that may attract them naturally by avoiding using insecticides.
Signs of a Rose Sawfly Larvae Infestation
How dangerous a rose sawfly infestation is to your rose bush depends on the size of the infestation and how well established/healthy the rose bush is. A large infestation can be fatal to a rose bush that is newly planted or already suffering from another problem (for example, a disease). Likewise, an infestation can make a compromised plant more susceptible to a disease in the future.
The surest way to know you have an infestation is to detect the larvae on the leaves. Short of that, look for leaf damage. Rose slugs, which will begin their attack in late spring, chew into the tender parts of a leaf (the areas between the veins). The result will be a skeletonized leaf. The "bones" (veins) then turn brown.
What Causes a Rose Sawfly Infestation?
There are natural enemies to the rose sawfly and its larvae outside of the insect world as well. These include birds. Whether it be these predators or beneficial insects, their presence on your land can keep the population of harmful insects down naturally. However, when they are absent, this is an open invitation to harmful insects such as rose sawflies and their larvae.
Create a predator-friendly environment in your landscape so that predators will stay/come and thrive. For example (besides avoiding the use of chemical insecticides), understand that birds prefer a landscape with cover (trees and shrubs) to an open landscape where there are no places to hide. You can also make your yard more hospitable to bird predators by providing water where they can drink and/or bathe.
How to Prevent a Rose Sawfly Infestation
A plant well cared for is a plant that, generally speaking, will be better able to ward off pest problems. So begin by giving your roses the basic care that they need to thrive.
Systemic insecticides have the benefit of allowing you to be proactive and take preventive measures, rather than waiting for an infestation to occur. Apply a systemic insecticide to the soil around your rose bushes in early spring to mid-spring for rose sawfly control. An example is imidacloprid. However, this type of insecticide, too can harm beneficial insects.
Another way to minimize the likelihood of an infestation of rose sawflies (short of not growing rose bushes at all) is to take away the food source of the adults. For example, the adults feed on the nectar and pollen of common tansy (Tanacetum vulgare), and most gardeners would gladly give up growing tansy if it meant having an easier time growing roses. However, if a neighbor of yours is growing tansy, adults can easily enough fly into your own yard and deposit their eggs on your rose bushes. Other food sources for adult sawflies include cow parsnip (Heracleum sphondylium) and wild angelica (Angelica sylvestris).
The more numerous and diverse the bird population is that visits your yard, the more likely that you will have a bird ally that will do the job of eating adult rose sawflies for you, thereby preventing them from laying eggs.
Another way to prevent an infestation (or a further infestation, at least) of rose sawfly larvae depends on understanding the insect's life cycle, which is similar to that undergone by many different types of insects. The wasp-like adult emerges from pupation in early spring. The female lays her eggs on the underside of a leaf of a rose bush. The resultant larvae come out in several weeks to begin their month-long decimation of the plant's leaves. Finally, they drop to the ground and enter the pupation phase in the soil near the base of the rose bush, which lasts till early spring, bringing the cycle full circle. For purposes of prevention, take advantage of the pupation stage. If you lightly cultivate the soil around the base of the rose bush, you will expose the pupa not only to the harshness of winter's cold but also to hungry birds.
Arge ochropus Larvae vs. Similar Pests
There are many caterpillar-like larvae that attack your ornamental plants. The good news, if you do not have the time and energy to distinguish between all of them, is that the control measures you need to take to fight them are largely the same. The rose is just one ornamental plant that is attacked by sawflies. There are different types of sawflies that attack fruit trees, ash trees, oaks, pines, and dogwoods. There is even more than one type whose larvae eat rose bush leaves:
- The other large rose sawfly (Arge pagana): Bearing the same common name as Arge ochropus, this pest is, indeed, similar to its namesake. The adults look largely the same except that the wings of Arge pagana are black; its larvae can be distinguished from the larvae of Arge ochropus by their black spots.
- Bristly rose slug sawfly (Cladius difformis): True to its name, the rose slug of this pest has bristle-like hairs sticking out of its light green (sometimes black) body.
- Curled rose slug sawfly (Allantus cinctus): Here the larvae have a light green body with white spots; the head is greenish-yellow with black eyespots (an anti-predator defensive marking). When not actively feeding their bodies curl up, thus their common name.
- Eurasian rose slug sawfly (Endelomyia aethiops): The larvae of this type are similar in appearance to those of Arge ochropus, but the adults are different, being a solid black.
Even in cases where you are unable to arrive at a positive identification on the larvae, the damage they cause will have a similar appearance.
Do rose sawflies sting or bite?
No, they are quite harmless to humans. Their larvae do not bite either.
Why are they called sawflies?
The females of this garden pest have saw-like genitalia that allow them to penetrate the rose plant tissue where they lay their eggs.
Are rose sawflies wasps?
No, but they are related to wasps. They differ from wasps in not having a "waist."