Gardenbite: Why are there holes in my roses? Roseslugs | #GoodGrowing

Welcome to the good growing podcast. I am Ken Johnson, horticulture educator with University of Illinois Extension coming to you today from Jacksonville, Illinois with a gardenbite. And on this week's gardenbite, we're gonna talk a little bit about one of the more common pests we find on roses, rose slugs. So roses are grown by many people in their gardens for their large, showy, and usually fragrant flowers. And while they may not be as popular as they used to be, they are still quite popular garden plants.

And if you've grow your own roses or you've enjoyed roses, looking at roses, you may have noticed small holes in the leaves or windowpane like feeding damage on the leaves of rose plants. And we often start seeing this sometime mid to late spring and into the summer. When we get later into the summer, a lot of that damage is caused by Japanese beetles. But earlier in the year, this is usually caused by rose slugs. So what exactly are rose slugs?

We've got three different species of rose slugs that can be found here in Illinois. So we've got the European rose slug, the curled rose sawfly, and the bristly rose slug. And rose slugs are caterpillar like insects. But despite their common name and their appearance, they're not slugs nor are they caterpillars, but they are sawflies. And sawflies are related to bees, ants, and wasps.

So these are gonna be in the order Hymenoptera, and this will become important when we get into managing these pests. And we often consider them pests because they are feeding on plants. Unlike bees, which are pollinating and and wasps, they're feeding on other insects and pollinating. Sawflies are kind of the group of insects in that Hymenoptera that we generally consider pests because they will feed on plants and cause damage. And they get their common name, sawflies, because they have a saw like ovipositor, which is the egg laying structure that they have, and they're gonna use this ovipositor to cut slits into stems or leaves of plants to lay their eggs.

And like I mentioned, these these sawflies, they look like caterpillars. And there's a few ways you can tell a caterpillar and a sawfly apart. So for caterpillars, their eyes are very small and difficult to see. Whereas on sawflies, they have relatively large eyes, and and they can be seen pretty easily. So if you think about in in children's books, stuff like that, caterpillars are usually drawn with these big visible, easily noticeable eyes.

What they're really drawing is a soft light. And another thing we can look at, that's probably the easiest way to tell them apart is is to look at the eyes. Another way to tell them apart is looking at the the pro legs of the of the insects. These pro legs are fleshy structures on the abdomen. They look and act like legs, but they are not actually true legs.

So caterpillars are gonna have five or fewer pairs of pro legs, whereas sawflies are gonna have six or more pair of pro legs. An easy way to remember that is if they have enough to spell sawfly, it's a sawfly. Back to the ones that are causing damage on our roses. So again, we've got three here in Illinois that are typically causing problems. And all of these are going to be introduced either from Europe or Asia.

European rose slug, these larvae are light green, they're gonna have orange heads, and it'll grow to be about one half inch long. Typically, they're feeding on the upper surface of leaves, and they will leave behind the veins or the lower leaf surface. So you can get some of this window painting type damage from them. And they're usually feeding from May through June. When they're done feeding, they're gonna leave the plants and they will pupate in the ground, and they're gonna stay there until the following spring.

So we only have one generation per year for the European rose slug. The adults will then emerge in the spring, and the eggs are gonna be laid in pockets near the edges of the leaves. Next, we've got curled rose sawflies. And again, these are green, but they're gonna have white spots and have yellow heads with a dark stripe down the middle. And they're gonna grow to be about three quarters of an inch long as larvae when they're full grown.

When they're small, again, they're gonna skeletonize leaves. And as they grow larger, they can eventually eat entire leaves except for maybe the main mid vein of the leaf. For these, in addition to their feeding damage, when they pupate, they will bore into the twigs of rose plants, particularly those that have been pruned, and this can cause some further damage to the plants. And the reason we call them curled rose slugs is when they will often curl up when they're when they're resting. And for this particular species, we have two generations per year.

The the final one and probably the one we most commonly encounter is gonna be the bristly roast slug. And these are green and they're gonna have fine hair like spines on them. And you have to get somewhat close in order to be able to see these. And they'll also grow to be about half an inch long when fully grown as larvae. Again, when they're small, they're gonna typically feeding on the undersides of leaves and causing that windowpane like damage.

So again, only the upper leaf surface is left behind there. And then as the larvae get larger, they can eat larger holes and make it all the way through the leaf tissues there. And if you get a lot of them feeding those those feeding holes, they can end up eating an entire leaf except for maybe the large mid vein in the middle there. When they are done feeding, they're gonna spin a cocoon and plant debris or sometimes on the rose plant itself. And for these, there can be six or more generations per year.

And this means they are capable of causing, they can be capable of causing extensive damage, especially towards the end of the growing season, as this feeding damage keeps building up and those populations, potentially grow higher and higher. And the eggs for these are gonna be laid in the midribs of the rose leaves. So if you have damage from rose slugs, there's there's several different things we can do. And again, remember, European and curled rose slugs, we only have one or two generations of these, so they're only present early in the growing season. Because of this, management usually is not necessary.

So this is where identification is going to become important. Again, however, because bristle roast slugs can be present throughout the growing season, again, we have up to six generations per year, maybe more, potentially on where you're at in The States. Man we may need to manage these if the populations become too high. So if you have small infestations, we can remove these by hand, hand picking them, or we can use a forceful spray of water can be used to knock often the small larvae off of plants and making we need to make sure we're getting both the upper and lower sides, undersides of the leaves if we're gonna be doing this. If populations get too high, insecticides can also be used to manage the populations.

And then but remember, even though they look like caterpillars, they're gonna be related to bees, ants, and wasps. So some of the insecticides we use for to manage caterpillars may not be very effective, or something like the bacillus thuringiensis gristachiae, BTK, that only works on caterpillars. So you can put that onto rose plants, it will not affect sawflies. So again, identification is going to be very important when we're trying to manage these. So we're using the proper insecticides in order to manage these.

So some insecticides that we can use to manage roast slugs, something like horticultural oil, insecticidal soaps, acidophate bifenthrin, carbaryl, and cyclothrin are some of the active ingredients you would want to look at for managing these. Again, if you do decide to use pesticides, make sure we're reading and following all the label directions, while roses may have limited, if any, pollen, depending on the type of flower you have. It's a full double. But our more open flowers will have nectar and pollen available to pollinators. We wanna make sure we're avoiding getting insecticides on flowers, because anything that we are going to be applying to kill rose slugs is more than likely going to kill pollinators too.

So again, make sure you're reading the label following any restrictions that may be on there for applying to flowering plants because these are flowering. You may wanna remove flowers or spray this in the evening when pollinators are going to be less active. But again, there are other things like hand picking or using a a strong spray of water to remove these insects off of plants without the use of insecticides. Well, that's all I've got for you for this week's garden bite. Hopefully, you have a little better understanding of how we can manage rose slugs in our landscape, or if you've got some damage on your rose plants, hopefully, now you have an idea of what may be causing it.

Good Growing podcast is a production of University of Illinois Extension. Thank you for doing what you do best and that is listening or if you're on YouTube watching. And as always, keep on growing. Hi.