Rose pests that are out of sight but not out of mind: leaf miners and chilli thrips

In a perfect world, a rose garden would have no pests. This is wishful thinking and is never going to happen. So, the second-best option in a rose garden is that the pests be easy to spot. “Plain view” pests are easier to manage, and we feel a definite satisfaction when we can see their successful elimination.

Hoplia beetles destroy our rose blooms in the spring, but the battle we wage with this pest is fairly easy to win. Dark brownish-gray in color, they are very visible when they feast on their preferred, light-colored blooms. Similarly, the metallic-green fruit beetles that gorge on our ripe peaches, figs, apples and roses in the summer don’t steal into the garden surreptitiously. They loudly announce their arrival into the garden, zooming and buzzing right past us in an iridescent flash of color. Both varieties of beetles really focus as they devour their favored plants, which makes it gratifyingly easy for us to catch them. All that remains is to drop them into a bucket of soapy water.

We are usually hard-pressed to find a single positive thing to say about spider mites and rose slugs, but here are two points in their favor: We can see these pests when we look closely on the underside of damaged leaves. And because we can see them, we do get the gratification of being able to target and dispense spider mites with jets of water, and we can squish rose slugs with a press of our fingers.

But some insect pests just don’t fight fair. The damage they do is very apparent, but we literally can’t see them.

There are two rose garden pests — leaf miner larvae and chilli thrips — that use invisibility to their advantage. The leaf miner ranks low on the list of problem pests, but chilli thrips are another matter. In San Diego, they top the list as the most unwanted pest in the rose garden.

Leaf miner (genus Liriomyza) adults are small moths and flies. The female adult lays an egg just below the surface of a leaflet with her sharp ovipositor. When the egg hatches, we are unable to see the damage-causing larva as it feeds in twists and turns in a serpentine trail within the leaf for two to three weeks. Other plant families have their own leaf miners with the host species and characteristic form of the larva’s leaf squiggles helping to identify the insect species; this includes the larvae of some sawflies and wasps.

This pest is easy to relegate to the “out-of-mind” category as its leaf “doodles” may seem innocuous and easy to ignore. But failure to remove and dispose of affected leaves means an ongoing, perpetual graffiti on the inside of our rose leaves. This is because when the larva matures, it eats its way out of the leaf and drops to the soil, where it pupates. When it becomes an adult, it emerges from the soil and mates, and the life cycle repeats with the female adult laying an egg just below the surface of a leaflet. There are several generations annually.

Chilli thrips have the distinction of being our most massive pest problem in the San Diego rose garden. Part of the problem is that we cannot see chilli thrips at any stage of their life cycle (egg, first and second instar larvae, prepupa, pupa and adult) with the naked eye. (Chilli thrips measure 0.016 to 0.024 inch in length, about one-quarter of the size of the western flower thrips.) During the course of their life cycle, chilli thrips are in the unopened bud, in the leaf, on the leaf, in the bloom, in the grooves where leaflets attach to the stems, and in the soil or litter on the soil. And, at every stage, they are out of sight. But this is not a pest that can be out of mind.

In hot weather, the life cycle can repeat from egg to egg in 11 days, and within that period, female chilli thrips can lay 60 to 200 eggs. Because of this rapid-fire reproduction and because chilli thrips can be transported 60 feet on a breeze, delay and hesitation with this pest means that damage can become horrific and spread to roses and other host plants all over the garden.

Chilli thrips larvae and adults extract sap from new growth and tender buds with piercing and sucking mouthparts. The first sign of damage is quite innocuous and does not generate much alarm to rosarians unfamiliar with this pest. The damage consists of a slight marbling of red and green on the back of fresh new foliage, and sometimes there is a slight crinkling, curling or puckering in the new foliage. Within days, we notice worrying, dirty-brown streaks on the back of the new leaves and brown or bronzed-tinged buds.

The finale is chilli thrips’ unmistakable signature: scorched and deformed blooms. Unfortunately, unlike western flower thrips that feed mainly on light-colored blooms, chilli thrips have an indiscriminate palette and attack blooms of every color.

In the management of chilli thrips, we are handicapped in our mechanical control as we cannot squish a pest we cannot see, and we cannot see if blasts with jets of water are even effective. Minute pirate bugs and the larvae of the lacewing and syrphid fly do attack chilli thrips; however, chilli thrips immigrated from Southeast Asia without their natural predators, so they are pests without sufficient biological enemies here.

Hot summer days and dry Santa Ana winds are prime weather conditions for chilli thrips infestations. If your new foliage or buds are looking even remotely iffy, and especially if we are experiencing hot weather, don’t hesitate — just prune out all questionable damage, and bag up and dispose of all the cuttings.

To combat chilli thrips, some rosarians spray the new foliage on their roses preventively in the hot months. This can be ineffective: Chilli thrips can evade pesticide application because much of their life cycle and feeding is within the leaf, and within the terminal buds and unopened bloom. If you decide to spray, read the pesticide label carefully. Some pesticides can harm minute pirate bugs, lacewings and predatory mites or spiders, which are some of the very beneficials that help us in our fight against this and other pests. Spinosad is an organic pesticide that targets chilli thrips, but it can be toxic to bees for three hours after application, so spraying with products that include this active ingredient must be done in the evening, after the bees are done working for the day. Also, be aware that if you choose to use pesticides, you must rotate them with pesticides that have a different mode of action to prevent pest resistance.

Some insect pests wear the cloak of invisibility. They might be out of sight, but the damage they do is very much on our mind. When we battle these pests, we must be very vigilant to the very first signs of their damage and nip their life cycle right away.

Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension.