Bagworms, Soft Scales, and Fall Webworm Remedies

Updated: Jun 21

Tis the season for bagworms and soft scale insects to start feeding on your trees. Bagworms and soft scale are partial to Thuja, Pines, Chaemacyparis, and many other conifers (not gymnosperms necessarily). The bagworms will attack those listed and others of the conifer group having short needles and scales that can be used by the "worm" to create their puncture-proof bags. Right now is the time to treat the bagworms because they are in a vulnerable stage, moving around in soft bodies that can succumb to predators or pesticides. Once the bag is formed the best one can do is pick off the bags. I prefer to use biological controls because there is no insecticide that I know of, that doesn't both kill the young bagworms while also exposing us to neurotoxins. I'm fortunate to have great clients who let me release tiny little hunting wasps who have, year after year, been effective in controlling bagworms on their properties. These wasps are so tiny they can't sting and are easily overlooked buzzing around low to the foliage hunting their prey (see picture below from They consume more than bagworm larvae and can thus treat other pests in your garden and yard. If you use beneficial bugs and have regular pesticide treatments it is important the company treat inside the house only. Or at the very most, only around the perimeter of your house or building. Otherwise, that pesticide is likely to move in the wind (we can't see this) and settle on plants and other objects, including in water nearby, and do damage to these kinds of helpers as well as pollinators.

This is also the time of year to see and treat the early stages of soft scale that will feed on multiple conifer species. There is a Bacillus thurengiensis (Bt, a bacterial predator) effective against soft scale. BUT beware Bt can also be a generalist predator attacking beneficial insects. There are different strains of Bt and their impact on beneficial insects seems to vary among strains. Strains btw, are like different subspecies of bacteria. In a study by Parasetyo et al. 2018 (J of Oil Palm Res) on Bt use in palm oil plantations, they found no decrease in the abundance of diversity of beneficial insects when Bt subsp kurstaki was used. However, work by Pinheiro and Valicente (2021, Neotropical Entomology) found that strains of Btk having two particular genes (Cry and Vip) while effective against their target insect were also generalist predators of lepidoptera (butterflies and moths). Work by Leroy et al 2022 (J of Applied Ecol) found that oak canopy arthropod diversity was significantly and negatively impacted by the use of Bt kurstaki. Bt caused a lower biodiversity loss than the insecticide Diflubenzuron but still, it caused a loss.

I have been using for two consecutive years a Bti (see below) to control the mosquitoes in my yard. It's been very effective against mosquitoes. A favorite pastime of mine is to sit outside and read all year long. But this year I have very few lepidoptera or bees and a yard built for them with over 30 species of flowering native ephemerals and a couple of dozen native host plants for local lepidoptera larvae. A friend from UTK forestry, Jaq, told me on a hike about how Bt may be a generalist biocide (bacterial predator). Has my application of Bt for mosquitoes caused a general die-off of beneficial bugs? Gotta read!

The strain used for mosquito control is Bt israliensis (Bti). Work by Bruhl et al. 2020 (Science of the Tot Environ) did find some evidence of low arthropod abundance in areas where Bti was used. Their study was inspired by previous studies looking at massive declines in bird abundances correlated with the use of Bti. Because ya know, birds only feed their offspring insects. No insects, no well-fed baby birds, and thus low numbers of birds in subsequent years. We've lost more than three BILLION birds from our continent (USA + Canada; Rosenberg et al. 2019, Science) between 2017 and 2019. What are the numbers now (head in sand thank you very much)? Biosevert and Boisevert (2000 Biocontrol Sci Tech) found negative effects of Bti on Diptera (which includes a lot of pollinating flies and there are a lot of these that look just like bees), Lepidoptera (pollinating butterflies and moths), Plecoptera (stoneflies critical to healthy freshwater systems and fish habitat), and Nematocera (beneficial soil nematodes). I am no longer using or recommending the use of Bt israliensis for control of mosquitoes as of three weeks ago. I've planted scented mints, lemongrass, geraniums, and lavender on my porch and in my garden to control their numbers and I keep a bottle of organic insecticide on the porch for my bare feet. Mosquitoes carry West Nile virus which causes a neurological disease that is no joke. I still recommend the use of Bti for clients with children. The good news is that only 9 cases of West Nile have been reported as of June 14 2022 in the whole of the US (excluding Alaska and Hawaii). For me, the importance of helping to maintain beneficial bug and bird populations that we are losing at alarming rates far exceeds the 0.0000001% probability of my being bit by a West Nile virus-carrying mosquito and the 0.0000000001% probability of contracting meningitis or encephalitis. Being bit does not mean becoming infected and even if infected only 22% of the 9 people with the virus also caught meningitis or encephalitis. This is a personal decision and I think we each have to decide what is best for us so no judgment from me if you do or don't use the Bti for your mosquitoes!

Back to soft scales and biological control. There's a soft scale predator I have used for the past three years for clients and it has worked beautifully. I've used it to treat every kind of soft scale. It does the job and if there is a sufficient diversity of plant material to attract soft scale year after year then my predators are going to have enough to eat and feed their babies and return year after year as well. Maybe not the best money-making business model but I'm in this business for the birds and bees and my bank account. We can all cohabitate!

Finally, back to the fall webworms and sponge worms (treat them the same) - they are in high densities north of TN. This could be a big outbreak year for the south as well. The sprays we buy from our local hardware and big box stores (except the organic ones) have been repeatedly shown to cause neurological damage and mess with hormones. Have you or friends or family developed neuropathy? Well, it isn't just the glyphosates causing neural damage, it is also several insecticides. I often wonder because I did so much horticultural work prior to going back to school, did I expose myself to endocrine disruptors leading to my very early onset of menopause. Forgive the TMI but my ovaries gave out at 36 and I always thought it was because I lived and gardened next to a superfund site while at school in Bloomington but maybe it wasn't just that (I didn't know for a year btw that I lived a skip away)? Maybe it was also the years of landscaping and being young and flippant about PPE?

What do I use to control webworms in my yard? Nothing usually because I have been lucky enough to not have anything but a handful of webworms "nests" in my trees since moving in 2020 to this tree-lined paradise. This year however I am relying on the paper wasps. What?!? Yes! They chew up the webworms to feed to their larvae. As long as the wasps are not building a nest near the front or back door where they might get confused about my intentions, I'm good. Contrary to myth, they are not aggressive. After all, they're more likely to die getting in a tangle with a human compared to vice versa. So the evolutionarily smart thing to do is to avoid the big humans unless cornered. Now, if you or your family member are allergic - use the Bt or something called NPV (a baculorvirus) to control your webworms.

I miss my butterflies. I've bought what I can to try and replace what I may have inadvertently killed with the Bti. Carolina sphinx moth, gulf fritillary, and the painted lady butterfly larvae are currently growing big and fat in my kitchen. Once they metamorphose I'll put them outside to pollinate, lay eggs, and pray I see them next growing season. It's been lovely to watch them grow and a couple have made their cocoons. Wow! In those cocoons there make a slurry of their cells to reform into the beauties we all love.

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