I knew that my sweet potatoes were infested with something, but I assumed it was wireworm. The blackened tunnels snaking through many of my sweet potatoes looked similar to those I had seen years ago at one of my volunteer projects, a Plant a Row for the Hungry garden.
However, today, as I was poking through my basket of sweet potatoes, looking for a "good one" to cook for lunch, a critter crawled out of one of the sweets. Here he/she is:
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| A little blurry, because it was moving, but the critter is definitely a weevil. |
A weevil!
Note -- there is another weevil on the right side of the tuber that is not moving, because I smashed it -- a reflex action -- before I remembered that I needed to take a picture, to share with my gardening friends!
Damage caused by sweet potato weevils
My sweet potatoes that have black tunnels inside also have holes in the outer skin, where the pests have gone in and out of the tubers.
This is the inside of the tuber that the full-grown weevils crawled out of:
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| Weevil damage in my sweet potatoes |
I sliced away a bit of the side on that left-hand piece, to show the snaking tunnels more clearly.
What I didn't realize back in the summer is that holes in the leaves, that I saw at that time, could have been caused by weevils. Honestly, the thought that weevils might be responsible for holes in the leaves didn't even enter my mind. This is a new pest for me!
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| Pest-predators in the garden, like this frog, could not keep up with the hole-making critters. |
What makes figuring out which pest ate holes in the leaves difficult is that many pests, sweet potato weevils included, mostly eat at night, when I am not likely to see them.
It also is new to me that a leaf-eating pest could also be a tuber-eating pest. My experience is that the two groups are usually completely separate.
The pictures in University of Florida's "Featured Creatures" article on sweet potato weevils are pretty horrifying. They show a tuber with an entirely blackened interior spotted with maggoty-larvae. For the non-squeamish, there are close-up pictures of the larvae, to aid in identification.
Getting rid of sweet potato weevils
The weevils are unlikely to just disappear without some kind of "help". Since I am an organic gardener, the kinds of "help" I can give, to remove the weevils from my garden, are limited.
However, I am hopeful. The UF article linked above does include multiple actions that organic gardeners can take to reduce or eliminate a sweet potato weevil infestation. These are suggestions that UF offers:
Get rid of other plants in the morning glory family
Sweet potatoes are in the morning glory family, and the sweet potato weevils feed on all related plants, apparently. Unfortunately, I live within a half mile of the beach, which is home to a lot of beach morning glory vines. Those are not mine to remove (and I wouldn't want to, because they support a lot of pollinators).
I also have some morning glory vines climbing one of my clothesline supports. This is going to change.
I will be looking for another flowering vine to replace those morning glories, and I will pull up all the new morning glory vines that sprout in that area in spring.
Keep the soil moist
Dry soil opens up as it cracks, which makes it easy for the weevils get into the ground to infest sweet potato tubers. Last year (2022) was very dry here, but I did not worry much about keeping the sweet potatoes watered, because they are so drought tolerant. Even though the garden is mostly sand, cracks probably formed.
Moist soil also helps promote growth of a fungus that fights the weevils underground.
The previous year had been super-wet. We ended up with more than 100 inches of rain in 2021. If the weevils were already in the garden back then, the constant wetness (no cracks, plus happy fungus) may have kept them from doing obvious damage.
Rotate crops
Always, gardeners are told to rotate crops -- making sure that no crop is grown in the same spot within three to five years. For home gardeners, this is mostly not practical, or even possible. All we can do is try, and be hopeful.
However, for many pests, including sweet potato weevils, this recommendation might not be super-useful.
Clean up old vines and tubers
Removing every bit of vine and tuber from the garden at harvest time removes food that weevils might survive on over the winter.
However, cleaning up every single bit of vine and tuber is difficult. No matter how many times I re-dig an area, it seems like I always find one more small tuber or length of vine. Still, I will hunt for leftover bits of vine with even more diligence as I prepare the garden for spring planting.
If I had known about the weevils in fall, when I harvested the crop, I would have put all the vines into the landfill instead of my compost pile. There are probably some weevils humming happily in my compost as I type, waiting for me to spread them back onto the garden.
Luckily, there is another option for getting rid of weevils that will make my oversight ok.
Add some parasites
This is where I am headed. There are wasp and ant predators on sweet potato weevils that may already be working in my yard, but beneficial nematodes seem to offer the best hope for reducing the weevil population by a significant amount.
Calling them beneficial nematodes makes them sound totally wimpy, but these little guys act as parasites on pests, finding a way inside, injecting bacteria that kill the pest, then "eating" the remains of the dead pest.
Even better, beneficial nematodes do not harm frogs, birds, salamanders, squirrels, or other garden (or nearby) wildlife, unlike many chemical control options.
I had already been looking at nematodes as a treatment for wireworm -- when I thought that was the cause of my sweet potato damage -- and also for small hive beetles, for my beehives.
Back in north Georgia, treating the Plant a Row for the Hungry garden with beneficial nematodes worked like a miracle to get rid of the wireworms. It took two years before the wireworms were completely managed, but the first crop after applying the beneficial nematodes showed BY FAR less damage than the previous year's crop.
I am looking at a "Combo Pack" of beneficial nematodes from Arbico Organics. This is a pricey option, but I am expecting it to be totally worthwhile.
Beneficial nematodes in both products are listed as attacking sweet potato weevils. Applying both seems like a good option for making sure that the weevils get attacked. The smallest combo pack covers a lot of ground, so I will also apply it around my fruit trees and other places in the yard.
The nematodes need to be applied after the garden has warmed up a bit, and they have a super-short shelf-life, so it will be a few weeks before I order these.
I will let you know how this all goes! At sweet potato harvest time, I will have some kind of result to share.
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