Eggplant Webworm Late Instar with frass and silk Rhectocraspeda periusalis. Photo taken by FOL in The Serene Forest
It has been pretty tricky trying to identify this new critter in The Serene Forest. Our recent plantings of ichiban eggplant have become borderline infested with these "worms" which are actually caterpillars for a moth. Numerous internet searches and even a specimen send off to the Florida Department of Agriculture has left us with a hesitant identification of this worm.
For now, we are identifying this worm as the larvae of the Eggplant Webworm, Rhectocraspeda periusalis. A differential diagnosis (as we say in the medical field) would be Omiodes indicata, the bean-leaf webworm also called the soybean leaf folder. Due to the fact that this critter was found on our eggplants, the Eggplant Webworm would sound more likely. To me at least. This post will be updated if any new information contradicts this.
Regardless, these little guys have been very destructive to our eggplants. Eggs are laid solitarily on leaves. Once hatched, new instars mine the eggplant leaves before becoming big enough to exit and start munching on leaves from the outside. Younger instars are typically yellowish without any distinctive markings. Later instars begin to exhibit two reddish brown dorsal stripes. The head is noticeably different in color - like a light brown. At this point, you can see the larvae pretty easily. They begin to use a silk like substance to "roll' or fold leaves over themselves in protection from predators. This characteristic is what give the name "webworm" or "leafroller" or similar common names.
Host plants include:
Some other Nightshade plants
Damage appears more commonly nearest to the midrib of leaves. Younger, more tender leaves are preferred. The damage often times causes leaves to turn brown and eventually fall off the plant. Just priot to pupation, the larvae will roll the leaf over their chosen site where a cocoon is formed (cocoon for a moth, chrysalis for a butterfly). The cocoon is short lived. In about 3 days they will emerge as the Eggplant Webworm Moth.
At this point we obviously have pointed out why this critter can be considered a gardening foe. Once a moth, it is a nocturnal pollinator bringing benefits to the garden - a gardening friend. So it is complicated on distinguishing it as a friend or foe.
Like any other gardening foe, we highly recommend allowing them to exist in your garden's natural ecoystem. Natural predators will eventually show up and help control their numbers. Sacrificial planting is a technique that is proven to be successful in managing these worms. By allowing a few eggplants to be hosts, you can help allow the moths to emerge. By moving larvae from your chosen "winners" to your sacrificial plants you can still harvest beautiful eggplants from the chose "winners".
Natural predators include parasitic wasps, other solitary wasps like the Red Marked Mason Wasp, assassin bugs, birds, lizards, among others. The beauty of allowing natural predators to regulate your garden is that they will continue to do so. Once their food source is found, the area is on constant patrol for more. And its free!
If all seems lost, there are chemical methods that can be used to keep Eggplant Webworm larvae from destroying plants completely. We suggest only using chemical methods if all seems lost. Keep in mind that by using chemical methods you will never allow enough time for predators to find and destroy these larvae for you. Therefore, future larvae will also likely not be found in time. Bacillus thuringiensis commonly known as B.t., is the most effective chemical treatment. B.t. is listed as an organic treatment against worms and caterpillars. B.t. is a bacteria that targets worms and caterpillars specifically. Therefore, it will not harm other beneficial insects. It also does not harm moths or caterpillars.
I would like to emphasize our stance on allowing the garden to be an ecosystem. Natural predators have proven to be effective enough in The Serene Forest that we never have to use pesticides.
Rhectocraspeda periusalis Eggplant Webworm. Photo taken by FOL in The Serene Forest