Q: My shrub dogwood is being eaten by white and yellowish caterpillars. Do I have two pests on my hands, and how do I stop them from defoliating the poor plant?
A: These are actually two color phases (based on age) of the same native insect: dogwood sawfly. While they look like caterpillars, these larvae are actually wasp cousins, not in the butterfly or moth group (the only true caterpillars). This is an important distinction because not all of the low-risk insecticides used on caterpillars work on sawflies. Refer to our sawflies page for more information and images of other common sawfly larvae in gardens.
Before you reach for a pesticide spray, though, try to just pick them off by hand or blast them off with a strong spray of water from a garden hose. Sure, they might just crawl back, but even if they are not injured in the throes of the “water park” ride, they are going to be vulnerable to ground predators like ants, spiders, and beetles, hungry birds, and other animals looking for a snack before they reach the relative shelter of the leaf undersides of their host plant. Plus, the more time they spend having to refind their feeding ground, the less time they can spend eating and maturing.
If you can’t reach enough to pluck them off or must resort to insecticide use, opt for an organic or low-toxicity choice like insecticidal soap, horticultural oil, or spinosad. Make sure all leaf surfaces are thoroughly coated as per product label directions. If a shrub is mostly denuded already, though, don’t bother … the larvae will soon move on to adulthood or starve, and the dogwood will regrow as long as its roots are still healthy.
Tolerance for occasional or moderate insect damage is important for sustainable landscape maintenance, especially when we grow native species in order to support our local ecosystem. A garden using diverse plantings with a mix of species will help you ignore a lone plant that gets hit hard once in a while by an insect outbreak as it takes time to recover.
Q: What can I use to treat my tomatoes? The bottom of some fruits look like they’re rotting and brown.
A: The dark or brown, leathery, sunken area or lesion on the end of the fruit where the blossom used to be is called blossom end rot. Despite “rot” in the name, this damage is not caused by a microbial infection and instead is brought about by growing conditions, so use of a fungicide will not help or provide a cure.
The primary cause is a deficiency of calcium inside plant cell walls, making them weak and prone to breaking down. That doesn’t necessarily mean that the soil itself is lacking enough calcium, however. Inconsistent soil moisture, such as from over- or under-watering or heavy rainfall, impacts how well the plant can absorb what it needs. Overuse of nitrogen fertilizer can also contribute to this tissue deficiency, as can soils that are too acidic (having a low pH) because this reduces calcium availability to roots.
Foliar and fruit sprays of a calcium supplement, marketed specifically for blossom end rot, might help as a temporary measure, but correcting any issues with soil pH, watering, and fertilizing are key to keeping the condition from recurring. You can learn a little more about this disorder on our blossom end rot of vegetables page, since the condition can also arise on pepper, eggplant, pumpkin, squash and watermelon.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.