When the Pest Becomes the Pollinator

I think this is an Eastern Black Swallowtail on my zinnia

Because I planted lots of flowers and vegetables this spring, I am especially interested in all the pests that are competing with me for the food and with the pollinators that are insuring that the fruit matures. Getting rid of pests, however, is a delicate balance because some creatures are pests at one time in their development and beautiful pollinators at another, such as butterfly caterpillars. Some are more voracious than others in their caterpillar stage, destroying one's fruit or vegetable crop within days if not exterminated.  For instance, the adult moth (Helicoverpa zea) of the tomato fruitworm is a pollinator, but the caterpillar stage of the moth has been a constant problem this spring and summer. We have used Bacillus thuringiensis to combat the caterpillars, but they return, and by the time I see them (for they start out very small), they've done some damage to the fruit and leaves of an individual tomato plant.

According to Wikipedia, the tomato fruitworm is prolific in the wild,  eats many different kinds of agricultural plants, and is variably susceptible to Bt.  It's called the cotton bollworm when it feeds on cotton and the corn earworm when it feeds on corn, but it eats more crops than these. Because the tomato fruitworm is so voracious and because it eats the summer vegetable we love the most--tomatoes--I have become its sworn enemy this season.

caterpillar of Eastern Black Swallowtail eating my dill this morning

But I'm not consistent when it comes to killing caterpillars that eat my herbs, flowers, and vegetables. This morning, as she was leaving for work, my daughter called my attention to one of my dill plants. "There's a caterpillar on it, Mom!" she said. I know, however, that the larval stage of the Eastern Black Swallowtail can often be found on dill and other plants related to the carrot family. Sure enough, there was the caterpillar of a black swallowtail chomping on my dill. I couldn't bring myself to kill it. The dill is heading now and will soon seed, and while dill is one of my favorite herbs, I also love to see butterflies in the garden. This year, I planted three dill plants, but next year, I'm planting many more in a flower bed that I will devote to butterflies. I plan to remove humanely any butterfly caterpillars from the plants I don't want eaten to the butterfly garden where pollinators can chomp to their heart's content.

closer look at the front of the caterpillar

It is also possible that the Bt that Tom sprayed before the rain of last week might still be viable, especially in the flower heads of the dill, which the caterpillar is now eating. I will be saddened, however, to see another dead caterpillar of the Eastern Black Swallowtail on my dill. I found some earlier after Tom had sprayed the nearby zinnias, trying to get rid of the tomato fruitworms which seem to love zinnia leaves as well as tomato leaves.

Although I have gardened all of my life, for years I didn't pay much attention to pests or pollinators, beyond remarking on the beauty of a butterfly or on the ugly results of the pest. We would sprinkle a little Sevin dust on our plants and go on our way. Sevin is the pesticide that my father used on our tomatoes, too, when I was growing up in Southeast Texas. But Sevin has its issues. Sevin, Bayer's trade name for the active ingredient carbaryl, is very effective, but it's a pretty nasty chemical and will kill honeybees just as quickly as tomato fruitworms.

Sevin is used so often and for so many purposes (to kill bugs in the garden, to kill mite infestations on chickens, etc.) that it's probably safe to say that it is over-used and probably not used carefully. The ecological description on TechPac's "Material Safety Data Sheet" is enough to convince me not to use it:

This product is extremely toxic to aquatic and estuarine invertebrates. For terrestrial uses, do not apply directly to water, or to areas where surface water is present or to intertidal areas below the mean high water mark, except under the forest canopy and use on rice. Discharge from rice fields may kill aquatic and estuarine invertebrates. Do not contaminate water by cleaning of equipment or disposal of wastes. This product is highly toxic to bees exposed to direct treatment or residues on blooming crops or weeds. Do not apply this product or allow it to drift to blooming crops or weeds if bees are visiting the treatment area.

C'mon, does anyone really think that folks who use Sevin are following the advice in that last sentence?

Back to squishing the tomato fruitworms and using biological controls on pests.