We planted on Tuesday and yesterday morning when I was out in the garden I noticed a few munched on leaves…as well as the culprit:
A dastardly cucumber beetle! Rather, there were several beetles on several different plants. Normally sighting a single beetle on a plant would not trigger the need for spraying. However, on young transplants and seedlings, cucumber beetles can quickly devour the whole plant. I believe that 1 beetle per plant is in fact the “economic threshold” that triggers spraying. (“Economic threshold” is a term for the population level of insects at which there is likely to be an economic loss if treatment is not done. Determining economic thresholds helps farmers know when to spray and when not to waste the time, money, and chemicals. It is an important concept in sustainable agriculture.)
Another reason to spray with only the sighting of a few beetles (with beetles there’s no guarantee you are going to be able to find them all, and they eat a lot quickly!), is that while these guys are currently enjoying our nightshade family veggies (tomatoes, eggplant, peppers), we just planted a whole bunch of different kinds of cucurbits (vine crops – cucumbers, squash, pumpkins, melons, etc.). As the name “cucumber beetle” might suggest, their preferred snack is cucumbers and other vine crops.
Cucumber beetles can make a good dent in the tomatoes and eggplant, but they will eat the entire plant right down to the ground on a newly germinating cucumber or melon. If we can control them now before the cucumbers start to come up, we should be in good shape.
Another reason to spray for cucumber beetles on young vine crops (in case you needed another) is that these early season cucumber beetles have overwintered from last year, and because of that it is possible that they are carrying a disease called Bacterial Wilt that they can transmit to the new crop. Bacterial wilt is NOT a fun disease to have in your melons (and cucumbers, to a lesser extent). As soon as a cucumber beetle vector (one carrying the disease) takes a bite out of your melon plant it transmits the disease. You won’t know it until mid-summer when your vines have nice, unripened fruit on them, and they suddenly collapse in a wilted heap. And there is nothing you can do about it. The only thing you could have done about it was to spray those cucumber beetles in the early season to prevent them from spreading the disease.
Cucumber beetles are one of the prettier vegetable garden insect pests, and I really kind of like them, in a twisted way. However, to my way of thinking, this is one pest that you want to hit with some type of pesticide early, rather than waiting. If you kill off the first generation (the one that may be carrying disease) then you should have a much lower population for the rest of the summer and a much lower risk of bacterial wilt. The worst case scenario would be waiting to control these insects until you have a large population in early to mid-summer that has been passing Bacterial Wilt to the plants, back to the next generation of beetles, infecting more plants, etc.
So what are your spraying options?
Organic Options: Rotenone (organic, but definitely NOT non-toxic), rotenone/pyrethrin combos, pyrethrin, pyrethrin/neem oil combos, neem oil. (This list is from most toxic and most effective to least toxic and least effective.) On a very low population, the neem oil can work okay, but it is a contact spray, which makes it less effective.
Synthetic Options: Permethrin (or pretty much any product that ends in -thrin and is labeled for use on vegetables). This is the synthetic form of pyrethrin/pyrethrum.
Obviously you would choose your spray based on your personal preferences, your willingness to spray multiple times, and your tolerance for insect feeding and other damage. This is a situation where, still several weeks away from any harvest, I would probably use a stronger product in hopes that I only needed to use it once.