OSU Master Gardener volunteers utilize objective, research-based information to diagnose plant problems and offer sustainable solutions. This blog will highlight scientific studies that may be of interest to OSU Master Gardeners (and others) who would like to know more about the art and science of home horticulture. Any opinions expressed in this blog are the author's and not necessarily those of Oregon State University.
This weekend, I noticed small holes chewed in my potato plants. Whenever I see holes chewed in leaves, I immediately think 'it must be a beetle'. In fact, loopers, armyworms, cutworms and grasshoppers also chew holes in leaf tissue. However, the small, tiny holes in my potatoes were a clear indication that flea beetles were to blame. Flea beetle damage has been described as a leaf looking like it has been shot with several shots from a bb gun (albeit, very small bb's). Whitney Cranshaw describes the damage as looking like the leaves have been hit by fine buckshot - but not knowing what buckshot is, I default to my past experience with bb's. This is very different than the wholesale destruction (often called 'skeletonization') caused by loopers, armyworms and cutworms. It is also different than the large 'chomps' taken from leaves by grasshoppers.
It didn't take me too long to locate the offending chewers. Flea beetles are small and black. When disturbed, they jump off of the leaf much like a flea might jump from a dog. Flea beetles, however, are not blood suckers. They're leaf chewers that locate their host plants using the chemical cues that the plants emit.
Herein lies my problem. I prefer not to use pesticides in the garden. This is a personal preference - due to my being (1) lazy and (2) cheap. I don't like to spend money on pesticides. I don't like to take the time it takes to calculate application rates. Although I love math, I try to keep my garden a math-free zone. Unfortunately, to judiciously use pesticides or fertilizers in the garden requires that I pull out a tape measure - document the area on which I will use the chemical(s) - carefully read the pesticide or fertilizer label - and then translate the area where I will use the chemical(s) to an amount that will come out of the container. I'm just not that motivated.
So what is the problem? Flea beetles are very, very, very, very (you get the idea) difficult to control. They're strong fliers. They locate their host plants by sensing ('smelling') the plants' chemical cues. Worse yet, research strongly suggests that some flea beetles can use the chemical cues in the plants on which they are feeding, to make an aggregation pheromone (Peng and Weiss 1992). This sets the poor gardener (e.g. me) up for failure. One male flea beetle lands on my potatoes and starts to feed. He incorporates the chemical constituents of the leaf tissue, and re-manufactures at least some of these into Axe Body Spray for beetles. Basically, he turns my poor potatoes into his singles bar. No wonder I noticed so many mating beetles on my potatoes this morning, despite my efforts to pick the plants clean just 12 hours earlier. Given this type of chemical wizardry, it would be virtually impossible for me to control the flea beetles by hand-picking them off of the leaves - my preferred pest control method.
Weighing my pest management options, I refer to the PNW Handbook, and look up the entry for flea beetles. The PNW Handbooks are what we use in the Oregon Master Gardener Program, for research-based and reviewed pest control options. The PNWs don't always list the full suite of cultural, physical and biological controls available. But, if chemicals are necessary (which they may be, to control the flea beetles on my potatoes), then the PNWs provide a list of reviewed products. From the list of home use products for flea beetles, I see that neem oil is listed. Neem oil is an organic pesticide that can be used to control a variety of insects and pathogens. Because it is an organic pesticide, it tends to degrade quicker in the environment, relative to synthetic pesticide options. (Please note that organic pesticides are still pesticides. Organic pesticides are not necessarily less toxic than synthetic options, and in some cases are more toxic.)
But, I'm not yet keen to use neem. This is my personal preference, and not my professional recommendation to other gardeners. My own aversion to using chemicals, at this time, is because along with the flea beetles that were feeding on the tops of my potato leaves, I found aphids feeding and giving birth on the underside of the leaves. During Master Gardener training classes, I've often lectured about the benefits of 'popping' aphids. Squish an aphid, and you magically attract parasitoid wasps. Akin to how male flea beetles can use a plant's chemistry to attract the ladies, parasitoid wasps are able to 'smell' the chemicals that are released when an aphid is crushed. Some species of aphids release alarm pheromones when they are crushed. The alarm pheromones send a signal to nearby aphids: 'Abandon ship! We've been found! Parachute to safety, or else be eaten!'. The cool thing about alarm pheromones is that parasitoid wasps, ladybugs and other aphid enemies can hone in on a group of aphids, using released alarm pheromones as a guide (Micha and Wyss 1996). Yesterday, I crushed aphids. Today, I found parasitoids crawling all over my potatoes. A coincidence? Probably not.
So I'm left with potato plants that are infested with flea beetles, and that also have a few aphids (the bad guys). But, these same plants have parasitoid wasps and ladybug eggs (the good guys).
I want to get rid of the bad guys, and will most likely need a chemical solution to take care of the flea beetles. But, even the organic option - neem oil - is known to have negative effects on parasitoids, ladybugs an other natural enemies (Lowery and Isman 1995). The same is true for other insecticides listed for control of flea beetle.
What will I do? Because: (1) my potato plants are relatively large, (2) my tolerance for damage is high, and (3) my aversion to garden math during non-working hours is even higher - I will continue to vigilantly hand pick pests off of my plants. But, if I appear to be losing the hand-picking battle, I'll pull out my calculator, work through garden algebra, and judiciously apply a pesticide on affected leaves. I'll follow label directions, will continue to scout my plants, and will turn to floating row covers to keep the beetles off of my plants while they're looking for places to feed and mate. Once the flea beetle danger has passed, I'll remove the row covers, and look forward to harvesting healthy spuds in the fall. Although, it's hard to predict when the flea beetle danger may pass, since there can be 2-3 generations per year in Western Oregon.
Wish me luck!
Top: Flea beetle and flea beetle damage (photo taken by: Gail Langellotto) Middle: Parasitoid wasp (photo taken by: Gail Langellotto) Bottom: Ladybug eggs (photo taken by: Steve Rhodaback)
Lowery, D.T. and M.B. Isman. 1995. Toxicity of neem to natural enemies of aphids. Phytoparasitica23: 297-306.
Micha, S.G. and U. Wyss. 1996. Aphid alarm pheromone (E)-B-farnesene: a host finding kairomone for the aphid primary parasitoid Aphidius uzbekistanicus (Hymenoptera: Aphidiinae). Chemoecology 7: 132-139.
Peng, C. and M. J. Weiss. 1992. Evidence of an aggregation pheromone in the flea beetle, Phyllotreta cruciferae (Goeze) (Coleoptera: Chrysomelidae). Journal of Chemical Ecology18: 875-884.