Over-informed on IPM - Episode 025 What cucumber beetles want
I don’t know if this happens to the rest of you entomologists out there but when I meet someone new and they learn that I study insects for a living they will often express how careful they are about not killing insects. It’s really very sweet the lengths that most people will go, the complexity of their strategies, to extract a bug from their home. Sometimes people will even apologize to me, if they recount a story of killing a bug. At some point or another I’ll remind them that, actually, a big part of my job is killing bugs…and not just for entomologists involved in pest management. The study of insect biology and ecology usually results in lots of sampling that results in lots of dead bugs. For the good of science and for the good of insect collections.
Well, I do spend most of my time here talking about how to kill bugs but not in this week’s episode! Well there will be a little killing – there’s always a little killing - but mostly behavioral manipulation! Directing pest behavior away from the crop you are protecting rather than trying to eliminate any bug that dare tread on your crop plants.
We are returning to a topic we covered last year, striped cucumber beetle! You know it, you love it, it’s probably in your cucurbit crops right now (I’m recording this in July)…adults are munching on your leaves and your flowers, pooping out the pathogen that causes bacterial wilt… cucumber beetle babies are below ground munching on your roots or pupating and about to emerge to do some above ground munching.
This is a pretty hard pest to control but we know a whole bunch about its likes and dislikes and there’s lots of potential for manipulating its behavior, diverting these insects away from cash crops. The three things cucumber beetles love the most – other than destruction –they love flowers, they love the bitter taste of cucurbits, and they love hanging out with their friends.
These beetles love flowers. Cucumber beetle overwinters as an adult, under leaf litter, in wild habitat, probably in the woods. In the spring they emerge and feed on the pollen and nectar of a rather wide array of host plants. Not until they’re ready to meet someone special and start a family do the turn to cucurbit crops, like cucumber, summer squash, winter squash, gourds, pumpkins and melons. But movement of striped cucumber beetle into cultivated crops really jumps up when those crops start to flower. They really respond to the bright yellow color and the floral volatiles from those squash flowers.
They also love cucurbitacins, the family of chemical toxins that cucurbit plants produce as general defense against herbivory. Well cucumber beetles have evolved a tolerance to these chemicals and so the taste is a cue for them to stay on that plant make a meal out of it. We call this a phagostimulant or a feeding stimulant. Cucurbit crops vary quite a bit in the types and amounts of cucurbitacin in different plant parts. So hubbard squashes comes from a group of cucurbits with really high cucurbitacin, particularly high content in its roots. So this is a preferred crop used as a trap crop because cucumber beetle has evolved to recognize this as a good host plant for reproduction. They’re more likely to hang out and start a family on hubbard squash plants than – say – butternut squash. So you can plant a little hubbard near your butternut squash and the beetles will hang out on the hubbards instead of the butternut squash plants.
And cucumber beetle love hanging out with other cucumber beetles. Male striped cucumber beetle produce an aggregation pheromone called vittatalactone, which is attractive to male and female cucumber beetles. It’s even attractive to other cucumber beetle species, like spotted cucumber beetle…the folks who are studying this pheromone are finding that this pheromone is even attractive to squash bug. Another pest of cucurbits that’s not even remotely related to striped cucumber beetle. So you can see that vittatalactone is a pretty powerful chemical for making a cucumber beetle party…more on that later…
So we know how to attract them. The bigger challenge is what to do after you’ve got them. I’m going to pull in some expertise in this area from a buddy working with watermelon crops on Maryland’s Eastern Shore.
Simon Zebulo UMDES
Anna: Watermelon is not susceptible to bacterial wilt so it generally tolerates a few cucumber beetles. Relying on behavioral manipulation for pest management usually works best when you have a high acceptable threshold. Another key aspect here is that Simon works with growers who are producing for the Washington DC market and the demand is quite high for organically-produced or insecticide-free produce. These farmers are very open to labor-intensive ways of avoiding the use of pesticides. Trap cropping, or what Simon refers to as sacrifice plants or sacrificial crops, it’s a great approach for this market.
Simon: trap cropping is a very old kind of pest management method. And different people have different names for trap crops. Some people say catch crops, some say sacrifice or sacrifice plants, because you normally sacrifice some plants for insects. Which insects, they like to stay or hang around them to eat, to lay eggs, to raise their kiddos stuff like that. So, trap crops are something that you grow away in a garden or hedge to try to attract pests, and then you trap them there or you can control them there easy.
But when you have, you know the summer squash as a trap crop, cucumber beetles prefer squash, especially summer squash over watermelon, cantaloupe and other cucurbits. Why do we use squash as trap cropping watermelon seeds? Because squash produces very nice, big, good looking flowers. These flowers act to trap cucumber beetles. Whenever you go to any cucurbit field you will find the cucumber beetles all out on the flowers. So these squash have the really big yellow flowers which attract many, many cucumber beetles.
We really need to keep them producing flowers for an extended period of time. What we do is we keep harvesting the fruit as soon as they emerge so this would force the squash to keep producing flowers so especially during those initial growth stages for watermelon, we need squash plants with flowers in the perimeter or intercropped with watermelon so most of the cucumber beetles would get attracted towards squash. Because if the plant produces, especially squash, if the plant produces enough fruit they don’t produce flowers.
Anna: Once you get the beetles on the trap crops you have them there, they are attracted to those big beautiful flowers that you’ve forced by picking the little fruits off. What happens once you get there? Like are you worried about them spilling over to the crop? How are you managing them once they get on the trap?
Simon: I think they are, they are, that’s why I’m in this. If you just let the plant produce food, the insects will go to the main crop, the watermelon plant. So that’s why you have to make sure, always flowers has to be there in order to keep the insects in the squash plant.
Some farmers after they trap they control them right there. When they get attracted in the squash they can do whatever. That can be pesticide, biological pesticide, organically accepted pesticide. They will spray to the squash not the watermelon.
Anna: So Simon has found a rather clever way to keep cucumber beetles on his summer squash trap crop, or in this case probably a sacrificial crop. He knows how much these beetles love flowers so keeping flowers in the trap crop keep adults there longer.
There has been some work demonstrating mass trapping by taking advantage of this affinity for flowers. Jaime Pinero (now of UMass) found that yellow bucket traps baited with floral lures captured and killed enough cucumber beetles to keep the number on nearby plants below threshold. Investigations were pretty much stalled because we really need a more specific lure. The combination of yellow visual stimuli and floral olfactory stimuli makes these cucumber beetle traps pretty effective bee traps too. Not only do we want to avoid hurting local bee populations, cucurbits are insect pollinated crops. I asked Simon what he saw when he was studying these systems where an insecticide was not applied to the cash crop but just to the trap crop.
Simon: You know, interestingly in the trial that we did, we had controls without trap crops and fields with trap crops. And the watermelons from our trap crop plots are much bigger in size than the watermelon in our control. The reason that you know, the size of the watermelon depends on the number of flowers getting pollenated by pollinators. The more pollenating that you are doing, you will have more seeds and more seeds mean a bigger fruit. So that it was really fascinating to see the size of the watermelons. Even my farmers really love this method and even they adopt this method. They grow last summer, they grow very nicely.
Anna: So a system that works nicely for these farmers. Perhaps knowing what beetles love can help you with your system? For those of you who take the time to hand pick cucumber beetles, you should do this at night if you can. They are most active at dusk. Maybe focus those efforts on a preferred host, like hubbard squash if you grow it, on the crop that’s flowering the most.
For those of you growing in a larger scale, you probably already know that these beetles display an edge effect in the spring. Early season chemical control applied too just edge rows is likely just as effective as applications to the whole field. Avoiding chemical applications, especially early in the season, can be very important to conserving the natural enemies that help keep pest populations lower, more manageable.
So that’s it for now. Thanks to Simon Zebulo of UMD Eastern Shore. I also want to thank Rebecca Smyth, Don Weber, and Lauren Brzozowski, who taught me everything I know about cucumber beetles. And a special thanks to Jason Lightbown who wrote and performed our theme music.
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