Thursday, July 19, 2012

Bean Leaf Beetle (Such a Pest!)

 The bean leaf beetle, Cerotoma trifurcata, is one of the garden pests I did not cover in my previous post. Unfortunately, it is also one of the garden pests which infiltrated our vegetable garden.

The beetles went after the snap beans (also called string beans or green beans) in our vegetable garden. They are also fond of soybeans, clover, dry edible beans, and several leguminous weeds. High numbers in recent years have been attributed to milder winters or adequate snow cover that insulated and protected overwintering adult populations.

Both bean leaf beetle larvae and adults possess chewing mouthparts. Larvae may feed on roots and root hairs but show a preference for root nodules. Even though nodule damage can diminish the plant's ability to fix atmospheric nitrogen, it is the adult feeding damage to foliage and particularly pods that is economically important. Defoliation by adult bean leaf beetle is identifiable by the small round holes between the major leaflet veins. This damage differs from the larger, irregular holes or jagged leaflet margins caused by caterpillars and grasshoppers. 

 Based on current research, pod damage by adult bean leaf beetles is the most important type of injury. This damage can cause complete pod loss when adults feed at the base of the pod. This type of injury is referred to as "pod clipping." Adult damage to the outer pod wall also leads to the formation of pod lesions. Moisture can then enter through these pod lesions, and this increased moisture level permits the entry of secondary pathogens. Seeds damaged by these pathogens become shrunken, discolored and moldy. Several primary pathogens (bean pod mottle, cowpea mosaic and southern bean mosaic viruses) also can be transmitted by adult beetles.

Luckily, Shannon and I found an organic fix that seems to have gotten rid of them quite nicely.

We chopped up a small turnip and a large clove of garlic, then tossed them in the food processor with a handful of marigold seeds and 2/3 a cup of corn oil. Once it was ground into an oily paste we watered it down (about a gallon of water) and sprinkled it on our beans. We waited until evening since we didn't want the sun beating down on the oil splattered leaves and the humidity was low enough we weren't worried about any fungal possibilities from the water. It has now been over two weeks and we haven't seen a beetle since and our beans are recovering quite nicely!

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