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
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
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!