Don’t worry about those brown trees!
By Glenn Palmer –
Every year around this time people start asking about “Those trees that are already turning color”, like fall has come earlier and earlier. Generally they’re referring to roadside stands of Black Locust trees (Robinia pseudoacacia). The discoloration is caused by the offspring of a small beetle called the Locust Leafminer (Odontota orsalis). The female beetle deposits eggs inside Black Locust leaves where the larvae hatch and feed, creating “mines” between the upper and lower surfaces that eventually turn the leaves brown.
This is an annual occurrence, but there seem to be cycles where some areas are particularly hard-hit and entire mountainsides do indeed look like fall has arrived. Although a systemic insecticide applied at the right time might reduce the damage there doesn’t seem to be any long range ill effects on the trees.
Dirr, in his Manual of Woody Landscape Plants, says “Black Locust is an ‘alley cat’ type of tree which can survive under the toughest conditions like strip-mined areas, highway cuts and fills. Not recommended for the home landscape.”
So, our suggestion is to find something else to worry about. No action needs to be taken regarding those browning trees.
By the way, Locust wood is a survivor, good for ground contact like fence posts or raised garden beds where it generally will last much longer than cedar lumber.