Black stem borer management in spring

The time to manage black stem borer is in spring when females are colonizing new trees. A few adults have been active since late April and we anticipate flight will surge as it warms up in May.

Incidents of black stem borer injury to young apple orchards are on the rise in Michigan. Adults are attracted to stressed trees although they have been known to infest trees that do not appear to be stressed. Young trees near the perimeter of orchards, especially near woodlots, are at greatest risk of injury. Signs of infestation include 1 millimeter diameter entrance holes, sawdust “toothpicks” protruding from the holes, dark discoloration on the bark, oozing sap and dry, blistery bark.

The time to manage this insect is during the spring flight when females are searching for new trees to attack, wanting to establish brood chambers for egglaying and raising her young. Once they enter the tree, they are out of reach for pesticide applications, so timely control measures are essential.

We placed traps in several infested sites in late March and captured the first adults in southwest Michigan during a warm spell in late April. Catches have been low since that initial burst of activity. Peak catch in the past two years has occurred in mid- to late May. We anticipate the same timing this year with adult flight surging as it warms up in May. This will be the optimum time to apply an insecticide.

There are a limited number of insecticides available for borer control. During the past two years, we conducted chemical efficacy trials with several borer insecticides in search of information growers could use to make effective control decisions. Unfortunately, what studies like ours, and similar trials in other states, have shown is that the effectiveness of borer control in apple orchards is highly variable.

The materials we tested did not perform significantly different from each other, or an untreated water-only control, when compared statistically, but there were some general trends. In 2015, we made trunk drench applications to trees in two commercial orchards, comparing lambda-cyhalothrin (Warrior without Zeon Technology) at 5.12 ounces per acre and zeta-cypermethrin (Mustang Maxx) at 4 ounces per acre. The results from this trial showed that Warrior insecticide had fewer entry holes.

Last year in 2016 we used apple wood loglets treated with each material and baited with ethanol to attract beetles. Loglets were placed in woodlots on edges of infested orchards, and the number of entry holes created by black stem borer were counted over time. Also in 2016 we applied insecticides using two methods: as a trunk drench or as an airblast spray. The loglets treated with the two chemicals from the previous year along with 10 ounces acre of permethrin and 12.8 ounces acre of bifenture. The bifenture and permethrin-treated loglets had the fewest cumulative entry holes in the two runs of the test conducted in 2016.

In addition to chemical treatment, growers who remove black stem borer-infested trees should destroy them, preferably by burning, immediately.

Dr. Gut’s work is funded in part by MSU’s AgBioResearch.

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