After getting off to a late, slow start this season, Japanese Beetle (Popillia japonica) numbers have risen dramatically with high populations being reported in various parts of the state. However, not everyone is seeing heavy damage. Thus far, the population distribution is remaining highly localized.
Curtis Young reported on our BYGL Zoom Inservice yesterday that northwest Ohio is experiencing a double-whammy. Basswood Leafminer (Baliosus nervosus) beetles are hammering their namesake host, American basswood (Tilia americana), and Japanese beetles are skeletonizing their favorite host, little leaf linden (T. cordata).
Japanese beetles were flying well below our radar prior to 2016 with rare reports of damaging populations. That began to change in 2016 when we saw some heavy localized populations in many areas of Ohio [see BYGL Alert! "Japanese Beetles Making a Comeback," July 7, 2016]. The beetle revival continued in 2017 and 2018.
Each year was marked by Japanese beetle populations being highly localized. A short drive could take you from a location with high numbers to another location with virtually no beetles. This localized population distribution appears to be continuing thus far this season.
Japanese beetle adults may be found feeding on over 300 species of plants. Although the damage may appear significant, in most cases it does not produce harm to the overall health of established plants. However, newly planted trees and shrubs may be at risk and require some form of beetle management.
Will this season's high Japanese beetle populations translate into heavy white grub damage? That's impossible to predict because one of the most important factors is water. Rainfall or irrigation is needed to keep Japanese beetle eggs and 1st instar grubs – the stage the hatches from the eggs – from dehydrating. Although we had a very wet spring in Ohio, we're now seeing a reversal with soils rapidly drying out.
Japanese beetle leaf-feeding is often used as the poster child for showing what leaf "skeletonizing" looks like. However, other leaf-feeding insects may produce very similar damage including the basswood leafminer.
On the other hand, although the beetles are considered leaf skeletonizers, their damage is most accurately described as "surface skeletonizing" or scarifying. The small adults have mandibles that are too small to chew completely through the leaf; they can only remove the leaf surface. Adults may feed on the upper or lower leaf surfaces. The epidermis on the opposite side of the damage dehydrates and degrades leaving behind a fine-textured matrix of skeleton-like leaf veins.
Basswood leafminer adults are wedge-shaped and around 1/4" long; females are slightly larger than the males. There is a single generation per season; however, adults are present at two different times during the growing season. Adults spend the winter in the leaf litter beneath host trees. They emerge in the spring to feed as skeletonizers on newly expanding leaves. The damage may be noticeable when viewed up close, but it's seldom severe enough to cause entire trees to change color.
Eggs are laid in June with females inserting single eggs beneath the upper epidermis at the edge of an area where they have skeletonized. The resulting grub-like larvae feed as leafminers between the upper and lower epidermis. The blotch-like leaf mines contain a single larva at first, but may eventually house several larvae as mines coalesce. Pupation occurs within the leaf mines and new adults emerge in August. The leaf-feeding damage from this new crop of adults combined with the larval leafmining activity is responsible for the annual flaming of basswoods.
The leafminer is most often associated with American basswood (a.k.a. American linden) as well as other members of the Tilia genus such as little leaf linden and silver linden (T. tomentosa). However, the scientific literature notes the beetle may attack a wide range of hosts including birch (Betula spp.), elm (Ulmus spp.), American hophornbeam (Ostrya virginiana), maple (Acer spp.), oak (Quercus spp.), willow (Salix spp.) and some fruit trees such as apple (Malus spp.) and cherry (Prunus spp.). There was even a paper published in 1982 assessing the damage potential to soybeans.
Curtis reported that thus far, Japanese beetles and basswood leafmining beetles appear to have reached a host truce with no cross-over between basswood and little leaf linden. However, if this doesn't last, the two beetles seem to divvy-up their host trees with Japanese beetles dominating the upper canopy and basswood leafminers feeding in the lower canopy.