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Friday, December 4, 2015

White Grubs –the larvae of scarab beetles

 
White Grubs –the larvae of scarab beetles –are a growing concern in field crops in Indiana, Michigan, Ontario, and Ohio (I-MOO). Early in the season, grubs feed on germinating seeds and small plants, reducing plant stand. Later in the season, grubs feed on corn and soybean roots, reducing root mass and impacting water & nutrient uptake and standability. Grub-feeding on tuber crops such as sugar beets directly impacts tuber size and quality.
Grub identification is important because there are multiple species present in the Midwest, and these species differ in life cycle and damage. Most species have a single-year (annual) life cycle, with only one generation per year, but some June beetles have a multi-year life cycle. Annual grubs stop feeding in the spring, and may be avoided by adjusting planting time, but a field infested with June beetle may be infested for several seasons. Some grubs pupate earlier in the season (European chafer) than others (Japanese beetle). Adults of the different species also differ in behavior and damage. European chafer beetles are a nuisance around lights, but do not feed. In contrast, Japanese and Asiatic garden beetles do feed and cause damage in the adult stage.

Grubs are naturally C-shaped, and it is sometimes difficult to make them lay flat. Unfortunately, important characters for identification are on the butt-end of the grub (arrow in pictures). These includes the shape of the anal slit(the anal opening of the insect) and the raster pattern(pattern of hairs and spines in front of the anal slit).
The easiest way to view the anal slit and raster pattern is to cut off the end of the grub and put it upright in a dish of water or alcohol. If it is important to keep the specimen intact, set the dead grub in a small dish of fine sand (such as play-ground sand). This allows you to position the grub with the hind end facing up.
Grub set upward in sand
…or cut off the back end.
Asiatic garden beetle Annual grub
Chestnut colored & Barrel-shaped
Adults feed on many plants.
Anal slit: strongly Y-shaped
Raster: crescent-shaped row of spines
Japanese beetle Annual grub
Metallic green/ purple. Adults feed.
Note tufts of white hairs along abdomen
Anal slit: Crescent-shaped
Raster pattern: Triangular
European chafer Annual grub
Adults do not feed
Anal slit: Y-shaped
Raster: Diverging rows of spines
(= opening zipper)
May/ June beetlesMulti-year grub
Large beetles, often dark, some green
Anal slit: Y-shaped
Raster pattern: Parallel rows of spines
(= closed zipper)
False Japanese beetle Annual grub
Abdomen lacks white tufts
Anal slit: Crescent-shaped
Raster: Short row of converging spines
Oriental beetle Annual grub
Multiple color forms
Anal slit: crescent-shaped
Raster: 2 rows of spines
–one small & one large
Manure (Aphodius) grubs Annual grub
Grubs are small; adults are small dung beetles
Common in manured fields
Raster: undefined, with “pads”
D. Shetlar
C. Krupke
D. Shetla
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