White grubs

The larvae of many scarab beetle species are often collectively referred to as white grubs – Phyllophaga, several species (spp.); Coleoptera: Scarabaeidae – and can be found in soil, decaying wood and manure.

In Minnesota, larvae of the genus Phyllophaga (Harris, Figure 1) are the scarabs most often associated with damage to corn root systems. These are the true white grubs. The adults (Figure 2) are the common May beetles, also known as June beetles and June bugs.

A true white grub
Figure 1: A true white grub, Phyllophaga species (sp.).
A May/June beetle
Figure 2: A May/June beetle.
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Characteristics

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Life cycle

Like all beetles, true white grubs undergo complete metamorphosis. The insects progress through the egg, several immature (larva) stages, and a non-feeding intermediate (pupa) stage to the adult stage. As it grows, the larva must shed its exoskeleton (molt) between each of three distinct larval stages or instars (Figures 4 and 11).

The same process occurs as the insect transitions to the pupa and adult stage. The bodies of white grubs grow considerably between each molt. Meanwhile, the width of the larva’s head remains constant between each molt and can be used to determine the instar (Figure 11).

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Predicting damage potential

Knowing what type of grub is present in the field, its life cycle and the life-stage of the grubs are all helpful in predicting the damage potential the following year.

Small white grubs
Figure 5: Small white grubs can be present in heavily manured fields. These are manure grubs (Aphodius sp.). They seldom, if ever, cause stand problems in corn. Photo: Mark Schramm.

Where they live

The larvae of numerous Scarab beetle species are known as crop pests worldwide.

Phyllophaga is a scarab genus native to the New World. There are more than 200 Phyllophaga species native and endemic to various parts of the United States and Canada, relatively few of these are known crop pests.

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Natural enemies

The larvae have numerous natural enemies including birds, small mammals, and other insects.

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Damage symptoms and characteristics

White grub and injured corn seedling
Figure 9: White grub and injured corn seedling. In addition to root feeding, the mesocotyl has been cut and this plant will die. Photo: Jim Boersma.

During rare, extreme outbreaks, the feeding of May beetles is known to defoliate deciduous trees. More typically, the beetles don’t cause significant damage to trees. Plus, unlike Japanese beetles, the adults of true white grubs don’t injure corn or soybean crops.

Historically, localized, severe outbreak infestations of Phyllophaga, or true white grubs, have occurred in Midwestern states, including areas of Minnesota.

The larval stage damages crops. White grubs use their strong chewing mouthparts to feed on the root hairs and lateral roots of grasses, corn, soybeans and other crops (Figure 9).

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Scouting

Scout for white grub as part of spring stand evaluations and post-emergence weed-control scouting.

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Managing grub infestations

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Products are mentioned for illustrative purposes only. Their inclusion doesn’t imply endorsement, nor does their absence imply disapproval.

Always read and follow the pesticide label. 

Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center and Phillip Glogoza, Extension educator

Acknowledgments

Unless otherwise cited, photos are by Bruce Potter, University of Minnesota.

We appreciate the assistance of Emily Evans, University of Minnesota Southwest Research and Outreach Center in reading and formatting initial drafts.

The document benefited from the reviews of Drs. J.A. Coulter, University of Minnesota Agronomy and Plant Genetics; W.D. Hutchison, University of Minnesota, Entomology; P. Meints, Minnesota Corn Research and Promotion Council and Kerry Katovich, University of Wisconsin-Whitewater.

Reviewed in 2018

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