Dead plant patches indicate the presence of root-feeding insects (Picture 1). Pull dead vines and search through the root zone and soil for grubs.
White Grubs, Phyllophaga spp.
In New Jersey cranberries, white grubs (Phyllophaga spp.) are the most common scarab grubs with the most abundant species being P. georgiana. These are the most damaging species attacking cranberry roots. Larvae tend to feed gregariously on roots, are C-shaped, over 35 mm (1 3/8″) long when full grown, and often found near bog margins. Only the larval stage is considered a pest in cranberries. Adults are reddish brown and nocturnal.
Most likely, Phyllophaga grubs have a 3-year life cycle (based on previous reports and our own observations). Eggs are laid in late-June and July, and will hatch in July. First instars can be found in late-July and August and turn into second instars by the end of August-September. The second instar grubs will overwinter. These grubs will feed the following year until June and molt to third instars, which will overwinter. The following year, the third instars will feed for some time and then begin to pupate in June. Pupation will take place in July–August. Adults will appear in the soil in August-September. Adults will remain in the soil and overwinter. They will emerge in June, mate, and begin to lay eggs, completing the cycle.
Cranberry Rootworm (Chrysomelidae)
This is an important pest in cranberries in New Jersey. Feeding damage is often severe in limited areas, causing brown spots in the bog. Although the adults may feed on small berries, most of the damage is caused by the larvae. Larvae live in the soil under the vines, where they feed on the cranberry roots. Larvae prefer to feed on the bark of roots.
Mature larvae overwinter deep in the soil in cranberry bogs. Normally in New Jersey, pupation occurs in late May. Adults emerge during early June and remain active throughout July. They can be easily monitored using a sweep net. Adults are 5–6 mm (1/5″) long, with dark-brown, bronzed, and shiny coloration. Eggs are laid in July on the soil surface. After hatching, young larvae start feeding on roots. Feeding lasts until October. Larvae are about 8 mm (5/16″) long, yellowish-white in color with a light brown head, and usually assume a curved position. In fall and winter, mature larvae move down in the soil to overwinter. Generally, this insect has a 1-year life cycle. Pupation starts in late May and beetles emerge in early June. Adults are nocturnal and hide in leaf litter. Adults can be easily picked up in a sweep net. Females feed on foliage before laying eggs. Young larvae will feed on roots, feeding continues until October. The mature larvae move down the soil in the fall to overwinter. Holding of the water will delay onset of pupation. Rootworm has a 1-year life cycle. Entomopathogenic nematodes (Heterorhabditis bacteriophora) can be used to control cranberry rootworm (see Picture).
Imidacloprid is labeled for the control of rootworm, root grub (Phyllophaga spp.), and other scarabs in cranberries. This insecticide is a contact and stomach poison that affects the insect nervous system. It is highly systemic and toxic to honey bees; therefore, can be used only as a post-pollination insecticide. This insecticide can be applied by ground or by chemigation. Aerial application of this product is prohibited. Admire Pro has a long residual activity (> 100 days) as long as the insecticide is not directly exposed to the sun and can be used at 7–14 fl oz/acre. A maximum of 14 fl oz of Admire Pro can be used per acre per season. Irrigate target area with 0.1 to 0.3 inches before and after the application of Imidacloprid. The pre-harvest interval is 30 days. Most scarab grubs in cranberries have multi-year life cycles; consequently, you may not be able to suppress a population of 2- and 3-year life cycle grubs with a single application. You may have to use imidacloprid two to three years in a row for most effective suppression. For this reason, it is useful to know what species you have before using imidacloprid. Root grubs can be recognized by a distinct C-shape (see pictures below) and represent a number of different species of beetle larvae.
The different species of grubs can be separated by the rastral pattern, an arrangement of stout bristles on the ventral side of the last abdominal segment, and anal slit (see Figure and table).
|Grub species||Rastral pattern||Anal slit|
|Phyllophaga||2 parallel rows||V- or Y-shaped|
|Oriental beetle||2 parallel rows||Transverse|