WASP and FLY PARASITES

of

White Grubs

Introduction

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The most effective wasp parasites are the Tiphia wasps. Introduced from Japan, Tiphia popilliavora and Korea, T. vernalis, these species attack the grub stage in thatch or soil. Historically, they were established throughout the mid-Atlantic region but now populations of both species are low with only scattered pockets of high rates of parasitism. These two species are most effective in areas that have had high populations of grubs (10-30 sq ft) over long periods of time. They do not appear to be effective at low population densities which is typical of 90% of our turfgrass areas today. However, they should be introduced into newly infested areas around the country.

When the adult wasps are abundant, people mistakenly view these low flying insects as another type of honey bee or yellow jacket, ready to attack and sting. In reality, they are not aggressive and rarely sting even if handled. Today these wasps are widely accepted throughout Japan without any fear of being attacted.

The only major adult Japanese beetle parasite is a fly imported from Japan, Istocheta aldrichi, sometimes referred to as the Winson fly. This species prefers adult female beetles and deposits an egg just behind the head on the prothorax. The egg hatches within 24 hours and the maggot enters the beetle body. The adult beetle eventually is consumed and dies within 5-6 days. After the adult falls to the ground or buries itself before death occurs, the parasitic fly larva pupates and remains in the soil until next year.

This fly parasite is limited in distribution to the eastern New York state and the New England states. The USDA and State Entomologists have been introducing this parasite into other states (Ohio, North Carolina, Maryland, Virginia and Kentucky). To date we have not recovered this fly in any of these new release sites. However, there may be areas locally that have undiscovered natural populations.

Biological Control

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The newest grub biological control agents are parasitize nematodes. These nematode species are selected for their ability to enter the soil and seek out all species of grubs and quickly parasitize them. Research to date shows that they must be applied when the soil moisture is relatively high for optimal results. Several experimental tests confirm that the percentage control has been generally good with the upper ranges near the levels expected with the best soil insecticides. They are applied at rates of 1 billion nematodes/acre. Under optimal conditions one application per season for grub control should be sufficient.

The entomophathogenic nematode, Heterorhabditis backteriophora, has shown good activity against white grubs and is commercially available as Cruiser.

Cultural Control

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Soil moisture levels are very important in assessing damage risks in turf. Japanese beetle eggs must acquire moisture from the soil in order to develop and hatch. Research has shown that soil moisture levels less than 10% are lethal to the eggs and newly hatched grubs. Irrigated areas not only ensure good survival but are also very attractive to egg laying females. If daily irrigation is maintained during these grub outbreaks, Kentucky bluegrass turf can sustain 20 or more grubs per sq. ft. without showing injury. However, once the water is withdrawn, even briefly, the turf dries out and dies.

Turfgrass species and variety selection can greatly influence the susceptibility to grub damage. Perennial ryegrass, creeping bentgrass, Kentucky bluegrass and fine fescues are the most susceptible to severe damage, whereas the warm season grasses like zoysiagrass and bermudagrass simply outgrow damage and tolerate higher densities of grubs. Tall fescue varieties are intermediate to bluegrass and warm season grasses.

 

by Dr. Lee Hellman, Department of Entomology and Dr. J. Kevin Mathias, Institute of Applied Agriculture