biocontrol of root weevils

There are various methods to manage root weevils by using biological control. Check the beneficial nematode link for more information on the use of entomopathogenic nematodes for various soil pests including root weevils. One of the biological control methods is the use of the entomopathogenic fungus, Metarhizium anisopliae. This fungus is available as a commercial product for use in nurseries, Met 52 (Novozymes Biologicals). There are several important factors to of relevance regarding this fungal product.

Link to Met 52 label

This product has a Caution label based on an oral and dermal LD50 >5000 mg/kg. The re-entry interval (REI) is 4 hours for this product. According to the label, there is no REI for uses that are incorporated. The product is composed of the fungal spores on a grain matrix. Once the product is incorporated into the growing medium, insects may come into contact with infective spores, which adhere to surface of insect, germinate, and cause an infection of the insect and death within 3-5 days.

At the USDA-ARS Horticultural Crops Research Laboratory in Corvallis, Oregon, Dr. Denny Bruck has investigated some interesting aspects of this soil insect-killing fungus. The fungus is rhizosphere competent, meaning the fungus shows enhanced growth around developing roots even in absence of a host insect. Inoculation of the roots of spruce, Picea abies, with M. anisopliae spores successfully infected 76% of last-instar black vine weevil that fed on the roots (Bruck 2005).

There is a gradual decline of fungal density over time but after more than four months, the decline was slower in peat-based media than bark-based media. Pre-mixing one-third of the total volume of the potting media with the full amount of product seven days prior to planting (at which time the remaining volume of potting mix was added), allowed for higher populations of the fungus than media not pre-mixed and that lasted throughout the length of his research trial (342 days).  One word of caution is that temperatures above 100F will kill the spores.  

In another experiment, Bruck and Donahue (2007) evaluated persistence of M. anisopliae, the granular Met 52 formulation was incorporated into a soilless potting medium and measured for infectivity of black vine weevils over time. The percentage of larval infectivity started at a high of 90% at week 3 and persisted with a mean infectivity of 50-60% for the duration of the experiment, which lasted 77 weeks. The infection rate tended to increase (75-89%) during the cool wet months of the fall and winter.

Fungicide compatibility was also assessed and while a number of fungicides had significant effect on spore germination and mycelial growth in vitro, no detrimental impact on M. anisopliae populations was seen in bulk soils following dual applications of the fungicides. Captan and triflumizolet (Terraguard), with shorter reapplication intervals (less than 14 days), did have a detrimental impact on M. anisopliae populations in the rhizosphere, and thus may not be compatible in a program using M. anisopliae treatments (Bruck 2009).


Bruck, D.J. 2005. Ecology of Metarhizium anisopliae in soiless potting media and the rhizosphere: Implications for pest management. Biological Control 32:155-163.

Bruckk, D.J. and K.M Donahue. 2007. Persistence of Metarhizium anisopliae incorporated into soiless potting media for control of the black vine weevil, Otiorhynchus sulcatus in container-grown ornamentals. Journal of invertebrate Pathology 95:146-150.

Bruck, D. J. 2009. Impact of fungicides on Metarhizium anisopliae in the rhizosphere, bulk soil and in vitro. Biocontrol 54:597-606.

Infected BVW Larvae (Photo: Denny Bruck, USDA-ARS)