Pheromone Technologies to Manage Prionus Root Borer
The larvae of Prionus californicus, a longhorned or round-headed beetle (Cerambycidae), feed on the roots of trees and shrubs, including those of fruits, ornamentals, and natives. It is also a pest of hops in the Pacific Northwest. Feeding damage causes decreased nutrient uptake, water stress, reduced tree growth, and loss of fruiting wood and scaffold limbs. In fruit orchards with high populations of prionus, significant tree decline and mortality may result. There have been few effective controls for the pest. Soil fumigation and fallowing orchard soils for 5-6 years are expensive, reduce site productivity, and have shown variable efficacy.
Interestingly, this insect is native to western North America. Most insect “pest” species are invasive and non-native. Its life cycle requires 3 to 5 years, and most of it is spent below ground as larvae chewing on roots. In Utah, the beetle has been found in sweet cherry, apricot, and peach orchards, especially along the mountain benches where soils are sandy. Porous soils are more conducive to the movement of larvae as the largest instars (mature larvae) can reach more than 4 inches in length and up to ¾ inch in width – one big grub! In studies conducted in fruit orchards in Box Elder County, it was found that younger, smaller larvae feed on smaller diameter roots toward the outer edges of the root zone, and tend to move inwards and upward as they develop and feed on larger roots. The majority of larvae in the tree crown are late instars. The insect pupates near the soil surface and adults emerge from approximately June through September in northern Utah. Adults are short-lived (several weeks) large, brown beetles, and are active primarily at twilight (crepuscular). Males can fly up to several hundred yards and have strongly serrated antennae to detect a sex pheromone released by females. Recently, entomologists identified the pheromone chemical and synthesized a mimic of the primary component, 3,5-dimethyldodecanoic acid (Rodstein et al. 2009).
Researchers, including ourselves (Barbour et al. 2011), have tested this pheromone in the field and found that it is highly effective as a trap lure. The best trap design that we tested is a sunken 5 gallon bucket topped with a large funnel and pheromone lure hung above. We have tested a commercial lure (Contech Enterprises, Victoria, BC, Canada) and an experimental mating disruption dispenser (Pacific Biocontrol, Vancouver, WA) to assess if these technologies show promise for monitoring and management of the prionus root borer in orchards. The answer to both of these questions is a resounding “YES”.
In 2011, we set up trials in two pairs of sweet cherry orchards (0.25 and 1.1 mile apart) with the upwind orchard serving as the untreated site and the other treated with 100 mating disruption (MD) dispensers per acre. Dispensers were stapled to the lower trunk because adult mating typically occurs on the ground. A second trial was established in two additional sweet cherry orchards to compare the attraction of a research vs. commercial pheromone lure (0.1 vs. 30 mg pheromone load), and to test the longevity of the commercial lure (2 vs. 4 week interval between replacement). Four bucket traps were placed in each orchard; half were baited with the research lure and half with the commercial lure. In the lure longevity test, half of the commercial lures were replaced every 2 weeks and the other half were replaced every 4 weeks. An astounding 338 P. californicus male beetles were caught in the six cherry orchards during 2011. Trap capture in the MD orchards was 90% less than in the paired untreated orchards. This indicates that the male beetles were much less effective in locating the traps when the MD pheromone was present throughout the orchard.
Trap shutdown (reduced trap capture) is used as a measure of potential disruption of mating and suppression of population growth. The 30 mg Contech (commercial) lures caught about three times more beetles than the 0.1 mg research lure in both the MD and untreated orchards. In the lure longevity comparison, there was no difference in attraction of beetles to Contech lures replaced every 2 or 4 weeks, confirming that this commercial lure should last at least one month in the field, and perhaps longer.
These results show great promise for use of pheromone technology for population monitoring, MD, and potentially masstrapping of males. Because the beetle has a long life cycle, treatment with either MD dispensers or a relatively high number of baited traps will be required for multiple years to suppress populations as only a small proportion emerges as adults each year. However, pheromone technology holds the first bright spot for effective management of the prionus root borer, and is good news for Utah orchardists. The Contech lure will be available for sale in 2012, and hopefully the MD dispenser will be available soon.
We thank Contech Enterprises, Pacific Biocontrol, and the numerous research assistants who helped with this project over the last three years.
-Diane Alston, Entomologist, and Mike Pace, Box Elder Co. Extension Agent
Rodstein, J., et al. 2009. Identification and synthesis of a female-produced sex pheromone for the cerambycid beetle Prionus californicus. J. Chem. Ecol. 35: 590-600.
Barbour, J., et al. 2011. Synthetic 3,5-dimethyldodecanoic acid serves as a general attractant for multiple species of prionus (Coleoptera:Cerambycidae). Ann. Entomol. Soc. of Am. 104: 588-593.
Featured Picture of the Quarter
A single lady beetle can lay 200-1,000 eggs in her lifetime, depending on the species and availability of prey. Eggs are typically laid in groups of 30-50 on the undersides of leaves. It takes about 3-10 days for the eggs to hatch, and the larvae have up to 60 hours to find food before dying. If food is not readily available, newly hatched larvae will cannibalize siblings, or feed on unhatched eggs. Young larvae feed on prey with a piercing-sucking mechanism, while older larvae develop a chewing action, consuming the entire prey.
-Image by Erica Stephens, graduate student in the USU Biology Department