- Author: Mark Bolda
Hillary Thomas
Post Doctoral Scholar, UC Davis
Mark Bolda
UC Cooperative Extension
Light brown apple moth (LBAM) is an introduced species of leafroller (Lepidoptera: Tortricidae) that was first detected in California in 2007, and is currently present in high population densities throughout the Central Coast. It is a class A pest with a broad host range that is subject to state and federal quarantines. This has resulted in strict regulation of its many host crops, mandating zero tolerance for larvae in fields or on harvested fruit, and occasioned the financially devastating closure of several fields of caneberries in 2009, and again in strawberries during the 2010 season.
Central Coast strawberry growers have a variety of management methods available to them, which they can deploy in their efforts to prevent LBAM from being detected in their fields. Foremost, monitoring for the presence of LBAM is key. Leafrollers may be identified using timed visual inspections or pheromone traps. Where any levels of LBAM are detected, available management tactics include (1) pheromone mating disruption, (2) insecticide use, and (3) cultural management techniques such as vacuuming and manual removal of leafrollers. Natural biological control by wasps has also been observed and may play a more central role in its control with time. The following is summary for growers of how best to integrate available control methods to achieve the greatest result.
Mating Disruption: Field trials currently being conducted by UC Davis began in early summer, 2010, and have confirmed that pheromone based mating disruption can be useful for managing light brown apple in strawberries.
Mating disruption with pheromone based twist ties is an attractive tool to growers, because of low toxicity, compatibility with biological control, high specificity to the target pest, reduced risk of resistance, and ease of application. However, the use of twist ties should be supplemented with well-timed insecticide sprays and cultural controls whenever possible, because it is not certain whether twist ties used alone will result in zero detection of light brown apple moth larvae in fields, as currently mandated by state and Federal regulatory agencies. The presence of any suspicious looking leafrollers (even if later confirmed to be native leafrollers such as garden or orange tortrix) can delay the shipment of berries from coolers after inspection, and could result in economic loss.
Pheromone mating disruption is a tactic that, when effective, interrupts male moths from detecting potential female moths to mate with. It is a target (species)-specific tactic that will not affect other lepidopteran (moth) pests. It is also not an insecticide or growth regulator, and its mode of action will not kill moths outright. Larvae or pupae that are already present in a field just prior to the application of twist ties will not be controlled by this tactic and should be managed by available chemical or cultural controls. Therefore it is important to think of mating disruption as one strategy within an integrated pest management program to target multiple generations of LBAM. Because its effects will reduce the number of viable eggs in the following generation, it may have increased utility for LBAM in strawberry fields maintained for two years of production, or when well timed to target peaks of emerging LBAM adults throughout the year.
The following summarizes our knowledge of LBAM’s life cycle and population phenology (generational timing) in California. Pheromone trapping by UC Davis and as reported by USDA surveys suggest that annual flights of light brown apple moth generally occur at peaks 3 times a year, although lower levels of moth activity continue year-round. They are anticipated in higher numbers in the spring between March-April, summer between June-July, and a fall population peak between October and November. The exact timing of the peaks is temperature and weather related, may vary by year, and may be site-specific, so it is important to monitor fields to ensure that mating disruption techniques are deployed PRIOR to an observed peak in adult LBAM presence.
Twist ties are expected to be effective for 4-6 months after their application. Environmental conditions such as temperature or UV exposure can affect their longevity so currently it is recommended that growers be conservative and replace them after 3 ½ to 4 months, depending on the remaining length of harvest or intentions to carry over to a second year of production. Mating disruption works best when applied over large continuous areas and over longer periods of generational time.
In strawberries, the first recommended timing is a winter application in late February or early March. This application will likely have continued efficacy through a June or July LBAM flight peak, but if there is heavy population pressure in the area or if it is desired to act conservatively, a second placement of twist ties may be made in June to carry the field to completion of strawberry harvest in the fall. For strawberries continuing to a second year of production, another placement of twist ties should be made in November and then again in March of the following year.
Twist ties should be placed all across the production field and distributed at an even rate. At a minimum, twist ties should be placed down every other row of the strawberry field at an even spacing between each tie to arrive at the recommended per acre rate. If possible, it is suggested to place twist ties as far out as the edges of the field or slightly farther where possible, to reduce the probability of a mated female moth flying in from external sources. The recommended label rate is between 200-300 twist ties per acre. In strawberries, mating disruption has been effective when deployed evenly at a rate of 300 twist ties per acre at a height of less than 18" above the strawberry canopy. This reduced male LBAM pheromone trap captures to zero for the first several months of a 2010 trial. Since in strawberries, there is no pre-existing support system such as a trellis, growers should mount twist ties on supports such as chopsticks (see picture below), pin flags or sharpened wooden dowels of at least 9 inches in length. Ties simply deposited on the bed surface may shift, become buried in the soil, or may be difficult to later collect for the purposes of field-sanitation, particularly if multiple twist tie applications are made each year. However, the presence of loose twist ties in the field does not pose any health or regulatory risk. If in-field monitoring is being conducted and there is a rise in adult moth finds 3-6 months after initial twist tie application, this could be an indication of reduced pheromone release by the dispensers.
While other leafroller species in Central Coast strawberries are not common, it may also be useful to manage similar looking leafroller species, to reduce the probability of economic loss due to delays for leafroller identification during cooler and field inspections. Many insecticides that target LBAM will also be effective for management of native leafrollers.
Insecticides: The use of twist ties should be supplemented with the use of insecticides where possible, to reduce the probability of an in-field light brown apple moth find. This also has the added benefit of targeting similar looking leafroller species, such as orange tortrix or garden tortrix.
Since leafroller populations are lower in February and March, it is recommended to take initiative at this point in time by commencing a program of pesticide application. This is especially true for producers of organic strawberry, since the selection and efficacy of materials available is quite limited. As there is a wide variety of good conventional leafroller pesticides available, growers should seek materials that have a lower impact on beneficials and the surrounding environment. Materials should also be rotated to mitigate the potential for resistance to a single pesticide.
Following the initial early season spray, it is important that growers continue to monitor the field for leafrollers. Growers should look for leaf surfaces that are webbed or rolled together, and underneath the calyces of the fruit (see photos below and see the May 26, 2010 post of this blog ). Concentrate monitoring activities in suspected or previously infested areas.
Any sign of leafroller activity should be a signal to protect the crop. It should be emphasized here that the economic threshold for leafrollers during this period of regulation is zero, and subsequently the threshold for spraying is much lower than one would deploy in an integrated pest management program.
Cultural Controls: Because of the zero tolerance mandated for light brown apple moth infestation in fields, it is recommended that growers impress upon crews involved in weeding, runner removal and harvest the importance of manually removing suspect rolled leaves, larvae and webbed fruit. Considering that crews are passing over every foot of strawberry row at least three times a week during the harvest season, they can be very effective in reducing LBAM and other leafroller numbers. It is worth noting that LBAM larval infestations tend to be clustered, so the presence of leaf webbing on one plant means that it is near certain that there could more webbed leaves nearby. An incentive program which rewards leafroller finds in the field can be implemented to encourage participation with such a campaign of larval removal.
In annual strawberry culture, sanitation of the field after completion of the harvest is not an issue. However, in strawberries which will be carried over to a second year, proper sanitation practices during the dormant season will be an essential part of light brown apple moth management since larvae and pupae will overwinter in leaf trash and surrounding weeds. Leaves from plants should be deposited in the furrow and later incorporated into the soil. Growers should be aware too of the potential of certain species of plants on the Central Coast to be major sources of infestation and the need to control LBAM there when possible.
There are pesticides mentioned for management of leafrollers in this article. Before using any of these products, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.