April, 1997
By Phil Schwallier and Gary Thornton, Michigan
State University Extension
The obliquebanded leafroller (OBLR) is the most damaging insect pest to apples in Michigan. It is not uncommon for growers to spend $100 per acre, or more, on insecticides in controlling this pest. Depending on how successful the grower is in controlling OBLR, he may also have additional packout expenses of $200 to over $500 per acre.
Control of OBLR on apple has become more difficult in recent years, mainly due to the resistance that this pest has developed to the organophosphate insecticides (OP's), in the Grand Rapids and southwest Michigan regions. Guthion and Imidan are the first OP's they gain resistance to, although they can acquire resistance to Lorsban and Penncap-M too. Growers from these two areas have been forced to use multiple applications of harsh insecticides to control OBLR. This has led to reduced or eliminated beneficial populations and explosions in secondary pests such as European red mite, white apple leaf hopper, aphids and spotted tentiform leafminor.
The behavior of this pest has allowed it to escape certain pesticide control applications. It will tend to:
1. Migrate to terminals and feed only on newly expanding tips;
2. Roll leaves and remain inside protected from certain spray applications;
3. Choose to feed in clusters of fruit where spray penetration is difficult to reach. In clustered fruit, the larvae feed on multiple fruits causing considerable damage with relatively low numbers of larvae.
Growers in regions where resistance is prevalent, need to be aware of the behavior of this pest and know how to manage it with cultural controls, as well as insecticides. To reduce the ability of OBLR to survive, and to increase the efficacy of insecticide applications, the following suggestions should be considered:
1. Reduce the presence of clusters on apple trees by aggressive thinning and moderate pruning. Single fruits can be covered by spray applications more thoroughly and it eliminates the space between fruits for OBLR to hide in.
2. Alternate use of various insecticide chemistries to reduce development of or strengthening of the resistance to OP's by OBLR. OBLR will revert back to a population that is again susceptible to OP's once they are discontinued in spray programs. This reversion will happen in just several generations. The fewer times an insecticide is used against OBLR, the fewer number of resistant individuals that will exist in the overall population.
3. Target control applications against only one generation per season. Reducing OBLR exposure to pesticides will delay resistance from further developing in the population.
4. Apply control applications against the first summer generation. This generation does most of the damage to the fruit during a season. Some minor damage can occur from the overwintering generation, especially during hot years, when the season is early. The second summer generation can cause additional damage just before harvest. Controlling the first summer generation will reduce development of resistance by OBLR by exposing only one generation per season to a direct targeted control application.
5. Control all broadleaf weeds in and around your orchards. Many broadleaf plants contain toxic or semi-toxic chemicals that only serve to select general feeders (like OBLR) for a resistance type mechanism. Cross resistance in OBLR appears to be quite common. Cross resistance occurs when the resistant population is also resistant to another class of compounds, as a result of their initial resistance. Exposure to pesticides or noxious weed chemicals seems to trigger the resistance mechanism and thus the resistant population increases.
6. OBLR problems are internal orchard populations; that is the population, for the most part, exists within the orchard itself. Most likely external sources are not important, unless another nearby orchard is providing some source. Typically the neighboring source will only have an edge effect, but will continue to increase with each generation. The use of horticultural and IPM practices that improve spray coverage and insecticide timings will give you the best chances for adequate control. Slow down spraying speed, use more water where appropriate, do adequate pruning and fruit thinning for improved impact. Apply full covers during critical times and alternate row covers where appropriate.
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