Sudan Gyawaly, UCCE Area IPM Advisor Butte, Colusa, Yuba-Sutter, Glenn, and Tehama
Jhalendra Rijal, UCCE Area IPM Advisor Stanislaus, San Joaquin, and Merced
Integrated Pest Management (IPM) is a decision-making strategy that uses cost/benefit analysis to guide growers and farm managers in making pest control decisions. A well-developed IPM program weighs the cost of a pest control practice against the expected return from using that practice and results in pesticides or control measures only being applied when the pest population reaches a density that will cause economic damage. This economic injury level (EIL) is the “break-even point” between the cost of the pest management and the value of the crop. As the crop value decreases, the EIL must increase. For example, consider an orchard yielding 4,000lb/per acre with 4% codling moth damage if untreated. If it costs $100/acre to control the moth damage, at a walnut price of $1/lb, using insecticide sprays would translate to a net return of $60 per acre (4000lbs x $1/lb x 0.04% damage – $100 of insecticide = $ 60 net return). However, if the price of walnuts is $0.50/lb and everything else remains the same, controlling the moth damage will translate to a loss of $20/acre (4000lbs x $0.5/lb x 0.04% damage – $100 of insecticide = -$20). In this case, the grower must increase their tolerance for damage, reducing the number and cost of economically justifiable sprays. This example illustrates how IPM decisions may change with changing circumstances, balancing the damage tolerance or EIL against the cost of pest management. However, it should also be noted that increased damage in a lot will likely decrease the price paid for that lot, so the economic impact goes beyond lost yield if quality is affected.
Most growers follow IPM practices in normal years, but in the current walnut price situation, management decisions that made sense in past years should be carefully reconsidered. To prioritize necessary pesticide sprays, it is helpful to consider the pest damage history of the orchard and the nature of the damage caused by the pest. Pests can cause direct yield loss by damaging nuts or indirect loss by impacting nut quality, or next year’s production by interfering with tree growth. In walnuts, major direct pests are codling moth (CM), navel orangeworm (NOW), and walnut husk fly (WHF). Indirect pests include spider mites, aphids, and scale insects, as they feed on non-harvestable parts of the plant.
Treatment decisions for codling moth are based on in-season trap counts, nut damage assessment, and the orchard damage history. For first-generation CM treatment, if the previous year’s damage was relatively low (<3%) and the moth counts in the traps during the season are light, it is reasonable to wait until the second peak (1B) or even the second flight. In addition, with low walnut prices, the damage tolerance level should increase to over 3%. One should also note that a high percent nut infestation (generally >2%) by direct pests, including codling moths, navel orangeworm and walnut husk fly, may result in higher loss of crop value due to penalties for failing to meet the required grade quality standard. In the last two years, codling moth pressure has been moderate to low in most orchards in the Sacramento Valley. For that population level and the low crop price, one treatment, most likely the second flight, may be enough. To guide management decisions for second and third generations of codling moth, use trap counts and nut damage assessments.
The IPM practices for managing NOW include mating disruption, orchard sanitation, and a potential one-time insecticide application at husk split, if monitoring suggests it is required. An isolated walnut orchard with excellent yearly sanitation practice is less prone to NOW damage. Practices such as winter sanitation and proper blight, sunburn, and codling moth management, followed by a timely harvest to avoid late-season NOW infestation should also help reduce the risk of NOW damage even without insecticide applications.
Walnut husk fly (WHF) is another important pest that impacts profit. Walnut husk fly is not an issue in every orchard; when they are a problem, they are likely to concentrate in a particular area within the orchard. If you dealt with higher husk fly pressure last year, they might warrant a timely insecticide application with bait. One cost-cutting measure for this pest is applying the insecticide to cover only the orchard areas or blocks that have a history of high walnut husk fly damage. Suppose only 10% of the orchard area historically (adjacent to the water source/river, etc.) has husk fly issue. In that case, making 1-2 applications during the peak fly season for that area saves 90% of the product cost and a significant amount on application cost. Also, consider the EIL concept and utilize traps counts and in-season nut damage information to make management decisions.
Spider mites, aphids, and scale insects cause indirect damage to walnuts by infesting leaves and foliage and have higher EIL for treatments. Spider mites are usually less of a problem in well-irrigated dust-free orchards. Mite predators can control the mite population for most, if not the entire, season, so it is important to monitor mites and their predators diligently. Most miticides which are non-disruptive to the natural enemies are relatively expensive, so taking advantage of the “free” biocontrol is beneficial. If the orchard has heavy mite pressure especially earlier in the season and has no to minimal predator presence, in that case miticide application maybe necessary to avoid severe defoliation that can impact the nut yield and quality. Aphid populations are generally kept below damaging levels by natural predators and are only a serious problem occasionally. However, outbreaks of walnut aphids may occur if broad-spectrum insecticides are applied early for other pests, such as codling moths. Relying on biocontrol and skipping treatment for the first flight of codling moth can help.
Scale insects, especially walnut scale, are common in the orchard; however, insecticide application to control scale insects can wait if you are not dealing with very high Botryosphaeria issues. Also, scale biocontrol agents are very active in most orchards; careful monitoring of dormant spurs should give you confidence that biocontrol is at work.
In summary, determining the priority for pesticide sprays for insect pests is challenging. Growers and farm managers need to consider several factors, such as the potential loss of crop value, possible multi-year impacts, and the overall goal and resiliency of the orchard to tolerate the loss. Experiences have shown that the best approach is integrated pest management (IPM), which involves balancing the pest populations and management costs to make informed business decisions.