Vegetable Growers: Don’t Take Lygus Bug Lightly
The native western tarnished plant bug (Lygus hesperus), or commonly called “lygus bug,” is a sporadic pest on vegetables. California’s Central Coast has perfect conditions for the lygus bug to be successful. Pest populations can survive on several host plants.
Coastal areas of California typically receive rain during winter months, which supports weed plants. The weed hosts (e.g., mustards and wild radish) are abundant during spring in the unmanaged areas surrounding farmland such as in the ditches, roadsides, etc.
A major pest of strawberry, lygus bug also threatens vegetables, especially celery, lettuce, and Brussels sprouts. In vegetable nurseries, lygus bugs can infest celery and Brussels sprouts seedlings and cause serious damage.
How To ID Damage
The mouthpart of a lygus bug is “straw-like” and is often referred to as “piercing and sucking.” There are four needles inside the straw that are arranged in different configurations to inject saliva and suck up the digested soup. In strawberry, lygus bug prefers seeds and while feeding, it injures the tissue around the embryo when the fruit is small.
As these injured fruits develop, they appear misshapen and often are referred to as “cat-faced” fruits. However, on vegetables, lygus bug injury symptoms are different. On lettuce and radicchio, the injury appears as isolated spots of dead tissue on the midrib, whereas it appears as elongated lesions on the celery stock or petiole. This injury could be due to egg laying. Lygus bug typically inserts the barrel-shaped eggs into the plant tissue, which cause mechanical injury to the plants. Sometimes the spots on lettuce can coalesce to form a larger necrotic lesion.
Lygus bug is considered a sporadic pest in vegetables. Although it is hard to pinpoint a specific reason to predict a lygus bug season, it could be a result of a combination
• Phenology of weed plants in the landscape. In California’s Central Coast, rain events are rare from April through November. As a result, the weed plants growing in the unmanaged areas get progressively water stressed during this period. Depending on when the rain stops in spring, the weed plants growing in the unmanaged areas gradually lose vigor and fail to support the lygus bug populations. It is likely this deteriorating quality of weed plants triggers the onset of lygus bug migration and the pest leaves these weed hosts.
In California’s Central Coast, vegetable crops and strawberries are intensively grown throughout the agricultural areas and the migrating adult lygus bugs easily stumble upon crops such as celery, lettuce, etc. If these crops are not protected, lygus bugs can lay eggs and can start a colony. Therefore, poor quality of the weeds can shift the lygus bug dynamics leading to migration and problems in vegetable crops.
• Resistant populations. Lygus bug is routinely managed in strawberry using organophosphate and pyrethroid insecticides. It is likely that lygus bug populations were repeatedly exposed to these insecticides and have developed resistance. Typically, if a lygus bug population has developed resistance to these insecticides, it becomes a challenge to manage this pest in vegetable crops using insecticides that fall within the same Insecticide Resistance Action Committee mode of action classification.
• Time of attack. Although vegetable crops are vulnerable to lygus bug attacks throughout the growing period, the last few weeks before harvest are the most challenging for management. For example, celery is one crop that can have a problem with this pest. If lygus bug invades the early stage celery, it is relatively easy to manage using insecticide sprays because the sprays will have adequate coverage on most of the plant surface. However, during the late stages of celery when the plant canopy becomes thick, adequate spray coverage is questionable.
Similarly, the canopy of the lettuce plants close out completely during the final weeks to harvest and adequate spray coverage becomes difficult. Lygus bugs exploit these unexposed, insecticide residue-free zones within the plant to feed and develop into several colonies. Thus, if the pest population moves in at the late stages of the crop, the plant architecture and canopy favor the lygus bug to evade insecticide sprays and colonize those plants.
• Lack of novel insecticides. Unfortunately, newer insecticides are not registered on crops such as celery. The older chemistries are restricted with one application early in the crop stage, leaving fewer options to manage lygus bug. Currently, pyrethroid insecticides are primarily used to manage this pest in California’s Central Coast.
Another factor that could trigger lygus bug infestation is movement of infested plant materials from a nursery to the field. In the Central Coast (especially in the Salinas Valley), the northwesterly winds can move lygus bugs a great distance downwind. Also, lygus bug development accelerates with temperature. Dry and warmer winter months are the right recipe for early buildup of pest populations. Thus, lygus bugs could emerge into a major problem in vegetables if the biotic and abiotic factors are in favor of population growth.