- Fruits: berries (brambles)
- Pest Management: chemical control, disease vectors, integrated pest management
In the Pacific Northwest, an emerging complex of viruses in raspberry has caused symptoms of crumbly fruit, resulting in lowered fruit quality, and shortened life of the field. One of the important viruses is Raspberry leaf mottle virus (RLMV), transmitted by the large raspberry aphid, Amphorophora agathonica. Infection rates of 100% are commonly seen in fields only four years old. Control of RLMV depends on effective management of the aphid by targeting life stages that are responsible for virus spread to new fields, such as the winged morph of the aphid, or exposed life stages, such as the overwintering egg. Additionally, late-season raspberry aphid populations can be lowered by naturally-occurring parasitoids, as many mummified aphids (indicating parasitism) are observed in the late summer period. Given the potential overlap of viruliferous aphids and parasitoids in the field, the interactions between the two can affect control.
In the Pacific Northwest region, an emerging complex of virus diseases in raspberry has caused symptoms of crumbly fruit, resulting in lowered fruit quality, crop loss, and shortened life of the field. As a result, control of these viruses has consistently been one of the top priorities of the Washington Red Raspberry Commission and the Northwest Center for Small Fruit Research. One of the viruses implicated in these symptoms is Raspberry leaf mottle virus (RLMV), a closterovirus that is transmitted by the large raspberry aphid, Amphorophora agathonica. RLMV is widespread in the top raspberry producing counties of Washington, which produce 95% of the processed red raspberries in the U.S. Infection rates of 100% are commonly seen in fields only four years of age and contribute to the loss of the raspberry crop and short life of the field. Control of RLMV depends on effective management of the aphid vector by targeting life stages that are most responsible for virus spread to new areas, such as the winged morph of the aphid, or broadly exposed life stages, such as the overwintering egg.
To improve current management of this aphid vector, this study: 1) determined if control of the fall flight period of A. agathonica is required for control of virus spread, 2) evaluated organic fungicides and oils for their efficacy in suppressing egg hatch, and 3) compared the development of parasitoids on aphids feeding on healthy or virus-infected plants.
Amphorophora agathonica has two flight periods throughout the season in northern Washington: in mid-June and September. Transmission of virus diseases into newly planted fields and uninfected fields occurs during these flight times, as aphids carrying the virus migrate into these fields. It is uncertain how important the September flight period is for new infections of raspberry plants. This is because the leaves on which the aphid is feeding may senesce and drop off before the virus has enough time to replicate and move systemically through the plant and root system. The first objective of this study exposed raspberries in the field to viruliferous (virus carrying) aphids to determine if there is a time after which new infections are unlikely. Defining this time eliminates the need to control aphids after this point and reduces insecticide inputs into the system without compromising plant health. Raspberry plants were systematically exposed to viruliferous aphids by caging the aphids to randomly selected plants weekly from September through November. After the plants overwintered, they were tested for presence of the virus via RT-PCR to determine if there was a point after which plants are unlikely to acquire the virus.
One of the challenges of controlling A. agathonica throughout the growing season is that it can be difficult to get effective insecticide coverage because aphids are protected on the undersides of leaves. During my PhD research, I found that the large raspberry aphid overwinters as an egg on the bare raspberry canes, primarily near the ground, where they are exposed. These eggs on bare canes may be a more vulnerable stage effectively targeted with an insecticide spray. Reduction of the overwintering population would result in lowered spring emergence and decreased aphid pressure at the beginning of the growing season. Current management practices, such as application of lime sulfur for control of fungal diseases before bud break, may affect aphid egg hatch. Additionally, horticultural oils and neem oils are organic products registered for use in raspberry, but their efficacy against aphid eggs is unknown. The second objective of the study was to measure aphid emergence rates in the lab when exposed to lime sulfur or oils. Aphid eggs were obtained and stored outside over the winter. Prior to egg hatch, the eggs were taken into the lab and exposed to lime sulfur, lime sulfur with dormant oil, neem oil, or a water control. Emergence rates were evaluated to determine which applications may be most useful in the field for decreasing overwintering success of A. agathonica.
Biological control of A. agathonica may play a significant role in aphid population suppression, as aphid mummies are frequently observed under field conditions. The most common parasitoid found is an Aphidius spp. (Braconidae). Because of the intimate relationship between plant viruses and their insect vectors, as well as the close relationship between aphids and parasitoids, the development of parasitoids may be impacted by the virus status of the aphids’ host plant. With the presence of both plant viruses and parasitoids in the field, interactions between the two may affect biological control of aphids. The last objective of this study examined whether the virus infection status of the host plant affected the development time and size of Aphidius spp. developing in A. agathonica. Aphid nymphs feeding on either healthy plants, plants infected with RLMV, or plants infected with Raspberry latent virus (RpLV, another aphid transmitted virus commonly found in raspberry cropping systems) were exposed to mated female parasitoids. The amount of time for the parasitoids to develop and the wing length of the newly emerged parasitoid (a measure of parasitoid size) were compared. Differences between treatments indicated that biological control is being impacted by the presence of raspberry viruses in the field.
Objective 1: Determine the late-season interval when control of aphids is important for reducing raspberry leaf mottle virus (RLMV) in raspberry, and if there is a date after which aphid control is unnecessary.
Objective 2: Evaluate the effectiveness of lime sulfur, lime sulfur with dormant oil, and neem oil for suppressing aphid egg hatch.
Objective 3: Compare the development time and offspring size of the parasitoid wasp Aphidius spp. (Braconidae) that develop on aphids feeding on infected versus healthy host plants.