Blueberry aphid management
For virus susceptible varieties, it is time to check fields and take action to reduce aphid infestations.
This season’s wet spring has stimulated higher than normal shoot growth in blueberry bushes, providing lush foliage that is ideal for aphid development. High aphid counts are being reported during this week’s scouting, approaching 50 percent of young shoots infested in some fields. Aphid colonies can generally be well controlled by natural enemies such as lady beetles and lacewings, but these high aphid populations indicate that growers should be watching aphid abundance in their fields and taking appropriate action in virus-susceptible fields. The blueberry aphid (Illinoia pepperi) is the vector of blueberry shoestring virus which can cause bush decline and significant yield reductions. It is also a potential vector of blueberry scorch virus that was detected in Michigan in the past few years. Because of the ability of aphids to serve as vectors of plant disease, they should be controlled to minimize virus spread in infected fields and in susceptible fields near to virus-infected fields.
Scouting for aphids
Blueberry aphids are most often found on the undersides of young leaves at the base of plants. To scout for aphids, examine two young shoots near the crown on 10 bushes in a field and record the number of shoots where aphids are found. Multiply by five to get the percent of infested shoots. Tracking this number through the growing season can help identify whether populations are increasing, remaining steady or declining. It is also a good idea to record the number of shoots with parasitized aphids to get a measurement of the level of biocontrol present in your field. Be sure to sample weekly from as wide an area in the field as possible to have a better chance of detecting whether aphids are present.
Varietal susceptibility to shoestring virus
Some varieties are resistant to shoestring virus. Resistant varieties include Bluecrop and Atlantic. Varieties with moderate resistance include Draper, Aurora, Liberty, Legacy and Brigitta. Aphid control should be considered in fields of susceptible varieties, especially if there are symptoms of shoestring virus present. Aphid control is most important in fields containing varieties that are susceptible to the shoestring virus, such as Jersey, Blueray, Burlington, Earliblue, Elliott, Jersey, Rancocas, Rubel, Spartan and Weymouth. If fields of these varieties contain symptoms of shoestring, aphid control should be a priority during the season and infected bushes showing symptoms should be tagged and removed in the late fall once aphids are not able to be spread through the field during removal.
Aphicides for control of blueberry aphid
There are some aphid control materials available to blueberry growers that have excellent aphid activity. These should be applied after bloom in June as aphid populations start to increase, with application by ground sprayers to ensure coverage of the lower parts of the bush. Good coverage is essential for effective aphid control, and this will be more challenging in weedy fields. Controlling the aphids now will limit spread of the virus, thereby reducing the loss of yield or need for removing infected plants.
The most effective insecticides for aphid control are the systemic neonicotinoid insecticides Assail 30SG (2.5 to 5.3 oz/ac), Provado (4 oz) and Actara (3 to 4 oz). Foliar application of one of these products will move in treated leaves, helping ensure that aphids receive a lethal dose. They also provide long-lasting control. Because these insecticides are very effective and blueberry aphids do not readily form winged individuals, getting excellent control early in the season typically provides season-long control.
Selection of an insecticide for aphid control may be made considering other pests that are present to get multiple insects controlled with one spray. For example, Assail and Provado are also labeled for blueberry maggot (but check the rates!), and Assail (5.3 oz/acre) is also very effective against fruitworms.
Soil-applied neonicotinoids Admire and Platinum can also be used to provide aphid control. These must be banded under the bush and watered in to allow them to get into the plant tissues. With the time needed for uptake into the foliage, these applications should be made soon after bloom to allow time for uptake before aphid populations get too large.
Broad spectrum insecticides applied after bloom for control of other pests such as fruitworms can also provide some control of aphids. Lannate and the various pyrethroid insecticides registered for blueberry are active on aphids if applied to target the lower shoots. However, these are also highly disruptive to natural enemies, so fields should continue to be monitored for aphids to ensure that the populations do not increase again later in the season.
In mechanically-harvested fields, patterns of virus infection are often along the rows, indicating spread by harvesters. Aphid control prior to harvest is particularly important in fields with a history of shoestring virus infection to prevent this method of spread. Washing harvesters before moving to the next field is a simple strategy to further reduce the spread of blueberry shoestring virus within and between blueberry farms.
Dr. Isaacs's work is funded in part by MSU's AgBioResearch.