Late season and overwintering management of the large raspberry aphid
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. Also, 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 virifulous aphids and parasitoids in the field, the interactions between the two can affect control.
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.
Objective 1: During two years, September – November 2011 and August – November 2012, 100 raspberry canes (‘Meeker’) were inoculated with RLMV by the aphid vector Amphorophora agathonica. Additionally, a set of 10 control plants were inoculated in the greenhouse each week with viruliferous aphids in 2012. In the following spring of 2012 and 2013, leaves from the floricanes and new primocanes of the plants were tested in the laboratory for RLMV. None of the plants experimentally inoculated in the fall tested positive for the virus in the spring. These results indicate either a) late season is a poor time for successful inoculation of the virus, or b) that the aphids were unsuccessful at inoculating with RLMV for some other reason. The fact that the control plants also did not acquire virus suggests that fairly high aphid densities are needed to inoculate plants with virus, which warrants further study on the transmission biology of the virus.
Work remaining: To study transmission biology of RLMV, inoculation experiments are planned with varying densities of aphids in spring 2014. To start, source raspberry plants were grafted with RLMV leaves, and the virus was recently confirmed to spread systemically. Uninfected test plants and aphids are being prepared. Plants will be inoculated with seven, 20 or 50 aphids and checked for virus presence after one to two months. If high densities of aphids are needed to transmit RLMV, then control measures in the field might be applied when higher aphid densities are observed.
Objective 2: Amphorophora agathonica eggs were obtained during fall and winter 2011 by caging adult aphids with potted ‘Meeker’ raspberry plants. Eggs laid on raspberry leaves were removed and surface sterilized. The eggs were treated in February 2012 with lime sulfur, stylet oil, lime sulfur + stylet oil, Neem oil, or water control in a Potter spray tower. Eggs sprayed with lime sulfur and lime sulfur + stylet oil had approximately 99% egg hatch suppression over eggs that were sprayed with water only.
Work remaining: Aphids were again caged with potted ‘Meeker’ plants in fall 2013 with plans to replicate the study in February 2014.
Objective 3: Development time, gender, and size (wing length) of emerging Aphidius spp. adults were recorded after parasitized aphid nymphs were reared on healthy plants or hosts infected with RLMV or RpLV. Plant infection status did not affect the number of days required for developing parasitoids to pupate (turn into a mummy) or emerge from the mummy as an adult. However, plant infection status did significantly affect the size of the adult parasitoids. Parasitoids developing in aphid hosts feeding on healthy plants were larger than parasitoids developing in aphids feeding on plants infected with RpLV. Parasitoids developing in aphids on RLMV were intermediate in size between the healthy and RLMV treatments. Oftentimes, size of adults affects their longevity and/or fecundity. Therefore, it is possible that the presence of virus in fields can negatively impact biological control.
Impacts and Contributions/Outcomes
This research benefits producers in the Pacific Northwest region by informing aphid management at two points in the growing season; in the fall and in the early spring. Aphid flight between fields in the fall could be an important contribution to virus spread if plants are still susceptible to virus inoculation late in the year. Objective 1 addresses this question by looking at how susceptible plants are to acquiring RLMV, an aphid transmitted virus, at the end of the growing season under field conditions. At the beginning of the season, aphids which survive the winter will be the beginning of the next generations. One way to lower early establishment of aphid populations is to prevent eggs from hatching. Objective 2 examines the effect of lime sulfur, an agent already applied in raspberry fields for disease control in early spring, as well as other organic products on the suppression of aphid egg hatch. Objective 3 examines the interaction between virus-infected aphids and parasitoids for understanding the potential impact on biological control.
Lightle, D. and J. Lee. 2013. Large Raspberry Aphid Amphorophora agathonica. A Pacific Northwest Extension Publication. PNW 648, August 2013. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/41195/pnw648.pdf
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