Aster leafhoppers in carrots
Live barley near carrots when leafhoppers may be present increases the number of leafhoppers and their potential to spread aster yellows phytoplasma
What is an aster leafhopper?
Aster leafhoppers are an important insect pest of fresh market vegetables, primarily because it transmits aster yellows phytoplasma, which is a disease of celery, carrots, lettuce and, occasionally, onions and potatoes. Disease symptoms vary from crop to crop, but affected plants typically have distorted, discolored foliage and a bitter taste and are therefore unmarketable (Figure 1).
Figure 1. Carrot on the left shows signs
of aster yellows: Bronzing of foliage
and a hairy, stunted root. Carrot on the
right is healthy.
How do leafhoppers pick-up aster yellows phytoplasma?
Leafhoppers pick up the phytoplasma after extended feeding (hours to days) on infected plant tissues. The phytoplasma circulates in the insect’s body and multiplies during a two to three week latent period, during which time the insect cannot transmit the pathogen. Once the leafhopper becomes infectious, it may infect healthy plants for the rest of its life, and this transmission process only takes a few minutes to a few hours of feeding. Once plants acquire the phytoplasma, nothing can be done to cure the plant. Control efforts should prevent further spread of the disease by stopping aster leafhoppers from feeding on healthy plants.
Why should I care about the interaction of non-crop plants with the leafhoppers?
Both the aster leafhopper and the aster yellows phytoplasma can use many plant species as hosts. In many types of agricultural situations, the crop field is also a home for other species of plants, whether they have been intentionally placed there or not. Many carrot growers use some type of cereal cover crop in the carrot fields to provide horticultural benefits to the carrots by improving soil conditions. The cover crops are usually killed as the carrots start growing in order to prevent competition for water and nutrients. There is usually some overlap when the two plants are growing at the same time in the field. On the other hand, non-crop plants are also present as weeds in many fields, especially late in the season, when it becomes difficult to enter the field with tractors to apply herbicides. The cover crops, as well as the weeds, provide alternative hosts for the leafhoppers and a source for picking up the aster yellows. It’s important to understand how the presence of these non-carrot plants affects leafhopper abundance in the carrot crop.
How do aster leafhoppers respond to the presence of barley?
In the summer of 2010, the Michigan State University (MSU) vegetable entomology lab conducted a small-plot carrot research trial, where we investigated the impact of barley borders on the abundance of aster leafhoppers. Carrots (var. Canada) were planted in 10-foot by 10-foot plots at the MSU Montcalm Research Farm in Entrican, Mich. Four treatments were assigned to carrot plots in a randomized, complete block design, and replicated six times (N=20). We counted weekly the number of aster leafhoppers on yellow sticky cards placed into the center of the carrot plots.
The four treatments were:
- Carrot plot without a border;
- Carrot plot with a two-foot barley border;
- Carrot plot with a two-foot barley border where barley was seed-treated with Cruiser® insecticide; and
- Carrot plot with a two-foot barley border that was killed with a herbicide after the plants grew to about a two-foot tall stage.
Results: The number of aster leafhoppers was significantly higher in plots that had a living, non-treated barley border. The lowest number of leafhoppers throughout the season was in those plots that did not have a barley border (Figures 2 and 3). The other two treatments (insecticide and herbicide treated barley borders) were in between the other two treatments, regarding the number of leafhoppers on the sticky cards.
The presence of living barley near carrots at a time when leafhoppers are present should be avoided because the carrots will have more leafhoppers on them.