Investigator: Phillip Fujiyoshi, Dept. of Plant Pathology, University of California, Davis
Project location: Davis, California

OFRF funding for this project was awarded in Fall 2008.

Project links: Complete project report (5 pp) submitted by Phillip Fujiyoshi.

Summary
The purpose of this project was to find a way to make aphid-infested broccoli heads marketable by inducing the aphids to walk away in response to alarm pheromone.

Organic broccoli can be made unmarketable when aphids infest the head. Aphid alarm pheromone is a compound or mixture of compounds that causes an aphid to leave its feeding site. Using compounds known to be released from cabbage aphids when they are crushed, we proposed to determine which ones cause alarm behavior in this species of aphid and then apply these compounds to infested broccoli heads to determine whether the head can be made marketable.

Photo of a huge cabbage aphid colony
on a bolted diCicco broccoli
inflorescence taken in early spring
at Good Humus farm.

Specific objectives were:

1. Establish the existence of aphid alarm pheromone;
2. Determine the compounds involved in cabbage aphid alarm response;
3. Remove the aphids from infested broccoli heads using alarm pheromone;
4. Develop plant-based alarm pheromone from plants that can be grown on-farm;
5. Document labor and material costs of aphid removal.

The project found no alarm behavior in cabbage aphids raised on crop plants.

All of the latter objectives were dependent on success with the first objective, and as no alarm behavior was seen on field crops, an intermediate objective was established, that of testing for alarm behavior in aphids raised on a variety of wild cruciferous species.

Methods. All research was conducted at the Good Humus farm, Capay, California, between February 2009 and January 2010.

Host plants for the aphids were diCicco broccoli (Brassica oleracea), Chinese cabbage (B. rapa), the second-generation offspring of an apparent cross of bok choy and rapini (B. rapa), field mustard (B. rapa), black mustard (B. nigra), the wild mustard Hirschfeldia incana.

Aphids naturally colonized the broccoli and Chinese cabbage. The other plants each received a single aphid raised indoors on radish and broccoli seedlings or found on market-purchased gailan (B. oleracea).  Spring-planted bok choy-rapini hybrid plants received aphids on 26 April and were then covered with floating row covers. Fall-planted plants received aphids on 9 January and were covered with organza fabric, anchored with soil.

In most instances aphids were crushed with forceps to induce release of alarm pheromone.  On 21 March several inflorescences of diCicco broccoli with infestations of silvery-powdery aphids were clipped and brought indoors for testing. An inflorescence was placed on white paper and a crushed aphid held beside the aphid colony for 5 minutes while the aphid response was observed.  The control was crushed fruit fly (Drosophila melanogaster) pupae.  Five inflorescences were tested with aphids and four with fly pupae.

On 28 March aphids on broccoli inflorescences in the field were induced to release alarm pheromone by crushing or by the feeding of a soldier beetle or a ladybug larva, and the aphids' behavior was observed.  No control was used on this date.

On 4 April varying numbers of aphids on Chinese cabbage inflorescences were crushed to induce alarm pheromone release and held next to nearby aphid colonies while the colonies' behavior was observed.

The spring transplants were inspected for aphids on 16 May, and the fall transplants inspected on 30 January.

Results. Of the aphid colonies on the clipped broccoli inflorescences, none came off in response to fly pupae, while a total of 2 started walking and 3 landed on the paper while crushed aphids were held nearby, out of colonies of dozens individuals, perhaps over one hundred.

Of the aphid colonies on the broccoli in the field, crushed aphids had no effect on one colony, and it was difficult to tell whether the few aphids seen walking in a second colony had started walking before exposure to the crushed aphids. The soldier beetle's feeding caused a few aphids on a colony to start walking, although the beetle also walked on the colony. The ladybug larva feeding caused no response from the other aphids.

The colonies of aphids on Chinese cabbage did not respond to a crushed aphid held nearby. When crushed aphids were placed directly on two other colonies, a total of six aphids began walking, but crushed fruit fly pupae placed directly on a colony caused three aphids to begin walking. A whole colony crushed and held next to a small colony of around 10 individuals caused only the lone winged adult to walk.

The spring transplants were exposed to predators and parasitoids due to tears in the row covers, and there were none of the silvery-powdery aphids surviving when the covers were removed. The fall transplants had a total of 6 aphid mummies and no living silvery-powdery aphids.

Conclusions. At this point the existence of alarm pheromone in cabbage aphids is unproven.  No statistical analyses were done because the effect would have to be large in order to be useful to growers.  A good example of the effect of alarm pheromone was shown by another aphid species on sowthistle (Sonchus sp.) on the farm.  When several crushed individuals were held near a colony, the living aphids leapt off as if they had been hit by a blast of air, an effect not seen when a clean glove was held near them.

The question of whether aphids can be induced to leave a broccoli head by application of alarm pheromone remains unanswered. If cabbage aphids truly produce no alarm pheromone under any condition, that would be a fascinating evolutionary finding, but not helpful for growers. However, if there are conditions under which cabbage aphids produce alarm pheromone, such as a certain host, then the possibility remains for duplicating the alarm pheromone and testing whether it is effective in cleaning aphids from broccoli.

A final report describing the results of this project was submitted in February 2010.