California red scale, Aonidiella aurantii, is a major pest of citrus grown in California. Fruit that are heavily infested with California red scale are often graded down in the packing house and may become unmarketable. Trees with large populations will become severely stressed resulting in poor vigor and loss of production.
Integrated pest management strategies for controlling California red scale are essential, because of the scale's ability to become resistant to insecticide treatments.
In the San Joaquin Valley, there are four generations of California red scale each growing season. Reproducing females release first instar scales, called crawlers. The crawlers travel a short distance and settle onto twigs, leaves or fruit where they will remain until maturity. Adult male emergence coincides with the development of third instar females, which then mate and produce the next generation.
California red scale males are very small, golden in color and identified by a distinctive brownish red band across the thorax.
Male California red scale can be monitored using pheromone traps. A rubber septa impregnated with synthetic red scale pheromone is attached to a white card covered with adhesive. The male scales are attracted to the female sex pheromone and become trapped on the card. The number of male red scale captured is recorded weekly. Using male capture data, a degree-day model can be used to predict upcoming events in the scale lifecycle.
Cellophane tape with adhesive on both sides can be used to monitor the emergence of first instar scales, called crawlers. The crawlers are captured on the tape as they move across a twig or branch. These tapes are used to confirm the arrival of the new generation.
Heat units, or degree-days, are used with many insects to predict their developmental rate. The use of an accurate degree-day model can greatly enhance the effectiveness of an insect management strategy through precise timing of application. The developmental threshold, or lowest temperature that the insect can still metabolize and grow at, for California red scale is 53º F. Degree-days are calculated by subtracting the developmental threshold from the average daily temperature. The remainder is the quantity of degree-days that have been accumulated for that day. By accumulating degree-days from a starting point, or biofix, upcoming events in the insect's development can be accurately predicted.
The biofix for California red scale is the date that the first males of the season are captured. This usually occurs in early March. Approximately 550 degree-days after the males fly and mate with the females, the first generation of crawlers (1st instar scale) will begin to emerge. The crawler stage is most susceptible to pesticides and so 1 to 3 weeks after they first begin to emerge is the optimal time to apply pesticides. The scale continue to develop to 2nd instars, then at approximately 800 degree-days later the females become virgin 3rd instars. This point in time is optimal for releasing Aphytis wasps because this is the growth stage that they prefer to parasitize.
Approximately 1100 degree-days after the first male generation began to fly, the 2nd generation of males will begin to fly. Each of the next three crawler generations will emerge 550 degree days after the preceding male flight. Twenty-year temperature data can be used to approximate when these events are likely to occur. Current temperature readings can then be used to adjust the model. Weekly pheromone trap counts and crawler tapes can be used to confirm that these events are happening as predicted.
| Stage of Development | Degree-days from Biofix | |
| Crawler emergence | 550 |
|
| Third instar virgin females develop |
800 |
|
| Beginning of next flight | 1100 |
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