There are two types of beetles which attack asparagus on my property. The orange colored back with black dots identifies the Spotted asparagus beetle (Crioceris duodecimpuntata). This beetle lays its eggs on or in developing seed pods. The larvae develop inside the seed pods, and emerge ready to pupate in the ground. The use of all male asparagus cultivars, like Jersey Knight for example, helps to reduce the overall beetle population, because the Spotted asparagus beetle has no place to lay its eggs. If the number of beetles of this type is very great, and there are no female asparagus plants available, it may be possible for them to change their behavior, but I am not aware of that having occurred anywhere as yet.
The more numerous type is Crioceris asparagi, the striped asparagus beetle identified by a red-orange back with white dots on black stripes. This beetle lays exposed eggs on tender young stems. The larvae emerge appearing like miniature dark gray caterpillars and begin to feed, coming to maturity in just a few weeks. It crawls or drops to the ground where it pupates. The beetle climbs to the surface in about ten days. There is no immediate need to fly as there should be an asparagus stalk nearby to climb. There are two to five breeding cycles per year, depending upon length of warm season, and both types of adult beetles overwinter in garden trash close by the asparagus patch. They emerge in the spring, and the cycle begins anew.
1. Grow only male plants, and if the number of beetles is not great, perhaps all of the spotted ones will go elsewhere.
2. Maintain a clean environment to force overwintering beetles to find suitable cover further away from your asparagus patch or to burrow into the ground where many predators reside.
3. Experiment with tansy clippings scattered around the base of the asparagus stems that were too skinny to harvest. I shear 3 inches off the tips of the fronds of my tansy (Tanacetum vulgare) every one or two weeks in the spring, and distribute these uniformly in the asparagus patch. I continue on during the summer every three or four weeks with small pieces; shearing the tansy in this manner usually prevents it from coming to bloom, but without harm. The reputed value of repellency in these clippings is subject to some doubt, but after four years of this treatment (through 1995), I can report a drop of approximately 90% of the former population of beetles. Before treatment began, the asparagus tops would literally dry up from feeding activity of the beetles and larvae. Now, after four years, the asparagus tops are full and ferny right up to first killing frost. This is a long term plan of treatment. Its aim is to deter or disorient beetles that are walking from finding an asparagus spear to climb. It cannot hope to deter flying beetles. It is hoped that a diminishing population of beetles will eliminate the need for chemical controls, because small numbers of beetles can be tolerated by the plants.
During the season, attempts to capture these beetles by hand, usually results in the beetle dropping to the ground to escape. Once on the ground, the tansy clippings may deter it from climbing back into the plant. The eighth year of this experiment will begin in the spring of 1999. With the onset, finally of warm weather, the beetles should begin to stir from their long sleep. I plan to continue using the tansy clippings.
4. Poke your face into your asparagus plants every day. Interfere with the social life of the beetles at every opportunity. It may be that the declining population that I have reported here is due to their great dislike of my face. After all, if you can get the beetle to fly instead of dropping to the ground, he may get caught in the wind and be unable to find his way back to your patch.
Tanacetum vulgare can be very invasive, and should never be planted in any "garden area", especially not right in with your asparagus. I recommend that a small strip or circle of lawn area be cleared for your tansy patch, so that its tendency to spread can be controlled easily with your lawnmower. A dense patch will prove to be a strong attractant for the social life of lacewings and ladybugs.
Asparagus developing during cool temperatures (50º to 55º F) is higher in fiber than spears harvested in warmer weather.This is due to reduced spear growth rate, but not reduced fiber cell development which continues during cool temperatures. Thus a fast growing spear out-paces fiber development. Harvest shorter spears (5 to 7") in cool weather, and longer spears (7 to 9") in warmer weather. Try to snap spears at ground level as this method leaves a higher proportion of the fiber-heavy lower stem in the ground. Cool the harvested spears to 36º, while maintaining high relative humidity in storage.