The D.C. Smith Conservatory is located on the campus of the University of Wisconsin - Madison. This 1600 ft2 facility is used regularly for both educational and recreational purposes (it's a great place to relieve stress or eat lunch!), making pesticide use a concern. A third of the floor space is ground beds for understory planting, and there is a small pond and benches. The overstory contains twelve trees in excess of ten feet. The average temperature in the plantscape is about 80°F and relative humidity is always 60% or above (so it's a really nice place to go in the winter!). There is no artificial lighting.
A research program was initiated in August of 1997 to identify pests, decrease the use of pesticides, establish a long-lasting biological control program, and use the information in educational programs.
During the first week of October, an overall census was taken of the plants to determine the type and location of pests in existence. In the overstory nine tree species were sampled. On each tree (or group of a species), ten external terminals and ten internal terminals (if present) were examined. The number of external and internal terminals with a given pest type were recorded. Care was taken to get an overall census of each tree, terminals being chosen at each side and various heights. In the understory, each species was inspected for a period of five minutes and the presence (and type) or absence of pests was recorded. Some species, if planted close together, were treated as a group. Ongoing surveys of both the overstory and understory were later conducted every 2-4 weeks. After natural enemies were introduced their presence was recorded in a like manner to pests. Approximately half way through the study, the protocol was changed for the understory in order to quantify levels of pest populations. Instead of doing the five minute inspection interval, ten leaves of each species, or group of species were examined. In addition to noting the presence or absence of pests, the plants were rated for severity of infestation using a scale of 0-3, with 0 = none present; 1 = a few; 2 = more than a few; 3 = many.
The plants harbored various pests including citrus mealybug, twospotted spider mite, aphids, whiteflies, thrips and scale. In general, the citrus mealybug was the primary pest in the overstory and twospotted spider mite was the main pest in the understory. Therefore, we decided to focus on these two pests.
We turned to the biological control literature and consulted with two of our University entomologists (Dan and Susan Mahr) to decide on the most appropriate natural enemies for these two pests. For control of citrus mealybug, the mealybug destroyer (Cryptolaemus montrouzieri) was overwhelmingly specified as most useful in indoor environments. The parasitic wasp Leptomastix dactylopii is an excellent addition for control of the mealybug.
The predatory mite Phytoseiulus persimilis was consistently cited as an efficient predator of twospotted spider mite. It is imperative, for survival of this predator, that relative humidity is at 60% or above. P. persimilis acts quickly, but cannot survive long without prey. Therefore, Neoseiulus californicus is an ideal partner for controlling P. persimilis. N. californicus is slower to act but can survive longer without prey.
For general predation we considered the green lacewing Chrysoperla rufilabris. Aphids are their main prey, but they also feed on mealybugs, spider mites, scale, thrips and small caterpillars.
An order was placed for 400 mealybug destroyers, 100 L. dactylopii, 5,000 P. persimilis and N. californicus, and 5,000 C. rufilabris eggs. All the natural enemies except L. dactylopii were received on the morning of 11 November 1997. The predators were checked in and immediately applied according to directions. The following day, at least one-half to one-third of the mealybug destroyers were found dead on the floor. A brief census failed to reveal any living beetles. Only a few predatory mites and green lacewing larvae were sighted.
The first complete ongoing survey (26 November) did not reveal any of the applied predators. Upon consultation with a staff entomologist at the insectary that supplied the natural enemies, we decided that the predators probably were killed due to bifenthrin residue, last applied on 16 September 1997. The back-order of L. dactylopii was canceled.
The morning of 15 December, the minimal number of predators that could be ordered were applied (1,000 P. persimilis and N. californicus; and 1,000 C. rufilabris) after the pesticide residue was thought to be gone. A survey on 6 January 1998 failed to reveal any sign of predators. On the afternoon of 14 January, 100 mealybug destroyers were released. The following day, almost all of the predators were found dead on the floor.
Throughout the month of April 1998, the conservatory was cleaned of dead plant material, floors and windows were washed, and plants were pruned. On 15 April, the plants were washed down to rid them of dead and live pests, honeydew, sooty mold, and dirt.
On the morning of 30 April, another 100 mealybug destroyers were received. This time, we attempted to determine what it was that was causing their extensive and rapid death. Possibilities included pesticide residue on foliage or prey and/or toxins in the air. Upon receipt, ten beetles were placed in each of four rearing cages. Two cages contained mealybug-infested foliage collected from the conservatory and 2 vials of 10% sugar water solution. The other two cages contained only two vials of sugar water. One of each of these two types of cage treatments was placed in the plantscape and the remaining two cages were placed in the manager's office. The predators remained in the rearing cages for five days and were then released into the plantscape. The remaining 60 predators were placed directly in the plantscape, after nightfall. None of the beetles were found dead the next day. All of the predators thrived inside the rearing cages. A survey on 6 May 1998 found the mealybug destroyers to still be thriving-none were found dead.
A likely reason why the predators failed to establish until the last application of mealybug destroyers is pesticide use and residue. Examining the record of pesticide use in the conservatory for two months before the first predator introduction, we noted both fenoxycarb and bifenthrin had been used. According to published literature, fenoxycarb is slightly harmful (25-50% reduction in control capacity) to mealybug destroyer and does not persist; not harmful (25% reduction in control capacity) to P. persimilis and does not persist; or slightly harmful to green lacewing larvae and persists for two weeks. Bifenthrin is harmful to all of these predators and persists for eight to twelve weeks. Therefore, bifenthrin is likely to have adversely affected our predator population.
We discovered the hard way that it is imperative that only pesticides that are known to be safe for biological control organisms should be used (if any). Although information on many pesticides' toxicity to beneficials and persistence is sketchy, as much precaution should be taken as possible. It is best that pesticides are not used at all while attempting establishment of natural enemies.
The overall pattern in pest populations for the overstory was a general decline through early spring and a rise at the time of the last census. The understory had no change in pest populations from October through mid March (although the only measure of comparison was presence/absence up to this point). Overall, populations in the understory declined through 22 April and rose around 6 May. Pesticide residue may have caused the prey population to decline, but it may have been due to slowed pest reproduction during the winter months. The final survey found a rise in the prey population and a hint that the mealybug destroyer may be establishing itself. Perhaps the absence of pesticide residues and the commencement of spring will give rise to a new cycle of life and death in the plantscape.
Whew! This project has been a challenge for all involved. We have learned a lot, but have so much more to learn about establishing biological control programs. The more venues (like this newsletter) we have to exchange information, on what works and what does not work, the better! With successful establishment of the mealybug destroyer in the conservatory, now would be a good time to release some of the other natural enemies we tried to use earlier. But as a student, I am moving on and so the responsibility for this project is being passed on to someone else who should have a wonderful opportunity to learn about implementing biological control.
- Cynthia Hawkinson, University of Wisconsin - Madison
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