Problems and Solutions

EVALUATING THE EFFECTIVENESS OF RELEASING BENEFICIAL INSECTS TO CONTROL TUSSOCK MOTH POPULATIONS AT STANFORD UNIVERSITY

INTRODUCTION

The western tussock moth, a native of California, is commonly found on Quercus agrifolia throughout the Stanford campus. Several characteristics of this insect have made it the target of control efforts, both chemical and biological. The growing larvae are voracious feeders of oak leaves; the resulting defoliation can weaken trees already stressed by soil compaction, summer irrigation and other adverse environmental conditions. The larvae are also a nuisance, together with fruittree leafroller, and California oakworm, when present in high use areas. Their frass rains down on picnickers, they crawl into buildings, onto play equipment and people, etc. Control measures are taken in areas where complaints are received. While chemical sprays were used regularly in the past, these have been discontinued and replaced by monitoring and biological and physical controls.

Two biological control measures were undertaken from 1998-2000: springtime releases of green lacewing larvae and summer releases of trichogramma wasps. These insects are predators and parasitoids, respectively, released to prey upon Lepidoptera pests. The insects were purchased from Beneficial Insectary of Oak Run, California. Cards containing lacewing eggs were placed in tree canopies upon first sign of tussock moth larval emergence in the springtime; the hatching lacewing larvae eat the tussock larvae and thus directly decrease the numbers of tussock larvae present. (Lacewing larvae only eat the young tussock larvae, less than ~1/4 inch.) In the summer, cards containing moth eggs parasitized by trichogramma wasps are placed into tree canopies. Adult wasps hatch from these eggs, mate and in turn parasitize existing tussock egg masses on trunks and branches. The results are not apparent until the following spring, when fewer tussock larvae emerge.

PROCEDURE

To evaluate the effectiveness of the trichogramma release, we collected tussock egg masses from pairs of experimental and control trees in late summer of 2000. The trees in each pair were of equal size and species and subject to the same growing conditions. On August 16, an egg mass from each experimental and control tree was collected to determine the level of background parasitism. At the same time, trichogramma were released into experimental trees (2 cards in small trees, 5 cards in medium trees and 7 cards in large trees), but not into control trees. Two weeks later, on August 31, egg masses were again collected and a second release of wasps was made. Two weeks later, a third collection and release of wasps took place, on September 14. Egg masses were collected one last time, on September 28.

The egg masses were stored in individual glass vials in a plastic sweater box containing moist paper towels. The box was kept outside in a bicycle locker, protected from direct sunlight but exposed to daily temperature fluctuations.

DATA

Beginning in late October, wasps began to hatch from the collected egg masses. However, these wasps were not trichogramma species. Instead they were Telenomus californicus , a native parasitoid of western tussock moth.
The number of wasps in each vial was counted at two-week intervals, from late October through mid-March. In March, western tussock moth larvae began hatching in the vials as well. The number of tussock larvae to hatch in each vial was recorded. No trichogramma wasps hatched from any of the collected egg masses.

RESULTS

Of the 55 egg masses collected, 50 were viable while 5 were duds (nothing hatched out from them). These five are not included in this analysis. Another three egg masses were taken to various labs for wasp ID before any tussock larvae hatched, so they are excluded from this analysis as well.

Adult Telenomus californicus wasps hatched out of both experimental and control egg masses of western tussock moth, beginning in October and continuing through December. Most hatching was complete by the first of January. 34 of 47 egg masses had been parasitized (72%). The number of wasps in parasitized egg masses varied from 1 up to 50. The average number of wasps in a parasitized egg mass was 13.5. Figure 1 shows the number of wasps to hatch from each collected egg mass, while Figure 2 shows the emergence over time.

47 of 47 viable egg masses had tussock larvae hatch (100%). The number of larvae to hatch in each egg mass varied from 22 to 189, the average number being 92. There was much variation in the number of larvae to hatch from each egg mass, as seen in Figure 3 .

ANAYLSAS AND DISCUSSION

The number of tussock larvae to hatch greatly outnumbered the number of Telenomus wasps to hatch. Thus, Telenomus wasps provided only partial control for western tussock moth. Up to 55 % of eggs in a tussock egg mass were parasitized, but the average percentage was 14.5%.

The egg masses collected on 8/16 and 8/31 had significantly higher Telenomus emergence than those collected in September. The reasons for this aren't clear.

In general, there was great variation in number of wasps to emerge from egg masses. This is also true of the number of tussock larvae hatching from each egg mass.

Telenomus wasps occur throughout the area that tussocks are distributed on campus. They occur in higher numbers in areas where tussocks occur in higher numbers. The distribution of sampled egg masses was not uniform. Data comes from sites with easily seen egg masses.

CONCLUSIONS

Releases of trichogramma wasps have no effect on the tussock population. A naturally occurring egg parasite, Telenomus californicus , is already present on campus, concentrated in areas with large populations of tussock moths. Telenomus wasps parasitized about 14% of eggs in a given tussock egg mass in our samples, with about 90 larvae still hatching out.

The effectiveness of lacewing larvae has not been reviewed. Anecdotal evidence suggests that lacewing larvae disappear shortly after being released. They do not appear to significantly lower the population of Lepidoptera larvae feeding in the canopies of oaks on campus. However, they appear to increase the lacewing population generally throughout the campus.

As an alternative to biological controls, a physical control known as power-washing, washing off tussock egg masses with water under pressure, has been used with much success to reduce the population of western tussock moth on heavily infested trees on campus.