Are Glassy-Winged Sharpshooters Significant Pests of Avocado?

By: Pascal Oevering, Christine Payne, Ben Faber and Phil Phillips
UCCE Ventura County

Introduction Glassy-winged sharpshooters (Homalodisca coagulata) have spread widely in Southern California since their introduction in 1990. There are two GWSS generations per year; adults reproduce in spring and this offspring develops into the first generation adults around April-May. These adults reproduce through the summer. Their offspring develop into adults in late summer/fall, and overwinter and reproduce again in spring. GWSS adults and nymphs feed on xylem fluids of host plants. These fluids are generally of low nutritional quality; the amount of liquid required for development and survival is large. More than 250 plant genera are listed as host plants for GWSS, including avocado.

In 2001, concerns were raised by avocado growers in Pauma Valley (San Diego County) and Fillmore (Ventura County), that large numbers of second generation adults were in groves, feeding on fruit stems (pedicels) and leaving a white exudate cover on the surface of fruit. Growers were concerned that GWSS feeding affected fruit development. In 2002, we studied the natural population of GWSS in both effected areas. We also confined adult GWSS to caged avocado trees to study their effect on tree growth and the population development of GWSS on avocado. The results of these studies are presented here.

Study 1. Naturally occurring GWSS populations in Hass avocado orchards adjacent to Valencia orange trees in San Diego (Pauma Valley) and Ventura County (Fillmore).

In each county, two groves were monitored from March 2002 to November 2002. For each of the field sites 20 Hass avocado trees were selected in a row adjacent to Valencia oranges, five of which were selected for observation. At plot Pauma Valley I (PI), the trees were young (2-5 year old) and up to 5 ft tall, and in another plot (PII) trees were 8 years old and 15-20ft tall. At Fillmore I (VI) avocado trees were 2-4 years old (up to 6 ft tall) adjacent to 25 year old Valencia orange trees (20ft tall), and Fillmore II (VII) were mature 10 years old avocado trees.

Each tree was observed once every two weeks; the number of egg masses, nymphs and adults on the branch tips (2 feet in length) of 5 branches were counted by visual inspection and beat sampling. The sampling method allowed for counting all adults, nymphs and egg masses on small trees. On mature trees this sampling method recorded about 1% of all the adult GWSS in the tree (following comparative study between this sampling and pyrethriod fogging of a mature tree in collaboration with Dr. Carlos E. Coviella, Dept. Entomology, UCR). The numbers of nymphs and egg masses have not been correlated to absolute counts, but the continuous observation using this method allowed for comparison through the season. Data was analyzed using ANOVA.

We found that the GWSS numbers were generally lower in Fillmore compared to Pauma Valley (Figure 1). The numbers of adult GWSS did not differ between trees on the edge of an orange grove and adjacent avocado trees. The number of nymphs in young orange trees in Pauma Valley (plot P I) were higher than in either young or mature avocado or mature orange trees. In Fillmore, the number of nymphs in orange trees in plot VI was higher than in the other plots in Ventura. The number of egg masses in mature avocado trees in Pauma Valley (plot 2) was lower than in young avocado trees or both young and old orange trees. In Ventura, the number of egg masses in orange trees was higher than avocado trees in both plots.

This shows us that GWSS adults move freely into avocado groves, and although egg masses are laid in both orange and avocado trees, they oviposit less frequently in avocado trees. Fewer nymphs were found in avocado, which may be related to the fewer egg masses laid and/or developmental difficulties for nymphs to develop on avocado trees.

Figure 1. Mean number of GWSS adults, nymphs and eggmasses found in beat samples and/or visual inspection of 5 branch tips per tree in two plots in Pauma Valley and two plots in Fillmore from April to November, 2002.

Sticky Cards In August and September, we used yellow sticky cards to monitor numbers of GWSS adults and GWSS parasitoids (Gonatocerus ashmeadi and G. morilli) in oranges and avocado in Pauma Valley. Cards were collected after two weeks and the numbers of adults trapped were recorded. Numbers of the GWSS and the two parasitoid species did not differ between oranges and avocado trees (Figure 2). This means that not only the adult GWSS but also the egg parasitoids move freely into avocado groves.

Figure 2. Mean number of adult GWSS, Gonatocerus morrilli and G. ashmeadi trapped on yellow sticky cards in two Pauma Valley plots.

GWSS exudate on avocado fruit In Pauma Valley, avocado fruit were scored both in September and October and the percentage of the surface covered by exudate, fruit size and position of this fruit in the tree were recorded. These observations were made on 160 fruit on trees in the row adjacent to (in both Pauma Valley plots) and 5 rows and 10 rows away from Valencia orange trees (in P II). In these observations we did not find any correlation between height of fruit on the tree and/or size of the fruit and the percentage of exudate coverage on the fruit (Figure 3). When correlating the distance between avocado trees and orange trees and the number of fruit with exudate coverage, and the amount of coverage per fruit, we found a strong correlation in September, with the percentage of fruit with exudate, and the amount of exudate per fruit decreasing with increased distance from orange trees. In October, the same plots no longer showed this correlation (Figure 4).

Figure 3. Relationship between the percentage exudate coverage on the fruit and fruit size (A) and height of fruit on the tree (B), numbers above bars are the total number of fruit in the category.

Figure 4. Relationship between distance from orange trees and percentage fruit with exudate coverage (A) and mean percentage exudate coverage on the fruit (B).

The data here indicates that adult GWSS feed on the fruit stems (photo) and that their exudate covers fruit. With heavy coverage of fruit we noticed sooty mould growing on the exudate (photo). The data does not substantiate the concern that GWSS affects fruit development as no correlation was found between fruit size and exudate coverage. The effect on marketability of exudate-covered fruit may be significant and depend on packing procedure. While not substantiated by this study, the internal quality of the fruit is not likely affected.

Study 2. GWSS Feeding and reproduction on Young Avocado Trees In Fillmore, three groups of ten two-year-old Lamb Hass trees were used for GWSS confinement studies. Lengths of all branches and tree height and size of fruit of all trees were measured and leaves with egg masses were removed before 20 trees were entirely caged with 70% shade cloth. 200 adult GWSS (collected from local lemon trees) were released in each of ten cages. The remaining 10 caged trees and 10 uncaged trees were used as controls. After 101 days (May 3 to August 12, 2002) the ten cages in which GWSS were released were sprayed with Tame 2.4 EC spray. After spraying, the numbers of GWSS caught on tarps placed underneath the caged trees were counted. After removal of the cages, the lengths of all branches and tree height of all trees were measured together with the number of egg masses per tree, the size of mature fruit and the number of fruit set during the caged period. Because mature fruit on uncaged trees was harvested during the trial period, data could not be obtained for that fruit. The same spray and cage procedure was repeated with 5 one-year-old lemon trees, each caged with 50 GWSS for 38 days (August 30 - October 7). Temperatures inside cages and in adjacent trees were recorded using HOBO® data loggers.

The tree measurements on height and length of branches and the fruit size of caged and uncaged trees showed no effect of treatment (Figure 5) although more fruit set on uncaged trees and significantly more egg masses were found on uncaged trees (Figure 6). Temperatures inside cages did not differ with those outside the cages (Figure 7). No effect can be related to the presence of GWSS, since no difference was found between caged trees with or without GWSS. Only 3 adult GWSS, no nymphs and no egg masses were found in the cages after 101 days, indicating that the released adult GWSS did not survive in the cages for long. To test whether this lack of survival was an effect of the cages or of avocado as a host plant, GWSS adults were also caged with lemon trees on which GWSS normally reproduce successfully. 38 days after confining 50 GWSS per lemon tree, 1.6 ± 0.5 adults, no nymphs and 6.2 ± 1.9 egg masses were found per tree. Of the egg masses 78% were parasitized.

Figure 5. Increase in length of branches and stems(A) and increase of fruit length (B) of caged and uncaged trees Lamb Hass trees during 101 days on in GWSS confinement studies.

Figure 6. Mean fruitset (A) and mean number of eggmasses (B) on caged and uncaged trees in GWSS confinement studies.

Figure 7. Temperature inside and outside the cages used for GWSS confinement studies.

This data therefore does not substantiate that GWSS can sustain themselves by feeding on Lamb Hass avocado. There is likely to be an effect of the cages, but since some reproduction was recorded on lemon, the lack of reproduction on Lamb Hass cannot be explained by cage effect alone. The GWSS did not survive well enough to study their impact on Lamb Hass avocado growth.

In conclusion, it is clear that GWSS occur in Hass avocado, and in 2002, numbers of GWSS in Pauma Valley (large orange acreage) were higher than in Ventura County (small orange acreage, some lemon). During the feeding of GWSS adults on fruit stems the plant fluids pass through the adult and cause a white exudate on the fruit, which may cover a substantial amount of fruit. During the second generation, the amount of exudate coverage may increase and cover larger areas of the fruit, and black sooty mould may be present on top of the exudate. The effect of this exudate coverage on fruit quality remains unknown. Our data does not support the idea that GWSS can successfully reproduce on Lamb Hass avocado. How this translates to Hass avocado can only be speculated. Considering the observations in this study, we think that GWSS is unlikely to develop into the pest in avocado that they are in citrus. The occurrence of GWSS in avocado seems related to the presence of citrus in the vicinity, making it unlikely to become a general problem in avocado groves. However, exudate coverage of fruit may be problematic to growers in areas with a citrus-avocado interface. Research on the quality of exudate-covered fruit is needed to determine advice on which actions to take - if any.