The goal of the program is to determine the relationship of host plant xylem chemistry, and leaf morphology on host selection, feeding and ovipositional behavior of GWSS and its parasites.
GWSS oviposits in many plant species, yet the majority of GWSS egg masses tend to be concentrated on a few select host species that apparently offer the quality of xylem fluid (food) required for survival of nymphs. Food quality for nymphs appears to be an important factor affecting the population increase of GWSS. We concentrated on developing field methods and data towards determining the host selection behavior and use for oviposition of known host plants preferred for feeding by the adult GWSS. Experimental sites that offered mixed host plants were established in isolated islands in an open field and a large planting of crape myrtle. Plants on the islands and the surrounding crape myrtle were examined for the presence of GWSS life stages weekly. Results from both sites indicated a statistically significant preference of GWSS for ovipositing on holly plants over all other plant species. The second host most frequently used by GWSS for oviposition was Bradford pear. A few egg masses were also found on other hosts. Pyracantha was chosen as oviposition host only very early in the summer and in the fall. Parasites were able to utilize the GWSS eggs on all hosts. The data suggests that proximity of adult and ovipositional hosts may greatly increase exposure of adult hosts plants to GWSS. We also worked on standardizing and quantifying methodology for rearing GWSS. Strict attention must be given to lighting conditions especially from late to early season. When diapause occurs, diapause can be terminated if three weeks of short daylength are followed by long daylength (we employ 16:8 light:dark regime) so long as the proper ovipositional hosts are present.
We have now begun the second phase of this experiment, which is to assess the suitability to GWSS based on xylem chemistry of California-relevant host species. We have collected xylem chemistry data on the host plants used in the field plots and these are ready for analyses. We are currently investigating Chardonnay grapes, Navel oranges, Spanish Pink Lemon and Crape Myrtle. Data from these hosts will be compared to rates of development on soybean in order to assess the value of each of these species as developmental hosts for GWSS. Quantitative analysis is needed to prioritize the role of each potential host for implementation of GWSS control measures.
By exposing GWSS eggs to parasite adults, we determined the duration of the susceptibility of GWSS eggs to the parasites Gonatocerus ashmeadi and Gonatocerus morrilli. Parasitoids can successfully parasitize 100%of GWSS eggs for at least 7 days after oviposition. We also investigated the overwintering behavior of Gonatocerus sp. and GWSS. We determined for the first time in the U.S. that GWSS as eggs and Gonatocerus sp. within parasitized egg masses can overwinter at north Florida winter temperatures. Laboratory experiments showed that parasitoids fed honeydew provided from excised leaves with live whiteflies lived twice as long as those fed simple honey solution. Perhaps parasitoid abundance could be enhanced by providing alternative food sources.