Thrips - TSWV/INSV Control Systems Progress Report — June 1999
Report on Progress of Thrips-TSWV/INSV Control System
Section on Biological Control
Kevin M. Heinz
Texas A&M University
Introduction
For the past year we have been concentrating our efforts on the use of the entomopathogenic nematode, Thripinema nicklewoodi for biological control of western flower thrips. Our report concentrates on these efforts. We have also just initiated work on several predaceous mites for thrips biological control. Progress on the mite research will be detailed in future reports.
As a prerequisite for successful development and implementation of an entomopathogen for biological control, it is essential that the biology of both entomopathogen and pest insect are well understood. There exists a comprehensive and detailed literature for WFT. There is, however, scant information available on T. nicklewoodi. Here, we present preliminary information on the biology of T. nicklewoodi, data that is essential for the successful evaluation and development of T. nicklewoodi for the biological control of WFT.
Culturing of Thripinema nicklewoodi
As T. nicklewoodi is an obligate parasite of WFT, we have concentrated on the development of a rearing system for this nematode species using its natural host. A mixed population of T. nicklewoodi infected and uninfected WFT adults are placed in a plastic container containing bean leaves supported in a non-deodorant feminine napkin saturated with purified water. The bean leaves are supplemented by streaking with honey and adding bee pollen to their surface. Using this system, the thrips larvae produced by the uninfected WFT are infected while feeding on the surface of the bean leaf by nematodes that emerge from the WFT. We have been successful in continuously maintaining T. nicklewoodi for over six months using this method. We now use this method to maintain a stock culture of T. nicklewoodi infected WFT. In a second method, infected WFT are confined individually within 1.5ml eppendorf vials which contain a small piece of moist tissue paper and a kidney bean leaf disc (11mm diameter) streaked with honey. Uninfected WFT larvae and female pupae can then be placed in the vials with the infected female from which nematodes are emerging. Using this method, we have obtained good levels of parasitism, up to approximately 65%. To obtain nematodes for experimentation, the inside of each vial (to which no uninfected WFT had been added) is rinsed with water and the nematodes collected.
Biology of T. nicklewoodi
(a). Life cycle
We have established that the time taken from infection of WFT with T. nicklewoodi to subsequent emergence of the next generation of nematodes is approximately 15 days (range: 12-20 days) at 71oF. The time for completion of the life cycle within WFT is therefore estimated to be approximately 13-14 days. This is being verified by following the developmental progress of T. nicklewoodi in WFT.
(b). Nematode emergence
Reproduction and development of T. nicklewoodi in WFT is asynchronous, thus, emergence of the free-living males and females occurs over a period of time (Figure 1).
In addition, significantly higher numbers of female than male nematodes emerge from the infected thrips (Figure 1 & Table 1). This confirms our earlier observations, obtained when dissecting thrips to examine prevalence of infection, where we noted that infected WFT harbored more female than male nematodes, the ratio ranging from approximately 4:1 to 9:1 females to males. This result is of significance as it is only the female nematodes are infective to WFT.
The mean number of days over which emergence occurs is typically approximately 14 days (Table 1). However, emergence has been observed to occur for up to 36 days. No difference in survival time between infected and uninfected WFT has been noted.
Table 1.
Emergence of T. nicklewoodi from WFT.
Nematode emergence (mean (± s.e.) number of days) |
Total mean (± s.e.) number of nematodes emerged |
mean sex ratio (female: male) |
Mean (± s.e.) number of nematodes emerging/day |
13.9 (± 1.0) |
208.6 ± 35.3 |
4.7: 1 |
16.3 ± 2.8 |
(c). Site of nematode emergence from WFT.
Previous reports on the biology of Thripinema species have suggested that nematode emergence occurs either via the thrips anus or ovipositor. We have now confirmed that T. nicklewoodi emerges via the anus in the faeces of WFT: nematodes have been observed actively moving within both the mid-gut and within the faeces.
(d). Nematode survival
The foliar habitat is an unfavourable one in terms of nematode survival. However, the flowers of ornamental plants should provide areas with relatively elevated moisture levels: conditions necessary both for nematode movement and survival. Indeed, we have shown that the primary site of transmission on ornamental plants for T. nicklewoodi is in the flowers (see below (e)). However, as the foliar habitat is unfavourable for the survival of free-living nematodes, it was expected that survival of T. nicklewoodi following emergence would be relatively short and that host seeking would be a very active process. Indeed, we had previously noted that the free-living males and females are highly motile following emergence, suggesting very active host seeking and, on visual inspection, appear to contain low levels of fat (primary nematode energy source). We examined nematode survival in water, in which nematode survival should be optimal, and at three temperatures (1.5, 8.5 and 21oC). Survival was low at the highest temperature, with 100% nematode mortality observed within 40 hours. In contrast, survival increased significantly at the lower temperatures, nematodes surviving approximately 3.5 and 5 days at 8.5 and 1.5oC, respectively. In addition, we have calculated estimated survival times in water for 50% of nematodes (Table 2). These estimates will likely be lower for nematodes emerging directly into the foliar environment.
Table 2.
Estimates for the survival times in water of 50% of free-living T. nicklewoodi (S50) following their emergence from infected WFT.
Storage temperature (oC) |
S 50 (hours)a |
1.5 |
71 (59 – 85) |
8.5 |
50 (46 – 54) |
21 |
23 (21 – 25) |
aThe S50s are followed by the 95% confidence interval in parentheses.
(e). Infection by T. nicklewoodi of WFT on chrysanthemums
In a preliminary study to examine on which plant structure infection of WFT does indeed occur on ornamentals, cut chrysanthemum flowers, with leaves and stems attached, were infested with both adult female WFT infected with T. nicklewoodi and uninfected larvae. All thrips were removed after four days and status of nematode infection assessed. Nematode-infected larval WFT were recovered only from the flowers, with those that had fed on the leaves not infected. Additionally, flowers and leaves were washed with water to check for the presence of nematodes. Approximately 88% of the nematodes recovered were found on flowers. The results from this study suggest that infection of WFT by T. nicklewoodi does occur primarily within the protected environment of the flower. However, we have also shown that, in the absence of flowers, transmission does occur on the leaves.
Conclusions
The preliminary results presented here provide much needed information on the biology of T. nicklewoodi and will allow us to proceed to critically evaluate the potential of T. nicklewoodi for the suppression of WFT populations.
