Management of genetically engineered potatoes resistant to potato tuber moth, Phthorimaea operculella (Lepidoptera: Gelechiidae)

Abstract

Potato tuber moth (Phthorimeae operculella (Zell.), PTM) is an important pest of potatoes. The larvae mine into foliage and exposed tubers, making them difficult to control with spray insecticides. Cultural methods can be effective; but are often difficult to implement. As a new approach to manage PTM, genetically engineered potatoes were developed using a cry1Ac9 gene under the transcriptional control of the CaMV 35S promoter. This gene was transferred into six potato cultivars: Iwa, Ham Hardy, Russet Burbank, Red Rascal; Rua, and White Delight, using the Agrobacterium-mediated transformation. Sixty-three putatively transformed shoots selected based on their ability to root in MS medium supplemented with 50 mg/l kanamycin were transferred to the greenhouse and evaluated based on appearance, and resistance to PTM larvae. Twenty-seven of these were described as phenotypically normal and resistant to PTM.

These 27 lines along with five transgenic lines developed prior to 1997 were grown in small field plots during the summers of 1998/99 and 1999/00. Twenty-five out of 34 lines in the 1998/99 and 1999/00 trials were categorised as phenotypically normal. Nineteen of these lines were resistant to PTM larvae when first tested in the field and exhibited a relatively strong correlation with greenhouse results (1998/99, r = 0.68, P < 0.05). Lines 107 (derived from Ilam Hardy), D61, D64 (derived from Russet Burbank); D2 and D53 (derived from Red Rascal) consistendy reduced larval growth by ≥40% in the greenhouse and throughout the different field trials.

Russet Burbank D61 and D64, and Red Rascal D2 and D53, were evaluated for their resistance to PTM larvae in larger scale field trials during the summer of 1999/00. Plants were enclosed and artificially infested with eggs from a laboratory population or surveyed for natural infestation by a wild population of PTM between February and April 2000. Virtually no larvae or pupae were collected from enclosed transgenic plants, while from control foliage 70 - 90% and 18 - 30% of those to develop into larvae or pupae respectively, were recovered. The majority of mines from the transgenic lines due to natural infestation were less than 200 mm². Larvae of different ages (zero, three, five, and seven days old) used in stage-specific bioassays, did not exhibit any overall age-dependent pattern of increasing or decreasing susceptibility to the foliage of greenhouse or field-grown transgenic plants from these lines.

In the laboratory, PTM larvae on transgenic (cry1Ac9 or cry9Aa2) or non-transgenic tubers were exposed to a parasitoid of PTM, Apanteles subandinus Blanchard, and then reared through to adults. Significantly fewer parasitoids emerged from PTM reared on the transgenic tubers with high resistance. There was no significant difference in the percentage of female progeny of the parasitoid parents to emerge from PTM larval hosts reared on transgenic or non-transgenic tubers. Larvae of the brown lacewing, Micromus tasmaniae Walker, an important aphid predator in New Zealand, were reared On the green peach aphid, Myzus persicae Sulzer, from transgenic (cry1Ac9 or cry9Aa2) or non-transgenic plants. There was no significant difference between the two treatments for fecundity parameters measured for lacewing adults. The growth of greenlooper (Chrysodeixis mosoma (Doubleday)) was not inhibited by the transgenic lines containing the cry1Ac9 gene, but was significantly lower when fed foliage transgenic for cry9Aa2 (line 32), although the percentage to pupate on this line was higher than the Iwa control.

Transgenic potatoes resistant to PTM were successfully developed and their efficacy under field conditions has been clearly demonstrated. They had minimal or no negative impact on the beneficial species investigated in the laboratory. Therefore PTM-resistant transgenic potatoes could prove to be compatible with integrated pest management strategies for managing this pest.