Advances and Challenges of Using the Sterile Insect Technique for the Management of Pest Lepidoptera

Abstract

Over the past 30 years, the sterile insect technique (SIT) has become a regular component of area-wide integrated pest management (AW-IPM) programs against several major agricultural pests and vectors of severe diseases. The SIT-based programs have been especially successful against dipteran pests. However, the SIT applicability for controlling lepidopteran pests has been challenging, mainly due to their high resistance to the ionizing radiation that is used to induce sterility. Nevertheless, the results of extensive research and currently operating SIT programs show that most problems with the implementation of SIT against pest Lepidoptera have been successfully resolved. Here, we summarize the cytogenetic peculiarities of Lepidoptera that should be considered in the development and application of SIT for a particular pest species. We also discuss the high resistance of Lepidoptera to ionizing radiation, and present the principle of derived technology based on inherited sterility (IS). Furthermore, we present successful SIT/IS applications against five major lepidopteran pests, and summarize the results of research on the quality control of reared and released insects, which is of great importance for their field performance. In the light of new research findings, we also discuss options for the development of genetic sexing strains, which is a challenge to further improve the applicability of SIT/IS against selected lepidopteran pests.

Keywords: SIT; cytogenetics; genetic sexing; inherited sterility; moths; pest control programs; quality control.

Figure 1

WZ sex chromosome system in Lepidoptera. Chromosomes were stained with DAPI (4’,6-diamidino-2-phenylindole; blue). (a) Pachytene oocyte of the codling moth, Cydia pomonella (L.), showing n = 28 meiotic chromosome bivalents; the neo-sex chromosome bivalent (WZ) is identified by fluorescence in situ hybridization (FISH) with W-painting probe (red), highlighting the W-chromosome thread [14]. (b) Mitotic oogonial metaphase of the cabbage moth, Mamestra brassicae (L.), showing 2n = 62 chromosomes [15]; the W chromosome is identified by comparative genomic hybridization (CGH) with both the female genomic DNA probe (green) and the male genomic DNA probe (red), resulting in yellowish labeling. Bar = 10 µm for (a) and 5 µm for (b).

Figure 2

Comparison of female and male meiotic chromosomes in Lepidoptera. (a) Female metaphase I bivalents (n = 30) of the Mediterranean flour moth, Ephestia kuehniella Zeller, stained with DAPI (white). Each bivalent consists of two homologous chromosomes arranged parallel to each other; the proteinaceous structure of the modified synaptonemal complex (SC) forms an unstained gap between the two homologues. (b) Male metaphase I bivalents (n = 14) of the vaporer, Orgyia antiqua (L.), stained with lactic acetic orcein. Two homologous chromosomes are maintained in bivalents by chiasmata; two arrowheads indicate two homologues of bivalents with one chiasma, arrows indicate bivalents with two chiasmata. Bar = 5 µm.

Figure 3

Highly polyploid nuclei of the Malpighian tubule cells from the carob moth, Ectomyelois ceratoniae Zeller, stained with lactic acetic orcein. (a) A nucleus from adult female showing a sex chromatin body (W). (b) A nucleus from male larva without sex chromatin. Bar = 50 µm.

Figure 4

Sperm bundles in the Mediterranean flour moth, Ephestia kuehniella Zeller, stained with lactic acetic orcein. (a) A eupyrene sperm bundle. (b) Two apyrene sperm bundles. Bar = 100 µm.

Figure 5

Computer-simulated curves for the induction of dominant lethal mutations in insect sperm. Shape of the curve changes as the number of hits required for a dominant lethal event increases. (Reproduced from [55] with permission of Elsevier).

Figure 6

Comparison of structure of holokinetic and monocentric mitotic chromosomes and consequences of chromosome breakage. (a) Holokinetic chromosome of Lepidoptera with two sister chromatids (A₁ and A₂), each with a large kinetochore plate (K) covering about 50% of the chromosome surface; spindle microtubules (MTs) are attached to the kinetochore. (b) Typical monocentric chromosome, where sister chromatids are linked in a primary constriction, the centromere (C); the kinetochore (K) is localized on the surface of the centromere (modified according to [11]).

Figure 7

Wild false codling moth (FCM) trap catches and the average percentage of infested fruits per tree in the Sunday River Valley, Eastern Cape, South Africa, from 2011 to 2016 (reproduced with permission from Nevill Boersma, XSIT, Citrusdal, South Africa).

Figure 8

Estimates of pesticide use against the codling moth from 1991 to 2015 in the OKSIR Program area, Okanagan Valley. Sales data for the 15 products registered for the use against codling moth are used to determine these values. However, a number of these insecticides are also used against other pests and/or crops. The proportion of total sales of a given product used against the codling moth is estimated by local tree fruit experts (Jerry Vakenti, BC Ministry of Water, Land and Air Protection; Hank Markgraff, Head of Field Service, BC Tree Fruits; Hugh Philip, IPM specialist, Independent Consultant). The estimates of active ingredients are divided by the area (ha) of planted pome fruit in the program area to account for changes in sales due to the amount of pome fruit under cultivation. From 1991 to 2015, there was an estimated 96% reduction in pesticides used against the codling moth. Other factors, such as changes in application rates in spindle vs. traditional plantings, new product formulations, etc. contribute in part to this reduction (reproduced with permission from OKSIR).

Figure 9

Transgenic female sexing strain proposed for the control of codling moth populations using the sterile insect technique (SIT) [145,153]. (a) Scheme of genetic sexing strain based on the use of transgenic females that possess an insert in their W chromosome, containing the tdTomato marker gene (red) and the codling moth ortholog (CpN^60g11; yellow; R. Čapková Frydrychová and F. Marec, unpublished) of the dominant cold-sensitive mutant allele of the Notch gene of Drosophila melanogaster, N^60g11. EL, embryonic lethality. Z and W, sex chromosomes. (b) Scheme of male-only SIT using transgenic female sexing strain. Wild-type males are mated with transgenic females, expressing the red fluorescent protein in their eyes under the 3xP3 promoter. The obtained egg collections are exposed to a temperature below 20 °C, resulting in the death of all transgenic female embryos. Non-transgenic male-only offspring are kept until adulthood, sterilized by ionizing radiation, and released to apple orchards.