The isolating effect of greenhouses on arthropod pests [and its significance for integrated pest management] : a case-study on Clepsis spectrana (Lepidoptera: Tortricidae)

Chapter 1: the environmental conditions in greenhouses differ in many respects from those in the open field. Both the climate and the crops are different. A free exchange between the fauna of the greenhouses and the open air is hampered by the glass walls and roofs. The isolating effect of greenhouses on arthropod pests contributes to the effectiveness of control measures, but also to the development and maintenance of pesticide resistance in greenhouses. Because of the special conditions a specific fauna exists in greenhouses, and the use of exotic predators and parasites for biological control is possible. The greenhouse environment acts as a "sieve" only allowing such species to thrive that are adapted to these special conditions. These are sometimes exotic species that cannot thrive in the open in the Dutch climate. Native species may penetrate into greenhouse cultures, but to pass the "sieve" they have to adapt to greenhouse conditions.The leaf-roller <em>Clepsis spectrana</em> Tr. is native in the Netherlands. It gives an example of the development of a greenhouse-adapted biotype. In Dutch greenhouses, especially on roses, it causes much damage. In heated greenhouses, where artificial illumination is not given, growth and reproduction of <em>C. spectrana</em> continue without diapause during winter, which is advantageous for the species in this environment, where the difference between summer and winter temperatures does not exceed a few degrees and suitable food is available all the year round.Chapter 2 deals with the morphology, bionomics, host plant range, and distribution area of <em>C. spectrana.</em> Chapter 3 describes the materials and methods.Chapter 4: the number of larval instars from egg hatching to pupation varied between 4 and 8 in both field and greenhouse strains. Each larval growth type had its own specific progression of head capsule width. The difference in head capsule width between the 4- and 5-instar type was already evident in the 1st instar, between the Sand 6-instar type in the 2nd instar, and between the 6- and 7-instar type only in the 3rd instar or even later. Females tended to develop through a higher number of instars than males, but 5 instars was the most usual in both sexes. The development duration was the same in field and greenhouse strains, in each of the immature stages. The upper thermal limit for development was between 30 °C and 35 °C in both field and greenhouse strains, whereas the developmental threshold was close to 10 °C. An adaptation of greenhouse populations of <em>C. spectrana</em> to development at higher temperatures did not appear.Chapter 5: short daylength induced diapause in field strains in the larval stage. The critical photoperiod was between 16 and 17 hours. The number of moults up to the onset of diapause varied between 2 and 6, and was determined by the photoperiod. Larvae from eggs hatching later in the season entered diapause after a lower number of moults. The time of resumption of growth after diapause termination in spring was not correlated with the date of egg hatching in the previous year, in outdoor experiments. The larvae underwent supernumerary moults after termination of diapause. The duration of post-diapause development was longer as diapause had been entered after less moults. The number of moults after termination of diapause, and the duration of post-diapause development, were sex-linked. The functional significance of these phenomena is discussed.Chapter 6: the photoperiodic response of 5 different greenhouse strains, originating from larvae and pupae collected in different rose houses at different times of the year (both in summer and in winter), was tested in the laboratory. These greenhouse strains did not enter diapause, at both 20 °C and 15 °C, regardless of the photoperiod, One greenhouse strain was also reared outdoors. Egg hatching dates were August 1, August 17, September 25, and October 14. At least the major part of the larvae did not enter diapause.Chapter 7: field strain larvae entered diapause in a heated greenhouse, but the absence of a period of chilling caused an abnormal growth pattern in these larvae, compared to larvae terminating their diapause outdoors: (a) larval development duration was extremely variable (larvae from field strain eggs that hatched at August 24 in the greenhouse, pupated between January 28 and July 15 of the following year), (b) The difference in mean developmental time between male and female larvae was greatly increased, and (c) the mean number of moults, and the variation in the number of moults, Were increased. The survival of diapausing larvae, however, was not essentially affected by the absence of a period of chilling. The time required for diapausing larvae to reach the pupal stage under conditions without a cold period, was genetically determined, and could be shortened rapidly by selection.Chapter 8: a difference in the composition of the female sex pheromone between field and greenhouse strains could not be shown, nor in the calling behaviour of virgin females in relation to the light-dark cycle. Release-recapture trials revealed that females of both strains attracted males of both strains in equal proportions. There did not appear to be a difference in mating preference.Chapter 9: the index of genetic identity (I) of a field population on stinging nettles from the middle of the country, and a rose house population from the West of the country, amounted to 0.994 (based on allozyme frequencies). This is the level for panmictic populations of one species. Field and greenhouse strains could readily be intercrossed, with a viable F2 generation.Chapter 10: field and greenhouse populations of <em>C. spectrana</em> are certainly still conspecific. It is adequate to speak of a "field biotype" and a "greenhouse biotype". The greenhouse biotype is characterised by absence of the ability to enter diapause. When the biotypes are brought together, hybridization and introgression certainly will occur. Immigration of the field biotype into heated greenhouses apparently is sufficiently low to maintain a separate greenhouse biotype with constant characteristics. The origin of the greenhouse biotype might be a non-diapausing geographic race of <em>C. Spectrana,</em> from the warmest parts of its distribution area, or the field biotype immigrated into heated greenhouses, and subsequently lost its ability to enter diapause. The last possibility seems the most likely.Chapter 11: pheromonal trapping of <em>C. spectrana</em> is less effective in greenhouses than in the open air. Probably the specific nature of the air movements in greenhouses reduces the effectiveness of the long- range pheromone-mediated behaviour of the males. However, male pheromone-mediated behaviour in short-range orientation, close to calling females, may be unaffected (chapter 8.3). This decreases the effect of synthetic pheromone used for monitoring and mass trapping. However, application of the communication disruption technique may be successful under glass, because the overall concentration of synthetic pheromone in the air can be made much higher in greenhouses than in the open field. Further research is recommended.Controlling <em>C. spectrana</em> in greenhouses by releasing sterile males seems feasible, as: (a) immigration of new adults from outside almost certainly is negligible, and (b) <em>C.</em><em>spectrana</em> is the only tortricid occurring in the Dutch floriculture; the danger that other leaf-rollers will take over when chemical control of <em>C. spectrana</em> ceases seems therefore small.Diapause can be crossed into greenhouse populations of <em>C. spectrana</em> by releasing males of the field biotype. This probably is an ineffective way of controlling these populations.Parasitised <em>C. spectrana</em> larvae were found in several greenhouses, even in winter. The parasites, which were not identified, apparently were not diapausing. It is recommended to make a survey of the parasite fauna of <em>C. spectrana</em> in greenhouse cultures.