Aspects of host-plant relationship of the Colorado beetle

Host plant choice, suitability of and conditioning to the host in Leptinotarsa decemlineata SAY were studied under controlled conditions.The literature on historical and geographical distribution of the Colorado beetle has been reviewed and an extensive survey is given of the literature on food plant range, host plant selection, orientation to the host, host plant recognition and conditioning of host plant preference concerning Leptinotarsa decemlineata.Definitions of host plant relationship and several other criteria are discussed. In this thesis 'feeding preference for a given plant' means that in a choice test, the quantity eaten from this plant exceeds the quantity consumed from each of the other plants. An 'oviposition preference for a given plant' means that the number of eggs deposited on this plant outnumbers the oviposition on any other plant at the disposal of the insect.All experiments were performed with insects reared in the laboratory; most of the plants used were grown in a greenhouse.The suitability of the plants to function as host was determined by rearing beetles ab ovo on Solanum tuberosum, Solanum dulcamara, Solanum luteum, Solanum nigrum and Solanum lycopersicum. The results appeared to coincide with the preference for food. Solanum tuberosum gives best results and is preferred to Solanum dulcamara, which gives satisfying results (as do Solanum carolinense and Solanum rostratum) and is readily eaten too. Solanum lycopersicum is normally avoided and only eaten when nothing else is available; it yields poor rearing results, which differs with different strains used.Solanum luteum and Solanum nigrum are avoided even when starvation is the only alternative. The avoidance of Solanum luteum apparently is caused by rejective properties and not by lack of phagostimulants or toxic compounds. This is shown by oviposition experiments, by alternate feeding, by sandwich tests and by food choice experiments with leaves impregnated with juices of Solanum luteum.The duration of larval development on the different food plants is not very variable and is dependent on experimental circumstances (e.g. temperature and physiological condition of the plants).Food choice experiments with larvae and adult beetles reared on Solanum tuberosum or one of the other experimental plants, gave no indication that preimaginal experience had any influence on host preference of the insect. Solanum tuberosum was preferred to all other plants used. Food choice was greatly influenced by the condition of the plant, the plant variety and the test conditions.With increasing temperatures the rate of food intake also increases, but more so with Solanum dulcamara than with Solanum tuberosum when offered simultaneously; Solanum tuberosum is preferred at lower temperatures, Solanum dulcamara at higher temperatures. This was true for both newly emerged and mature beetles, reared previously on either of these plants. Ambient temperature had no influence on the consumption of rejected plants. These effects as well as other factors influencing the food choice experiments have been discussed.Inbreeding on Solanum dulcamara resulted in a food preference for this plant species after eight and more generations.In screen tests and in olfactometer tests, performed with female beetles, besides olfactory stimuli, optical stimuli were introduced also. Both types of stimuli elicited searching behaviour, either separately or in conjunction and play a most important role in host plant recognition as well as in host plant finding. However, no conclusions about longer range attraction can be made yet.In oviposition preference experiments solitary female beetles were given the choice between Solanumtuberosum on the one hand and one of the experimental plants on the other. In control series the fecundity of the beetles was tested by rearing on both test plants separately. In comparison with tuberosum-reared beetles, fecundity of the Colorado beetles, reared on various food plants (as measured by the number of eggs deposited), was slightly depressed by Solanum dulcamara , more so by rearing on Solanum lycopersicum and heavily by rearing on Solanum luteum . It is very probable that the decline of fecundity is caused by a decrease of food consumption, owing to the apparently less palatable properties of these plants; renewed fecundity can be achieved by giving the beetles suitable food. Solanum tuberosum appeared to be preferred for oviposition to Solanum dulcamara and Solanum lycopersicum ; Solanum luteum , however, was preferred to Solanumtuberosum as an oviposition substratum, though this plant suppresses the fecundity and is avoided as food plant. Oviposition response to Solanum luteum could have been even stronger when the preference for Solanumtuberosum as food would not have interfered. In the choice experiments there is probably a decreased food intake and consequently a decreased egg production due to the daily period spent on Solanum luteum . Time appeared to be a limiting factor and, indeed, removal of Solanum luteum caused an increased fecundity. The conclusion is drawn that in host plant choice the selections of food and of oviposition substratum are based on two different mechanisms. Furthermore we conclude that this discrepancy will not be fatal for the insect, as a period of maturation feeding precedes the reproductive phase and consequently food plant choice by the adult beetle will determine the oviposition choice.Although it is possible to modify the host plant preference of the Colorado beetle, we conclude that habituation or 'learning' is not responsible for the host plant affinity of Leptinotarsa decemlineata SAY.