Feeding on roots in the humus layer by adult pine weevil, Hylobius abietis

1 The consumption by adult pine weevil, Hylobius abietis, of the bark of roots present in the humus layer was assessed in a field study conducted in southern Sweden during two years (1998 and 2002). The study sites were divided into two areas: (i) a shelterwood where 80–100 mature Scots pine trees per hectare remained after cutting and (ii) a clearcut where no trees were left. 2 On average, 3741 m² per hectare of root bark was present in the humus layer, of which 135 m² was not coniferous but comprised species such as bilberry and broadleaved trees. 3 The mean area debarked by pine weevils was 2.9 m² per hectare; 2.6 m² of conifer roots and 0.3 m² of bilberry roots. Roots of broadleaved trees were almost never consumed. No clear preferences for roots of a specific level of vitality were observed. 4 No consistent difference between the shelterwood and clearcut was found, either in the amount of root bark area available or in the extent of root feeding by pine weevil. 5 A weak negative correlation between debarked areas on roots and seedlings was found, indicating that root feeding may have reduced damage to seedlings. 6 It is concluded that conifer roots in the humus layer constitute a major food source for the pine weevil and can be utilized for a considerable period in both clearcuts and shelterwoods.     

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

• 1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species.
• 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration.
• 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits.
• 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed.
• 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding].
• 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

     

Effects of green tree retention, prescribed burning and soil treatment on pine weevil (Hylobius abietis and Hylobius pinastri) damage to planted Scots pine seedlings

1 Feeding damage and mortality caused to planted Scots pine seedlings by the pine weevils Hylobius abietis and Hylobius pinastri were studied on burned and unburned sites with 0, 10 and 50 m³ per hectare levels of green tree retention from the second to the fourth summer after logging and burning of the sites. 2 The rate of severe feeding damage to pine seedlings caused by pine weevils was higher on burned clearcut sites than on unburned ones, whereas burning did not increase the feeding damage rate on sites with groups of retention trees. The damage rate in the fourth summer was approximately the same on burned and unburned sites. 3 Pine weevil feeding was the major cause of mortality of freshly planted pine seedlings on unburned sites. On burned sites, mortality was higher than the rate of severe feeding damage, particularly in the second summer after burning, possibly owing to fungal attack and abiotic factors. 4 At a retention tree level of 50 m³, feeding damage to the seedlings was lower than on clearcuts and at a 10 m³ retention tree level. Furthermore, on sites with 50 m³ of retention trees, scarification of the soil was found to decrease feeding damage more effectively than on clearcuts and 10 m³ sites. If the seedlings were situated in the centre of scarified patches, scarification alone was as effective as insecticide treatment on unscarified soil for decreasing feeding damage and mortality. 5 The results suggest that when burning is applied as silvicultural treatment after clear‐cuts, retention of trees is recommended to reduce the damages caused by pine weevils on pine seedlings.     

Effects of green tree retention, prescribed burning and soil treatment on pine weevil (Hylobius abietis and Hylobius pinastri) damage to planted Scots pine seedlings

1 Feeding damage and mortality caused to planted Scots pine seedlings by the pine weevils Hylobius abietis and Hylobius pinastri were studied on burned and unburned sites with 0, 10 and 50 m³ per hectare levels of green tree retention from the second to the fourth summer after logging and burning of the sites. 2 The rate of severe feeding damage to pine seedlings caused by pine weevils was higher on burned clearcut sites than on unburned ones, whereas burning did not increase the feeding damage rate on sites with groups of retention trees. The damage rate in the fourth summer was approximately the same on burned and unburned sites. 3 Pine weevil feeding was the major cause of mortality of freshly planted pine seedlings on unburned sites. On burned sites, mortality was higher than the rate of severe feeding damage, particularly in the second summer after burning, possibly owing to fungal attack and abiotic factors. 4 At a retention tree level of 50 m³, feeding damage to the seedlings was lower than on clearcuts and at a 10 m³ retention tree level. Furthermore, on sites with 50 m³ of retention trees, scarification of the soil was found to decrease feeding damage more effectively than on clearcuts and 10 m³ sites. If the seedlings were situated in the centre of scarified patches, scarification alone was as effective as insecticide treatment on unscarified soil for decreasing feeding damage and mortality. 5 The results suggest that when burning is applied as silvicultural treatment after clear‐cuts, retention of trees is recommended to reduce the damages caused by pine weevils on pine seedlings.     

Testing associational resistance against pine weevils mediated by Lasius ants attending conifer seedlings

Natural enemies attracted to plants may provide those plants with protection against herbivores but may also protect neighbouring plants, that is through associational resistance. Ant attendance may be an important mechanism for the occurrence of such effects because ants can reduce the damage caused by a wide variety of herbivorous insects. Ants have been shown, in a previous field experiment, to decrease the damage caused by the pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae), a pest species that causes high seedling mortality in forest regeneration areas. In this study, we specifically tested whether seedlings planted close to ant‐attended seedlings experience associational resistance. We did this under laboratory conditions using the ant species Lasius niger (L.) (Hymenoptera: Formicidae). The feeding damage by pine weevils was significantly reduced on seedlings attended by ants. The neighbouring seedlings, however, did not experience associational resistance. Nevertheless, some associational effects were observed as the number of weevils recorded on both ant‐attended and neighbouring seedlings was significantly lower compared with ant‐excluded seedlings.     

The effect of red wood ant abundance on feeding damage by the pine weevil Hylobius abietis

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Feeding preferences of the weevils Sitona gressorius and Sitona griseus on different lupin genotypes and the role of alkaloids

The weevils Sitona gressorius and Sitona griseus are specialist herbivores on lupins in Europe. The adult weevils feed on the leaves, and the larvae on the root nodules of the plants. This causes severe damage to lupin crops. In the present study, the feeding preferences of lupin weevil adults on different lupin genotypes were examined with respect to a possible effect of lupin alkaloids on host selection. A total of 12 genotypes from the species Lupinus albus, L. angustifolius, L. luteus, and L. nanus were grown in a field experiment and the feeding damage on the leaves caused by naturally occurring lupin weevil adults was estimated. Additionally, a feeding choice test with S. gressorius adults was performed to examine feeding preferences under laboratory conditions. A gas chromatographic analysis provided information on the alkaloid content and profiles in the leaves of the tested lupin genotypes. In the field experiment, significant differences in the extent of the feeding damage within the 12 lupin genotypes were observed. The dual-choice feeding bioassay did not show discrimination of lupin species, but two L. angustifolius genotypes were significantly less affected than the standard L. luteus “Bornal”. The alkaloid analysis revealed large contrasts in alkaloid concentrations and profiles in the leaves of the tested genotypes. Correlation analysis with the results from the field and laboratory did not indicate a significant influence of the total foliar alkaloid content on the extent of weevil feeding.     

Feeding by the pine weevil Hylobius abietis in relation to sun exposure and distance to forest edges

Abstract 1 The intensity of feeding by adult pine weevils Hylobius abietis (L.) on the stem bark of Norway spruce Picea abies (L.) Karst. seedlings planted in rows with a north–south orientation across a clear‐cutting, was measured throughout a growth season. The feeding was then correlated to light interception, soil temperature and distance to the nearest forest edge. 2 Feeding was at least twice as intense on seedlings in the central part of the clear‐cutting compared to those at the edges. The decline began approximatety 15 m from the edge and was of similar proportions on both the sun‐exposed and shaded sides. 3 Measures of global radiation and soil temperature correlated well with consumption on the shaded side. However, on the sun‐exposed side, there were no apparent correlations with global radiation or soil temperature that could explain the decline in consumed bark area. 4 We conclude that the decline in feeding towards the forest edges was mainly due to factors other than the microclimate variables we monitored. We suggest that the presence of roots of living trees along the forest edge may reduce damage to seedlings, since they provide an alternative source of food for the weevils. This alternative‐food hypothesis may also explain why seedlings in shelterwoods usually suffer less damage from pine weevils than seedlings in clear‐cuttings.     

Interaction of specialist root and shoot herbivores of Alliaria petiolata and their impact on plant performance and reproduction

Abstract.  1. This study explored interactions of two spatially and temporally separated weevils and their impact on Alliaria petiolata (garlic mustard) survival, growth, and reproduction at different herbivore densities.
2. The root-mining weevil Ceutorhynchus scrobicollis attacks A. petiolata rosettes from October to April, and larvae complete development in May. The shoot-mining weevil Ceutorhynchus alliariae attacks bolting plants in April/May with larvae completing development in June–July. Priority effects were expected, with early attack of C. scrobicollis affecting the later attacking C. alliariae , mediated through changes in plant growth or chemistry.
3. Attack by C. scrobicollis significantly increased plant mortality and changed plant architecture, while C. alliariae only significantly reduced plant height. Attack by C. scrobicollis also increased nitrogen content of stems.
4. Root feeding by C. scrobicollis affected the feeding niche of C. alliariae , but increased stem nitrogen content did not result in increased stem miner survival. While reduced height and stem diameters as a result of C. scrobicollis attack reduced C. alliariae attack at the stem level, attack at plant level and recruitment was unaffected.
5. Weevil density had no effect on plant performance, most likely due to strong intraspecific competition, and there were no synergistic effects between the two herbivores.
6. Overall, attack by C. scrobicollis was more detrimental to A. petiolata growth, seed output, and survival than attack by C. alliariae . Consequently, C. scrobicollis has been prioritised as a potential biocontrol agent for control of A. petiolata in North America.     

Effects of two pheromone trap densities against banana weevil, Cosmopolites sordidus, populations and their impact on plant damage in Uganda

Abstract:  An on-farm study to evaluate the effect of pheromone trap density on the population of the banana weevil, Cosmopolites sordidus (Germar) (Col., Curculionidae) was conducted in Masaka district, Uganda. The pheromone used was Cosmolure+, a commercially available weevil aggregation pheromone. Forty-two farms were assigned to one of three treatments: 0, 4 and 8 pheromone traps/ha. Pheromone lures were changed monthly at which time the traps were moved to a different location within the stand. Adult weevil population densities were estimated by using mark and recapture methodology at 0, 6, 12, 18 and 21 months, while damage to the banana corm was assessed at 0, 3, 6, 12, 18 and 21 months since the start of the experiment. Pheromone trap captures were generally low: about 10 weevils per trap per month. There were no significant differences in mean catches of C. sordidus per trap per month except for February 2002 when doubling the pheromone trap density decreased weevil catches. Although not significant, decreased efficiency was also the trend in higher trap densities over all the data sets. Doubling the number of traps increased the number of weevils caught per hectare per month from 0.4 to 0.6%. There was no significant difference in plant damage between the pheromone treatments in low- compared with high-trap densities. There were generally no significant differences in weevil populations and plant damage between pheromone-treated and control farms. Possible reasons for the low-trap efficacy in this study are discussed.     

Adult large pine weevils Hylobius abietis feed on silver birch Betula pendula even in the presence of conifer seedlings

1 The feeding preference of the adult pine weevil Hylobius abietis (L.) (Coleoptera: Curculionidae) for Betula pendula Roth was studied in no‐choice and paired‐choice feeding experiments. 2 In the first no‐choice test, large quantities of silver birch bark in Petri dishes were consumed; on average, the daily consumption of each weevil was 67 mm². 3 In the second no‐choice test, the weevils were offered 1‐year‐old silver birch seedlings for 6 days. Initially, the weevils fed mostly on the stem bases; later, they moved upward to feed on other parts of the stems. In addition to the main shoots, scars caused by gnawing were also found on leaf bases, blades, veins and petioles. Feeding resulted in the death of the main stems in 15% of the seedlings. 4 In the paired‐choice tests, the conifers were preferred to silver birch, even though a large amount of silver birch was also consumed in the presence of conifers. 5 In the paired‐choice tests, equal amounts of Scots pine and Norway spruce were always consumed. When hybrid aspen was offered, only small amounts were gnawed.     

Diel behaviour and time budget of the adult pine weevil Hylobius abietis

The pine weevil [Hylobius abietis (L.); Coleoptera: Curculionidae] has a high economic impact on forest regeneration in Europe. The general biology of the pine weevil has received considerable attention, although there is insufficient knowledge about its diel behaviour and time budget. Therefore, in the present study, the feeding and locomotion behaviour of individual adult weevils on Norway spruce seedlings [Picea abies (L.) Karst.] is observed for 24‐h periods in the laboratory. Both girdled and nongirdled seedlings are used to assess how the behaviour of weevils is influenced by the physiological response of plants to the girdling. The locomotion pattern shows a distinct maximum during the beginning of the dark phase, whereas most feeding occurs during the second half of the dark phase and the first hours of the subsequent light phase. The girdling treatment increases the time that weevils spend on the seedlings during the first part of an observation session, although it has no effect on their feeding pattern. The time budgets of weevils on girdled and nongirdled seedlings are similar. On average, weevils spend 34% of their time in locomotion and 6% on feeding. Females spend more time feeding than males (7.1% versus 4.2%), possibly because they have higher food requirements (e.g. for egg production). Females also spend more time in total on the seedlings than males (26.3% versus 7.0%). The present study reveals, in high temporal resolution, the diel feeding and locomotion behaviour and time budget of male and female pine weevils.     

补充营养对枹栎象成虫寿命的影响

在饲养笼内分别饲喂枹栎(Quercus serrata Murray)幼果、枹栎嫩枝、清水和10%蔗糖水,以不饲喂任何食物为对照,研究营养补充对枹栎象Curculio haroldi Faust成虫寿命的影响。同时,还研究枹栎象的个体大小和性别对其寿命是否有潜在影响。结果表明:补充不同营养对枹栎象的寿命有显著影响,其中10%蔗糖水组的寿命最长,是其他营养组的3~4倍。另外,虫体大小(即体长)对枹栎象寿命也有显著影响,且雌虫寿命一般比雄性略长,但性别间无显著差异。研究结果说明,营养品质和虫体大小可显著影响枹栎象寿命,进而影响其繁殖以及对寄主植物的危害。     

Attraction of pepper weevil to volatiles from damaged pepper plants

Pioneer herbivorous insects may find their host plants through a combination of visual and constitutive host‐plant volatile cues, but once a site has been colonized, feeding damage changes the quantity and quality of plant volatiles released, potentially altering the behavior of conspecifics who detect them. Previous work on the pepper weevil, Anthonomus eugenii Cano (Coleoptera: Curculionidae), demonstrated that this insect can detect and orient to constitutive host plant volatiles released from pepper [Capsicum annuum L. (Solanaceae)]. Here we investigated the response of the weevil to whole plants and headspace collections of plants damaged by conspecifics. Mated weevils preferred damaged flowering as well as damaged fruiting plants over undamaged plants in a Y‐tube olfactometer. They also preferred volatiles from flowering and fruiting plants with actively feeding weevils over plants with old feeding damage. Both sexes preferred volatiles from fruiting plants with actively feeding weevils over flowering plants with actively feeding weevils. Females preferred plants with 48 h of prior feeding damage over plants subjected to weevil feeding for only 1 h, whereas males showed no preference. When attraction to male‐ and female‐inflicted feeding damage was compared in the Y‐tube, males and females showed no significant preference. Wind tunnel plant assays and four‐choice olfactometer assays using headspace volatiles confirmed the attraction of weevils to active feeding damage on fruiting plants. In a final four‐choice olfactometer assay using headspace collections, we tested the attraction of mated males and virgin and mated females to male and female feeding damage. In these headspace volatile assays, mated females again showed no preference for male feeding; however, virgin females and males preferred the headspace volatiles of plants fed on by males, which contained the male aggregation pheromone in addition to plant volatiles. The potential for using plant volatile lures to improve pepper weevil monitoring and management is discussed.     

Non-pesticide alternatives for reducing feeding damage caused by the large pine weevil (Hylobius abietis L.)

The large pine weevil (Hylobius abietis L.) is an important pest of young forest stands in Europe. Larvae develop under the bark of freshly cut pine and spruce stumps, but maturing weevils feed on the bark of coniferous seedlings. Such seedlings frequently die because of bark consumption near the root collar. We tested the effect of three treatments (the insecticide alpha cypermethrin, a wax coating and a glue coating) on the feeding damage caused by H. abietis on Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) seedlings under semi-natural conditions. In two experiments (one in 2016 and another in 2017) seedlings in cages were subjected to pine weevil feeding for 16 weeks under shaded outdoor conditions. The experiment in 2016 compared insecticide and wax treatments and an untreated control on Douglas fir and Norway spruce, and the experiment in 2017 compared insecticide, wax and glue treatments and an untreated control on Norway spruce. In both experiments, all treatments significantly reduced H. abietis feeding damage at week 8 at the end of both experiments (week 16); the effect of treatments was significant only on spruce seedlings. The damages on Douglas fir seedlings was less on treated seedlings than on untreated control seedlings but differences were not significant. Coating stems with glue and especially with wax was generally effective at reducing weevil damage and in most cases provided control that was not significantly different from that provided by insecticide treatment. Our results suggest that a wax coating has the potential to replace the protection of seedlings provided by insecticides.     

Olfactory responses of the vine weevil, Otiorhynchus sulcatus, to tree odours

Abstract A Y-tube olfactometer and a still-air olfactometer were developed to determine the attractiveness of several host plants for the vine weevil ( Otiorhynchus sulcatus (F.); Coleoptera: Curculionidae). Odours of weevil-damaged yew ( Taxus baccata ) and spindle trees ( Euonymus fortunei ) are attractive to the vine weevil, but Rhododendron and strawberry ( Fragaria  ×  ananassa ) are not. Undamaged Euonymus is attractive to the weevils in springtime but not in late summer. When clean air or undamaged Euonymus is the alternative, weevils strongly prefer weevil-damaged Euonymus foliage, and this preference is retained throughout the year. Hence, plant damage plays a role in attraction of the vine weevil. In contrast to the permanent attractiveness of weevil-damaged Euonymus , mechanically damaged plants gradually lose the attractiveness that they have early in the growing season. This suggests that emission of volatiles, produced by the plants in response to weevil damage, is important for attraction of the weevils because the weevils may use these plant odours to find suitable food plants throughout the season. Apart from weevil-damage-related plant volatiles, green leaf volatiles must also play a significant role, as indicated by the fact that weevils prefer: early season, undamaged Euonymus over clean air; early season, mechanically damaged Euonymus over undamaged Euonymus ; and, throughout the season, had no preference when mechanically damaged Euonymus is tested against weevil-damaged Euonymus . Thus, monitoring traps may be developed by the use of green leaf volatiles and/or herbivore-induced volatiles, as attractants.     

Pine weevil damage to Norway spruce seedlings: effects of nutrient‐loading,soil inversion and physical protection during seedling establishment

• 1 The pine weevil Hylobius abietis (L.) feeds on the bark of young conifer seedlings and is one of the most economically important forest pests in Europe.
• 2 In a field experiment, we examined the combined effects of the treatments: nutrient‐loading of seedlings, planting in scarified plots and protection of seedlings against pine weevil damage for either half a season or a full season.
• 3 Nutrient loading had no significant effect on the amount of pine weevil feeding.
• 4 Seedling mortality was significantly reduced when seedlings were protected from pine weevil feeding during establishment. This occurred even though the debarked area of protected seedlings 5 weeks after the shields had been removed was similar to that of the unprotected seedlings. This indicates that initial protection rendered the seedlings more tolerant to later wounding by pine weevil.
• 5 Planting in soil inversion significantly reduced feeding compared with planting in humus.
• 6 We conclude that nutrient‐loading of seedlings in the autumn before planting would not increase pine weevil feeding after planting. Mortality could be reduced by treatments that postpone the start of pine weevil feeding on seedlings. Such treatments, combined with planting in soil inversion, would result in increased seedling growth, vitality and tolerance to pine weevil attack.

     

Maturation feeding and reproductive development in adult pine weevil, Hylobius abietis (Coleoptera: Curculionidae)

Maturation feeding on conifer bark by newly emerged Hylobius abietis(Linnaeus) is essential for reproductive development. When feeding occurs on young conifer transplants, this weevil causes significant economic damage. Between emergence and oviposition however, weevils feed on bark from different sources but of unknown nutritional 'quality'. The factors influencing the rate of feeding by males and females and female reproductive development were determined in laboratory bioassays using two contrasting food sources - the bark on different species of seedling conifer and on logs of mature trees. The nutritional 'quality' of bark was characterized by the concentration of nitrogen, total sugars, total polyphenols and resin. Regression models were used to show that overall, the rate of feeding on the bark of both seedlings and logs increased with weevil size and was negatively related to nitrogen concentration. The nitrogen concentration in seedling bark (mean 1.1%) was about three times higher than that of logs (mean 0.4%). The rate of reproductive development increased with nitrogen intake during feeding and the preoviposition period for weevils feeding on seedlings and logs was ~ 13 days and 46 days, respectively. Analysis of weevil mortality and of the weight gain of surviving weevils suggests that a nitrogen content of around 0.3% may be limiting for H. abietis. The possibility that nutritionally adequate food resources may be limiting for H. abietis is briefly discussed.     

Population dynamics of the white pine weevil, Pissodes strobi, infesting jack pine, Pinus banksiana, in Ontario, Canada

1. Eight generations of white pine weevil, Pissodes strobi (Peck) , infesting the terminal shoots of young jack pine trees, were sampled as larvae, pupae, and adults between 1988 and 1995.
2. The density of adult weevils increased rapidly for the first 3 years of the study, then declined in the next 4 years. Between-generation rates of change in density of weevils were related to mean number of weevils produced per terminal shoot of the host tree. Net increases in population density followed years in which a relatively high number of weevils emerged per terminal shoot and vice versa.
3. The mean number of weevils emerging per terminal shoot was determined by survival of weevils between the end of the larval stage and the successful emergence of adult weevils.
4. There was no clear relationship between survival of the post-feeding stages of the weevil and rates of parasitism or bird predation on these same stages. There was, however, a strong negative relationship between survival of weevils within the terminal shoot and the abundance of the facultative dipteran predator Lonchaea corticis Taylor.
5. The relationship among generational rates of change in populations, survival, and predation was evident irrespective of the age of the trees infested, suggesting that population dynamics of the weevil may be influenced as much by local predation pressure as by physical and biotic changes associated with growth of the young host trees.     

Genetic divergence and ecological specialisation of seed weevils (Exapion spp.) on gorses (Ulex spp.)

Abstract.  1. Reproductive isolation of sympatric populations may result from divergent selection of populations in different environments, and lead to ecological specialisation. In Brittany (France), the gorse Ulex europaeus (Fabaceae, Genisteae), may be encountered in sympatry with one of the two other gorse species present: U. gallii and U. minor . A recent study based on morphological identification of seed predators of gorse has shown that two weevil species (Curculionoidea, Apionidae) infest gorse pods at different seasons and have different host ranges: Exapion ulicis infests U. europaeus in spring, whereas E. lemovicinum infests U. gallii and U. minor in autumn. Weevil populations may thus have diverged in sympatry.
2. As morphological identification of weevils is often difficult and some of the characters used may exhibit individual or environmental variation, mitochondrial and nuclear sequences of weevils collected within pods of the three gorse species in 10 populations of Brittany were used to reconstruct their phylogeny.
3. The results reveal that species differentiation based on morphological characters is confirmed by the two molecular data sets, showing that E. ulicis and E. lemovicinum are distinct species, and suggesting the absence of host races. Finally, E. ulicis was able to use U. gallii and U. minor pods in spring in some years in some populations, which appeared to depend on the availability of pods present during its reproductive period.
4. Divergence between E. ulicis and E. lemovicinum may have resulted from temporal isolation of reproductive periods of weevil populations followed by specialisation of insects to host phenology.