Conspecific Eggs Research Articles

Competitive Interactions Among Two Specialist Predators and a Generalist Predator of Hemlock Woolly Adelgid,Adelges tsugae(Homoptera: Adelgidae), in the Laboratory

Abstract Competitive interactions among two specialist predators, Laricobius nigrinus Fender and Sasajiscymnus (Pseudoscymnus) tsugae Sasaji and McClure, and a generalist predator, Harmonia axyridis Pallas, of hemlock woolly adelgid, Adelges tsugae Annand, were studied in the laboratory. The two specialist predators are part of a biological control program that has been initiated for A. tsugae, and the potential for competition among these species and polyphagous established predators is unknown. Laboratory assays were evaluated in environmental chambers at two conditions: (1) early spring, 9:5°C (D:N), 12:12 (L:D) h, and 50–75% RH; and (2) late spring, 18:10°C (D:N), 16:8 (L:D) h, and 75–90% RH. Predator feeding trials using conspecific and heterospecific eggs showed that L. nigrinus and S. tsugae eggs were consumed by all species, and predation was decreased with increased A. tsugae density. Eggs of H. axyridis were eaten almost exclusively by conspecifics, at high rates, regardless of A. tsugae density...

Evolution of egg size and fertilisation efficiency in sea stars: large eggs are not fertilised more readily than small eggs in the genus Patiriella (Echinodermata: Asteroidea)

Fertilisation kinetics theory suggests that, when sperm are limiting, the larger eggs of broadcast-spawning marine organisms ought to be fertilised more frequently than smaller eggs, because they provide a bigger target for searching sperm. Whilst this effect has been demonstrated within species, it is not known if this pattern holds among species. We tested whether a large difference in egg size between congeneric seastars with contrasting planktotrophic and lecithotrophic modes of development results in differences in the likelihood of eggs being fertilised in sperm-limiting situations. Measurement of egg sizes and sperm swimming speeds led to the prediction that the sperm–egg collision rate constant for Patiriella calcar (420-µm-diameter egg) should be nine times greater than for P. regularis (140-µm-diameter egg). Although the eggs of P. calcar should be fertilised at greater rates in low sperm concentrations, they were not. When gametes were allowed to mix for 10 s, the hypothesis that P. calcar eggs required less sperm than P. regularis to ensure 50% of eggs were fertilised was rejected. When gametes were mixed for 5 min, P. regularis eggs were more frequently fertilised, but the difference was not statistically significant. We conclude there must be a difference between these species in the likelihood that when a sperm finds a conspecific egg it can successfully fertilise. This apparent uncoupling of egg size and likelihood of fertilisation suggests that fertilisation is not a major constraint on the evolution of egg size in these seastars.

Role of surface chemicals in egg cannibalism and intraguild predation by neonates of two aphidophagous ladybirds, Propylea dissecta and Coccinella transversalis

Abstract: The role of surface chemicals in egg cannibalism and intraguild predation by neonates of two aphidophagous ladybirds, Propylea dissecta and Coccinella transversalis were examined. Neonates of both species prefer to eat non‐sibling conspecific eggs than heterospecific eggs, with higher preference in P. dissecta. Surface chemicals appear to play a major role for the preference of conspecific eggs, as the ladybird behaviour was reversed when these chemicals were interchanged. The surface chemicals present on the eggs possibly act as feeding stimulants to conspecific neonates but not for heterospecific neonates. Egg clustering enhances the effect of surface chemical and stimulate non‐sibling egg cannibalism and appears to reduce intraguild predation. Egg clustering is advantageous to ladybirds, as it can stimulate non‐sibling cannibalism by neonates. It appears that aggregation of chemicals present on the conspecific egg surface possibly attracts the hungry conspecific neonate, providing the first meal for its survival, while protecting against heterospecific predators. Neonates of both ladybirds were reluctant to eat heterospecific eggs.

An obligate brood parasite trapped in the intraspecific arms race of its hosts

Reciprocal selection pressures often lead to close and adaptive matching of traits in coevolved species. A failure of one species to match the evolutionary trajectories of another is often attributed to evolutionary lags or to differing selection pressures across a geographic mosaic. Here we show that mismatches in adaptation of interacting species--an obligate brood parasitic duck and each of its two main hosts--are best explained by the evolutionary dynamics within the host species. Rejection of the brood parasite's eggs was common by both hosts, despite a lack of detectable cost of parasitism to the hosts. Egg rejection markedly reduced parasite fitness, but egg mimicry experiments revealed no phenotypic natural selection for more mimetic parasitic eggs. These paradoxical results were resolved by the discovery of intraspecific brood parasitism and conspecific egg rejection within the hosts themselves. The apparent arms race between species seems instead to be an incidental by-product of within-species conflict, with little recourse for evolutionary response by the parasite.

Do these eggs smell funny to you?: an experimental study of egg discrimination by hosts of the social parasite Polyergus breviceps (Hymenoptera: Formicidae)

Social parasites exploit the behaviours of other social species. Infiltration of host systems involves a variety of mechanisms depending on the conditions within the host society and the needs of the social parasite. For many species of socially parasitic ants, colony establishment entails the usurpation of colonies of other species. This frequently involves the eviction or death of the host colony queen and the subsequent adoption of the invading queen. The social parasite queen achieves host worker acceptance by either manipulating the nest-mate recognition processes of the host or undergoing chemical modification. Little is known, however, about how host workers respond to social parasite eggs or whether host species defend against brood parasitism during parasite invasions. Host species are believed to adopt social parasite offspring because the recent common ancestry between many social parasites and their hosts may grant the sharing of certain characteristics such as chemical cues. Use of multiple host species, however, suggests other processes are needed for the social bond between host and parasite young to form. This study reports the findings of adoption bioassays in which eggs from a slave-maker ant, Polyergus breviceps, were offered to workers of two of its host species from unparasitised or newly parasitised nests to determine whether P. breviceps eggs generally elicit rearing behaviours from multiple host species. Comparisons of parasite egg survival until adulthood with conspecific egg survival reveal that workers of both host species, free-living or newly enslaved, do not typically accept slave-maker eggs. Both host species thus have sufficient discriminatory power to reject social parasite eggs although our hydrocarbon analysis indicates parasite eggs may be adapted to their local host species. Combined these results suggest that host rearing of P. breviceps eggs may reflect an evolutionary equilibrium that is maintained by probability and cost of recognition errors.

Experiments on egg discrimination in two North American corvids: further evidence for retention of egg ejection

In the absence of brood parasitism in North America, black-billed magpies, Pica hudsonia (Sabine, 1822), and yellow-billed magpies, Pica nuttalli (Audubon, 1837), may have retained egg-discrimination behaviour that evolved in Eurasian magpies, Pica pica (L., 1758), in response to parasitism by Old World cuckoos. We further examined this hypothesis by testing the egg-discrimination abilities of black-billed magpies and the American crow, Corvus brachyrhynchos Brehm, 1822, which has no history of brood parasitism. In addition, we tested an alternative hypothesis that black-billed magpies evolved or retained egg discrimination to counter conspecific parasitism by testing their ability to eject foreign conspecific eggs and by using a signal detection model to estimate the level of conspecific parasitism required for ejection to be favoured. Black-billed magpies ejected all non-mimetic eggs and 62% of mimetic eggs. Significantly more mimetic eggs were ejected during the incubation stage than during the laying stage. Magpies ejected significantly more non-mimetic eggs than mimetic eggs overall, but there was no difference in ejection frequency during incubation. American crows ejected 21% of non-mimetic eggs and 8% of mimetic eggs. There was no significant difference in ejection frequency of the two egg types. Black-billed magpies ejected 11% of conspecific eggs and a relatively high level of conspecific parasitism (22%–49%) would be required to select for conspecific ejection, which provides little support for conspecific parasitism as a current selection pressure for maintaining egg discrimination. Thus, black-billed magpies appear to have retained egg rejection in the absence of parasitism through speciation from Eurasian magpies.

Egg rejection by Iberian azure‐winged magpiesCyanopica cyanusin the absence of brood parasitism

The Iberian azure‐winged magpieCyanopica cyanusshows a remarkable ability to discriminate against great spotted cuckooClamator glandariuseggs. Here, I studied whether egg recognition in this species could be a derived feature resulting from intra‐specific brood parasitism. Azure‐winged magpies showed a very high level of discrimination and rejection of great spotted cuckoo models (73.7%), and of conspecific eggs (42.8%), even when no evidence of great spotted cuckoo or conspecific brood parasitism has been found in the population. Azure‐winged magpie discriminated more readily than magpies, the current favourite host of the great spotted cuckoo. The high rejection rate of conspecific eggs by the azure‐winged magpie suggests that it is quite possible that egg discrimination in this species evolved in response to conspecific brood parasitism rather than to cuckoo parasitism.

Are blackcaps current winners in the evolutionary struggle against the common cuckoo?

Blackcaps Sylvia atricapilla reject artificial cuckoo eggs, and their eggs vary little in appearance within clutches, whereas among clutches eggs vary considerably. Low variation within clutches facilitates discrimination of parasitic eggs, whereas high variation among clutches makes it harder for the cuckoo to mimic the eggs of a certain host species. These traits have most probably evolved as counteradaptations against brood parasitism by the common cuckoo Cuculus canorus, even though blackcaps are not regularly parasitised today. In this study, we investigated how fine-tuned the rejection of parasitic eggs is in this species by introducing three types of eggs into their nests: a real non-mimetic egg the approximate size of a cuckoo egg, an artificial mimetic egg the size of a cuckoo egg and a real conspecific egg. As the rejection frequency of both mimetic and non-mimetic artificial cuckoo eggs has been shown to be high in previous studies, the variation in rejection behaviour between individuals is low, indicating that most individuals within the population are able to reject parasitic eggs. Thus, we predict that (1) the intraclutch variation in egg appearance should be generally low in all individuals, and that (2) regarding conspecific eggs, rejection decisions should be highly dependent on the degree of mimicry between parasitic and host eggs. We found support for these predictions, which indicates that due to their highly sophisticated countermeasures against brood parasitism, blackcaps can probably be regarded as current winners of the arms race with the common cuckoo. Furthermore, the high and consistent rejection frequency of cuckoo eggs found throughout Europe for this species supports the spatial habitat structure hypothesis, which claims that woodland-nesting species breeding near trees, like blackcaps, presumably experienced a high level of parasitism throughout their range in the past and, therefore, their rejection behaviour, once evolved, spread rapidly to all populations.

Rejection of Conspecific Eggs in Chaffinches: The Effect of Age and Clutch Characteristics

AbstractPrevious experimental studies have found that the majority of chaffinches, Fringilla coelebs, are able to reject both non‐mimetic and mimetic cuckoo eggs and also non‐mimetic conspecific eggs. However, interestingly the frequency of rejecters of moderately mimetic conspecific eggs has been found to be only approx. 50%. We examined the possibility that acceptors of moderately mimetic conspecific eggs are first time breeders, because these individuals may lack the experience needed to reject eggs that deviate only slightly from their own eggs. Older individuals, with good knowledge of their own egg appearance, should therefore reject such eggs. We also examined the possibility that acceptors of moderately mimetic eggs have a higher intraclutch variation in egg appearance, which makes it more difficult to recognize such eggs when compared with rejecters. We obtained no support for any age‐specific pattern in rejection behaviour. Furthermore, there was no relationship between age and intraclutch variation, or intraclutch variation and rejection behaviour. As there is no evidence of intraspecific brood parasitism in this species, the rejection of any foreign eggs is most probably an adaptation to past cuckoo, Cuculus canorus, parasitism. Acceptance of good and moderately mimetic conspecific eggs is probably due to cognitive limitations, because evolution of a more fine‐tuned recognition ability is unnecessary in the absence of intraspecific brood parasitism.

Parental fish change their cannibalistic behaviour in response to the cost-to-benefit ratio of parental care

Partial filial cannibalism, the act of cannibalizing some offspring, has been explained as a response to the high energetic cost of care. I tested this hypothesis by manipulating the cost-to-benefit ratio of care in the scissortail sergeant, Abudefduf sexfasciatus, a tropical damselfish with male care. Background egg mortality was lower than the incidence of cannibalism, confirming that males did not just dispose of dead eggs. Investment in the current brood affected future investment, because males forced to skip a brood cycle put more effort into courtship during the following cycle and obtained larger broods than did unmanipulated males. Any factor influencing the cost-to-benefit ratio of parental care should also affect the incidence of cannibalism. I reduced the cost of care by supplementary feeding and reduced the benefit of care by simulating a decrease in paternity certainty through simulated intrusions by non-nesting males. Supplementary feeding significantly reduced partial filial cannibalism by parental males, a result compatible with the hypothesis that eggs are consumed to cover the energetic costs of parental care. Cannibalism decreased regardless of whether males were fed with conspecific eggs or crabmeat. Cannibalism was only reduced but not fully eliminated by supplementary feeding, and residual levels of cannibalism after feeding were similar to the background rate of egg mortality. Simulated intrusions by non-nesting males led to an increase in filial cannibalism and a decrease in parental effort.

Effect of intraguild predation on the survival and development of three species of aphidophagous ladybirds: consequences for invasive species

Abstract 1 Survival and development of hatchling larvae of three aphidophagous ladybirds (Coleoptera: Coccinellidae), Harmonia axyridis Pallas, Coccinella septempunctata brucki Mulsant and Adalia bipunctata Linnaeus, when fed their own and the other species eggs were recorded.2 In all three species, the larvae survived when fed conspecific eggs.3 The percentage of larvae of H. axyridis that survived decreased to 35% and 85% when fed eggs of A. bipunctata and C. s. brucki, respectively. All the larvae of A. bipunctata and C. s. brucki died after eating eggs of H. axyridis. None of the larvae of C. s. brucki died after eating eggs of A. bipunctata, whereas 46% of those of A. bipunctata died after eating eggs of C. s. brucki.4 In general, larvae were reluctant to eat the eggs of other species. However, larvae of C. s. brucki showed less reluctance than H. axyridis to eat the eggs of A. bipunctata.5 The consequence of this for invasive species of ladybird is discussed.

The relationship between dietary specialism and availability of food and water on cannibalistic interactions among predatory mites in protected crops.

The interplay between dietary specialism, the tolerance of food and water stress and level of cannibalism is likely to be important in determining the outcome of biological control using inundative releases of multiple natural enemies, such as phytoseiid mites in protected crops. The dietary specialist, Phytoseiulus persimilis, with a short immature development time (4-5 days) when plentiful food was available had a low ability to survive without food (5 days), even with access to water. The dietary generalists, Neoseiulus californicus, N. cucumeris and lphiseius degenerans, had longer immature development times (by up to 2 days) than P. persimilis. Survival ability differed amongst the generalist species when they were starved but provided with constant access to water. Both N. californicus and N. cucumeris survived the longest (8-10 days) and I. degenerans survived the shortest period (4 days). No negative intra-specific interaction between immatures was observed with P. persimilis when food was available and in the absence of food this species tended to starve rather than act cannibalistically. Both N. californicus and N. cucumeris showed a low degree of cannibalism between immatures, either when food was available, or when starved but given access to water. Even when food was available survival of I. degenerans fell by 30% in 4 days and remained at 60-70% for 3 further days; survival continued to decline rapidly when they were starved but provided with water. This indicates that immatures of I. degenerans could either feed on dead conspecifics or that they were capable of a degree of cannibalism. Adult females of P. persimilis did not feed on conspecific eggs even when deprived of food but provided with water. Adult female N. californicus and N. cucumeris did feed on conspecific eggs but at a low level (<1 egg per day), which occurred only after 48 h starvation. Although egg cannibalism occurred more consistently with adult female I. degenerans than with other mite species it was at a low level (<1 egg per day). If the tendency to cannibalism, not just of eggs but with more susceptible life stages such as larvae, is reduced when water is available freely this could be important in determining the interactions that occur under natural conditions.

EGG DISCRIMINATION IN THE YELLOWHAMMER

Abstract In a coevolutionary arms race between a brood parasite and its host, both species evolve adaptations and counteradaptations, such as egg mimicry and egg discrimination. The Yellowhammer (Emberiza citrinella) is a minor host of the Common Cuckoo (Cuculus canorus) in Europe. We studied egg discrimination in the Yellowhammer in the Czech Republic where it is parasitized only occasionally. To investigate host responses to parasitic eggs, we added either a nonmimetic (blue) or a mimetic (conspecific) egg to 50 nests. The hosts rejected nonmimetic eggs at a higher rate (92%) than mimetic eggs (32%). Neither intraclutch variation nor contrast between mimetic and host eggs had a significant effect on rejection behavior. There is no evidence for intraspecific brood parasitism in this species. The ability to reject mimetic eggs has therefore most likely evolved as an adaptation against interspecific brood parasitism and may be the reason why the Yellowhammer is parasitized only occasionally. Discriminacion...

Effect of substrate availability and conspecific cues on communal oviposition in the apple murex snail Phyllonotus pomum

Communal oviposition, the sharing of a single substrate by conspecific individuals for depositing clutches of egg capsules, is common among gastropod molluscs. In soft-bottom habitats, communal oviposition may occur because of a limitation of suitable hard substrate. Conspecific cues may be responsible for aggregating females. This study examined the oviposition behavior of the apple murex snail Phyllonotus pomum in seagrass beds in St. Joseph's Bay, Florida. Movement pat- terns obtained from a mark-recapture study conducted during the 5 mo prior to egg-laying season suggest that females are capable of moving, and often move, farther than the distance at which sub- strate might become limiting. In field surveys, 1 to greater than 14 possible suitable substrates were available within a 4 m radius of communal oviposition events, yet females still oviposited commu- nally. In a caged field-substrate manipulation experiment, females contributed to a single communal egg-mass even when additional substrates were available within 0.25 m. When given a choice between substrate without and with fresh egg capsules (< 8 d old) in a caged field experiment, females deposited an equal number of clutches on each type of substrate. Communal oviposition in P. pomum was not mediated by substrate limitation, and previously laid conspecific egg capsules did not attract females or induce them to oviposit communally. Alternatively, there may be an adaptive advantage to communal oviposition in P. pomum. It is possible that the process of egg-laying from 1 individual attracts, aggregates and induces oviposition in neighboring females.

Break-down of arms race between the red-backed shrike (Lanius collurio) and common cuckoo (Cuculus canorus)

AbstractThe red-backed shrike Lanius collurio once was parasitised by the common cuckoo Cuculus canorus in low frequency until the late 1960s in Hungary, but no case of parasitism is known from the last three or four decades. The cuckoo most probably abandoned this host species because its arms race had defeated, which may be indicated by the high level egg recognition and rejection ability of shrikes. However, mimicry of the cuckoo eggs in the last known cases of parasitism was significantly lower than between cuckoo eggs and host eggs collected from great reed warbler Acrocephalus arundinaceus nests within the same period. This suggests that red-backed shrikes won the arms race leaving no chance for cuckoos to develop better mimicry for their eggs. We tested these findings with experimentally induced parasitism using two types of real eggs: red-backed shrikes rejected 57.6% of the foreign conspecific eggs, but rejected 93.3% of the real cuckoo eggs transferred form nests of great reed warblers. This high level of rejection against real cuckoo eggs shows that there is no chance for presently occurring cuckoos to reparasitise red-backed shrikes in Hungary. Experimental eggs rejected by shrikes had lower mimicry than the eggs which were accepted, but intraclutch variation did not differ between accepters and rejecters. Hosts' aggression was significantly higher against the stuffed cuckoo than against the control species, the collared dove Streptopelia decaocto, and it was the least against female red-backed shrikes. Red-backed shrikes high aggression against cuckoo dummies also showed a highly developed antiparasite adaptation.

Do Common Whitethroats (Sylvia communis) discriminate against alien eggs?

In a coevolutionary arms race between a brood parasite and its host, both species are expected to evolve adaptations and counteradaptations, such as egg mimicry and egg discrimination. The Common Whitethroat (Sylvia communis) is a regular Common Cuckoo (Cuculus canorus) host in some parts of its breeding range. We studied egg discrimination in the Whitethroat in the south-eastern part of the Czech Republic in an area where the Whitethroat is parasitised only occasionally. To investigate responses to parasitic eggs, in 34 nests either a non-mimetic or a mimetic (conspecific) egg was added. All 21 non-mimetic eggs were ejected. Of the 13 mimetic eggs, 7 were ejected and 6 eggs were accepted. No significant differences were found in intraclutch variation in egg appearance between acceptors and rejecters. Ejected mimetic eggs, however, showed greater contrast with host eggs than did the mimetic eggs that were accepted. Our experiments show that Whitethroats are persistent rejecters of alien eggs at the study site. This fact is discussed in the light of the host selection and host-parasite coevolution hypotheses.

Do male golden egg bugs carry eggs they have fertilized? A microsatellite analysis

In the golden egg bug, Phyllomorpha laciniata (Heteroptera, Coreidae), both males and females carry eggs on their back. Although females cannot carry their own eggs, males may carry eggs that they have fertilized. If males carry eggs they have fertilized, their behavior may be interpreted as paternal care. In this article, we provide genetic data for paternity assignment of eggs carried by 40 males collected from the field. The males and the eggs were typed by using four highly polymorphic microsatellite DNA markers. Out of the 247 eggs typed, 87% were excluded from the father-offspring relationship based on single-locus (least conservative exclusion) mismatches. Under the more conservative (exclusion by at least two single locus mismatches) method, 78% of the eggs were nonpaternal. Relatedness estimates further supported our paternity analyses. The average relatedness of the eggs to the carrying males was low (bem = −0.052 ± 0.024 SE). Within the population, males were unrelated to each other (bmm = −0.004 ± 0.0002 SE), as were the eggs carried by individual males (beggs = −0.004 ± 0.0001 SE). This first genetic study on the breeding system of the golden egg bugs did not find any support for the claim that egg carrying functioned as paternal care, nor did it support kin selection as explanation for conspecific egg carrying.

Squid egg mops provide sensory cues for increased agonistic behaviour between male squid

Males of many species compete for access to females. In order to avoid performing potentially costly agonistic behaviour for their entire adult lives, many group-living males use environmental cues to limit agonistic behaviour to times when it will be of most benefit. Long-finned squid, Loligo pealeii, live less than a year and aggregate in mixed- and single-sex schools. Adults participate in several spawning events, then die. During spawning events, males actively compete for females. Winning males pair with females, which subsequently lay eggs in communal sites on the ocean floor (‘egg mops’). To determine whether males use sensory cues provided by egg mops to regulate agonistic behaviour, we conducted four laboratory experiments. We measured the agonistic responses of pairs of adult males before, during and after exposure to conspecific egg mops. In three experiments, egg mops were manipulated to provide differing sensory stimuli (tactile, water-borne, visual). The addition of conspecific egg mops to tanks of paired male squid dramatically increased agonistic behaviour above control levels within minutes. Male squid were first attracted to the egg mops visually, but contact with the capsules was necessary to increase agonistic behaviour. After initial contact, agonistic behaviour was almost continuous as long as egg mops remained present, even when squid touched the egg mops infrequently. Visual stimuli seemed important in maintaining elevated agonistic behaviour between egg mop touches. When egg mops were removed from the tank, measured agonistic behaviour declined within minutes. When egg mops were added to the tank while covered by an opaque and porous cover that allowed water-borne stimuli to circulate into the tank, squid did not approach the covered egg mop or show increased agonistic behaviour. This result suggests that water-borne stimuli are not sufficient to increase agonistic behaviour. It is unusual for male agonistic behaviour in any species to be increased by contact with fertilized eggs. In this species, however, egg capsules might signal that sexually mature, receptive females are about to lay eggs. Indirect evidence suggests that mating with a female immediately before she lays eggs increases male paternity. If this prediction is true, the presence of egg mops may indicate the optimal time for male squid to establish mating precedence through agonistic bouts. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Conspecific Eggs and Bertha Armyworm,Mamestra configurata(Lepidoptera: Noctuidae), Oviposition Site Selection

The oviposition biology of the bertha armyworm, Mamestra configurata Walker, was studied with emphasis on the effect of conspecific eggs on oviposition site selection. Bertha armyworm lay clusters of up to 700 eggs, and larvae have feeding and growth habits similar to those of other Lepidoptera that gain advantages from feeding aggregations. In a field-cage experiment, multiple egg masses per leaf were noted, although the vast majority (85%+) of leaves available for oviposition received no eggs. A series of dual-choice laboratory experiments was conducted using paired excised leaves with and without eggs or egg-wash extracts. Females strongly preferred to oviposit on leaves with eggs of a different female than on leaves without eggs. However, females did not prefer leaves with their own eggs over control leaves without eggs. Gravid females also preferred leaves treated with a methanol egg-wash over leaves treated only with methanol, indicating that the source of oviposition stimulation may be chemically based. The potential relevance of these observations is discussed in the context of host-plant distribution and their exploitation by bertha armyworm.

Oviposition in three species of limnephiloid caddisflies (Trichoptera): hierarchical influences on site selection

SUMMARY1. Oviposition site selection was studied in three trichopteran species; an undescribed species of Hydatophylax (Limnephilidae), Onocosmoecus unicolor (Limnephilidae) and Neophylax rickeri (Uenoidae), in two Coastal Range streams in California, U.S.A.2. Hydatophylax sp. egg masses were generally found at or above the water surface on substrata near the bank in pools, where undercut banks, overhanging vegetation and rocks shaded attachment sites and provided wind protection. Onocosmoecus unicolor females deposited their egg masses exclusively above the water on moist emergent wood. The egg masses of N. rickeri were almost exclusively found in high velocity areas of riffles, where females oviposited under water and attached their egg masses to the submersed undersides of unembedded, protruding stones with large emergent surfaces.3. For Hydatophylax sp. and N. rickeri, a hierarchical selection scheme is proposed in which females use cues at three different spatial levels (stream, stream subunit, substratum) to choose oviposition sites. Females of O. unicolor seemed to choose at only two different levels of spatial resolution (stream, substratum).4. The formation of large aggregations of egg masses in N. rickeri and O. unicolor suggests that females actively choose oviposition sites where conspecific egg masses are already attached. The clustering of egg masses may minimise the risk of predation through the dilution effect, because egg masses of N. rickeri and O. unicolor are both attacked by dipteran predators.5. Females of the three Trichoptera species studied are able to make a clear choice of oviposition site in the heterogeneous stream environment. In comparison with other stream microhabitats, these are characterised by stable and relatively predictable environmental conditions during the time of egg development, which is seen as the main selective pressure leading to the observed egg‐laying behaviour.