The Formation of Collective Silk Balls in the Spider Mite Tetranychus urticae Koch

Tetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and food resources become scarce, individuals gather at the plant apex to form a ball composed of mites and their silk threads. This ball is a structure facilitating group dispersal by wind or animal transport. Until now, no quantitative study had been done on this collective form of migration. This is the first attempt to understand the mechanisms that underlie the emergence and growth of the ball. We studied this collective behaviour under laboratory conditions on standardized infested plants. Our results show that the collective displacement and the formation of balls result from a recruitment process: by depositing silk threads on their way up to the plant apex, mites favour and amplify the recruitment toward the balls. A critical threshold (quorum response) in the cumulative flow of mites must be reached to observe the emergence of a ball. At the beginning of the balls formation, mites form an aggregate. After 24 hours, the aggregated mites are trapped inside the silk balls by the complex network of silk threads and finally die, except for recently arrived individuals. The balls are mainly composed of immature stages. Our study reconstructs the key events that lead to the formation of silk balls. They suggest that the interplay between mites' density, plant morphology and plant density lead to different modes of dispersions (individual or collective) and under what conditions populations might adopt a collective strategy rather than one that is individually oriented. Moreover, our results lead to discuss two aspects of the cooperation and altruism: the importance of Allee effects during colonization of new plants and the importance of the size of a founding group

Correlation between final volume of balls (if considering balls as spheres, volume on T_H-14 h) and number of mites (after dissection).

<p>The volume was correlated to the number mites (p<0.005, N = 15). The equation was Y = 0.0067x with R² = 0.76 (N = 15).</p

Proportion of experiments in which the balls emerged (<i>y</i>) after a cumulative flow of <i>x</i> mites.

<p>The curve presents a threshold (<i>σ</i> = 259.4 mites). A logistic function (solid line) was fitted to our experimental data (dots) with a logistic regression.</p

Experimental set-up.

<p>a). Plant infestation (15 mites on each leaf) <b>T_I</b>. b) Ascending flows of mites on the stick <b>T_F</b> c). Emergence of the ball <b>T_E</b> and d). Harvesting of the ball <b>T_H</b>.</p

Increase of the cumulated flow according to time.

<p>The data of each experiment was normalised according to maximum time and maximum cumulated flow. The fitted curve (Equation 2) has parameters <i>α</i> = 2.231(1.959; 2.502) and <i>β = </i>0.266 (0.223; 0.308).</p

Growth of balls volume for the two consecutive days (two phases).

<p>Both phases were roughly linear (phase 1: y = 0.321x+1.269, R² = 0.924 and phase 2: y = 0.546x+2.201, R² = 0.79). The slope of phase 2 was steeper than that of phase 1 (F-tests comparing slopes: F_1,13 = 4.72, p = 0.049) due to a higher flow and possibly because fewer mites came down.</p

Number of individuals (mean ± sd, N = 15) contained in a ball.

<p>The stage (mature or immature) and the state (dead or alive) were noticed per layers (outer or inner part).</p

The influence of oceanographic scenarios on the population dynamics of demersal resources in the western Mediterranean: Hypothesis for hake and red shrimp off Balearic Islands

The aim of the present paper is to study the relationships between some climatic indices and parental stock, recruitment and accessibility to trawl fishery of hake (Merluccius merluccius) and red shrimp (Aristeus antennatus) off Balearic Islands (western Mediterranean). Available annual catch per unit effort, recruitment and spawning stock biomass have been used as biological data. As environmental data, the meso- scale IDEA index and the large-scale North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO) indices have been used. To analyze possible links between these indices with the population dynamics of demersal resources, two non-linear approaches have been applied: (i) stock–recruitment relationships from Ricker and Beverton–Holt models, by sequentially incorporating environment factors; (ii) generalized additive modelling, both classical general and threshold non-additive models were considered. The latter simulate an abrupt change in explicative variables across different phases (time periods or climatic index values). The results have shown that two oceanographic scenarios around the Balearic Islands, associated with macro and meso-scale climate regimes, can influence the population dynamics of hake and red shrimp. This is especially true for recruitment, which seems to be enhanced during low NAO and IDEA indices periods. During these periods, colder-than-normal winters generate high amounts of cold Western Mediterranean Intermediate Waters (WIW) in the Gulf of Lions, which flow southwards and reach the Balearic Islands channels in spring, increasing the productivity in the area. This oceanographic scenario could also be favourable to the distribution of hake on the fishing grounds where the trawl fleet targets this species, increasing its accessibility to the fishery. Both spawning stock and abundance of red shrimp seems to be also enhanced by high MO index periods, which could reflect the increased presence of the saline and warm Levantine Intermediate Waters (LIW) in the study area, extending over the fishing grounds of this species. The proposed interactions can be useful to assess and manage these important demersal resourcesPublicado