最近一项研究发现，有很多瓢虫都会沦为寄生蜂——瓢虫茧蜂的受害者。目前已经发现瓢虫茧蜂为什么会把瓢虫当作孵卵器，巫毒瓢虫(zombie ladybugs)会对黄蜂易受攻击的幼虫起到保护作用，令食肉动物远离它们，增加它们幸存下来的可能性。

该研究成果6月21日发表在《生物学快报》上。这一研究发现，这种保护是要付出代价的：在仍活着的瓢虫(而非死瓢虫或者没有瓢虫的情况下)身上结茧的黄蜂幼虫在蜕变成成年黄蜂后，它们产的卵会更少。这表明黄蜂卵用来发育的相同资源，也被用来控制巫毒瓢虫。

瓢虫的悲剧

黄蜂的寄生方式早就受到科学家关注，它们并不是昆虫界唯一一种采用这种方式的昆虫。例如，寄生蜂Hymenoepimecis argyraphaga会在Plesiometa argyra蜘蛛的体内产卵。然后幼虫会咬破寄主的身体钻出来。寄主的心智也会被寄生虫牢牢控制住。被感染的P.argyra蜘蛛在死亡前，会在合适地点为黄蜂幼虫织出一张蛛网。被特定菌类感染的巫毒蚂蚁会在森林里四处游荡，直到正午它们才会停下来，用嘴死死咬住叶脉，把自己固定住。

日落时蚂蚁会死去，菌柄会从蚂蚁的脑袋上长出来。但是蒙特利尔大学的研究生范妮-穆勒及其同事注意到，即使瓢虫茧蜂(D.coccinellae)幼虫从瓢虫的腹部钻出来，它的寄主仍不会死，这时瓢虫处于半瘫痪状态，偶尔会抽搐一下。这些科学家猜测，活瓢虫或许能保护黄蜂幼虫不被食肉动物侵扰。为了验证这一理论是否正确，研究人员在实验室里养了4000多只瓢虫，并让黄蜂在不幸的瓢虫体内产卵。然后他们耐心等待黄蜂幼虫从寄主腹部钻出来，开始结茧。

瓢虫保镖

黄蜂幼虫从寄主体内钻出来以后，研究人员把这些巫毒瓢虫和黄蜂幼虫分成三组。他们把第一组的瓢虫从茧上取下来，只留下正在发育的幼虫。他们让第二组的幼虫继续与瓢虫呆在一起，不过他们会把瓢虫的脑袋碾碎，杀死它。第三组一切照旧，让黄蜂幼虫的茧继续留在活着的瓢虫身上。然后他们把这三个组合放到草蛉(一种食肉昆虫，喜欢吃黄蜂幼虫)周围。每只草蛉可以单独与一个茧呆15分钟，研究人员记下它们成功偷取黄蜂幼虫的次数。他们发现，由活瓢虫保护的茧被吃掉的可能性仅为大约35%。当黄蜂幼虫茧没有外物保护，或者与死瓢虫呆在一起时，它们被吃掉的可能性在85%到100%之间。

该研究还发现，与茧缚在一起的瓢虫活得时间越久，新生黄蜂的生育能力可能就越弱，这表明正在发育的黄蜂与它的寄主共享资源。有意思的是，幼虫从瓢虫腹部孵化出来前，瓢虫的储备越丰富，它存活和保护幼虫的时间就越长。研究人员还需要进行更多研究，才能确定黄蜂稍后是否会形成更多卵，以便弥补它们与巫毒寄主分享的资源。不过该研究还发现巫毒瓢虫存在的另一项令人感到好奇的事情：大约有25%的瓢虫会在寄生过程中存活下来，等到黄蜂幼虫离开后，它们会重新开始新生活。

原文出处：

Biology Letters   DOI:10.1098/rsbl.2011.0415

The cost of a bodyguard

Fanny Maure, Jacques Brodeur, Nicolas Ponlet, Josée Doyon, Annabelle Firlej, éric Elguero and Frédéric Thomas

Host manipulation by parasites not only captures the imagination but has important epidemiological implications. The conventional view is that parasites face a trade-off between the benefits of host manipulation and their costs to fitness-related traits, such as longevity and fecundity. However, this trade-off hypothesis remains to be tested. Dinocampus coccinellae is a common parasitic wasp of the spotted lady beetle Coleomegilla maculata. Females deposit a single egg in the haemocoel of the host, and during larval development the parasitoid feeds on host tissues. At the prepupal stage, the parasitoid egresses from its host by forcing its way through the coccinellid's abdominal segments and begins spinning a cocoon between the ladybird's legs. Remarkably, D. coccinellae does not kill its host during its development, an atypical feature for parasitoids. We first showed under laboratory conditions that parasitoid cocoons that were attended by a living and manipulated ladybird suffered less predation than did cocoons alone or cocoons under dead ladybirds. We then demonstrated that the length of the manipulation period is negatively correlated with parasitoid fecundity but not with longevity. In addition to documenting an original case of bodyguard manipulation, our study provides the first evidence of a cost required for manipulating host behaviour.