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2002), but certain studies showed no e ffects because of compensatory growth (Hawkes and
Sullivan 2001; Boalt and Lehtila 2007). In an experiment in which we simulated herbivory by
clipping l eaves o f B. juncea we also observed compensatory growth at low herbivory level
(Liu et al., 2009; 2010b). The effect of herbivory on pl ant growth and reproduction depends
on lots of factors such as plant species (Rogers and Siemann 2002), resource level (Hawkes
and S ullivan 2001; R ogers a nd S iemann 2002) , i nduced i nsect-resistance an d d amage t ime
(Agrawal 1998; 1999).
In t he current s tudy, t rBC2 a nd nt rBC2 w ere exposed t o i nsects onl y at pl ant l ategrowth
stage so that it resulted in poor effects. The timing of the insect attack is important.
Agrawal (1999) found that insect-resistance was induced by caterpillar herbivory at the fourth
true l eave o f w ild r adish, an d t his r esistance p ersisted i n n ewly f ormed l eaves o f d amaged
plants, reducing the mass of caterpillars feeding on induced plants compared to un-induced
controls. T hus, t he h erbivory at e arly-growth s tage of pl ants could i nduce t he r esistance t o
herbivory for i nsect-susceptible pl ants, which could unde restimate t he differences b etween
resistant and susceptible plants. In this study, the late flowering plants showed higher fitness
than the early flowering ones at the time of insect deposit. Most studies involving Bt insect-
resistant f ocused on t he e ffects of i nsects f rom t he ear ly-growth t o m ature, a nd t hey f ound
insect-resistant transgenic plants showed higher fitness only under moderate or higher insect
pressure (e.g. Ramachandran et al. 2000; Letourneau and Hagen 2009).
The impact of the insects on the seed weight and number decreased with the increased
proportion of t ransgenic pl ants. In c ontrast, w hen pl ants w ere not s ubjected t o i nsects, t he
growth and reproduction of both trBC2 and ntrBC2 showed apparently erratic variations, with
a maximum in populations composed by one half of resistant plants. This genotype proportion
in a l imited r esource e nvironment c ould c orrespond t o t he s patial pl ant di stribution t hat
maximizes an eventual fitness difference between the transgenic and non-transgenic plants. In
other words, in years without insect attack, the spread of transgenic plants would be stopped
by a l ow r eproduction va lue r elative t o t he na tive s usceptible pl ants i f t he t ransgenic
proportion r eaches 50% . O nce ove r t his t hreshold, t he t ransgenes w ould not c onfer a ny
reproduction c ost, w hich ope n t he w ay f or i ts f ixation i n t he popu lations. U nder i nsect
pressure, the growth and reproduction in both trBC2 and ntrBC2 showed linearly increasing
trends w ith t he i ncreased pr oportion of t ransgenic pl ants. T his c ould b e i nterpreted b y a
protection effect applied to all the plants in a cag e, perhaps because the insects leaved from
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