Efficiency of cotton bollworm ( Helicoverpa armigera Hübner) control of different Bt cotton varieties in North China

Table 1 The percentages and number exceeding the action threshold for each cotton bollworm generation from 2008 to 2015

Year	Number of varieties	2nd generation (> 10 larvae)^a	3rd generation (> 13 larvae)^b	4th generation (> 13 larvae)^c	Ratios^d per generation/%
Year	Number of varieties	2nd generation (> 10 larvae)^a	3rd generation (> 13 larvae)^b	4th generation (> 13 larvae)^c	2nd	3rd	4th
2008	35	0	1	2	0.0	2.9	5.7
2009	29	5	3	2	17.2	10.3	6.9
2010	22	0	1	0	0.0	4.5	0.0
2011	22	9	0	0	40.9	0.0	0.0
2012	21	1	0	0	4.8	0.0	0.0
2013	21	1	0	0	4.8	0.0	0.0
2014	23	2	3	1	8.7	13.0	4.3
2015	24	2	0	0	8.3	0.0	0.0

^a2nd (>10 larvae), the number of varieties that exceeded the 10-larvae per 100 cotton plants action threshold at the second generation of cotton bollworms
^b3rd (>13 larvae), the number of varieties that exceeded the 13-larvae per 100 cotton plants action threshold at the third generation of cotton bollworms
^c4th (> 13 larvae), the number of varieties that exceeded the 13-larvae per 100 cotton plants action threshold at the fourth generation of cotton bollworms
^d \( \mathrm{Ratio}\left(\%\right)=\frac{\mathbf{2nd}\kern0.5em \left(>10\ larvae\right),\mathbf{3}\mathbf{rd}\ \left(>13\ larvae\right),\mathbf{or}\ \mathbf{4}\mathbf{th}\ \left(>13\ larvae\right)\ \mathbf{generation}}{\mathbf{Number}\ \mathbf{of}\ \mathbf{varieties}}\times 100 \)